BE1008619A3 - Control method and equipment of a thermal printing head. - Google Patents

Abstract

The invention relates to a method of thermal printing on a support (14), in which a first heating element (56a) is excited by means of a first control pulse which is sufficient to independently produce an active area. on the support (14), and a second heating element (56b) is excited adjacent to the first heater (56a), by means of a second control pulse, this second control pulse being insufficient to independently produce an active zone on the support (14).

Description

       

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  Method and equipment for controlling a thermal print head.



   The present invention relates to a method and equipment for controlling thermal printers, preferably for forming binary images. More particularly, the invention relates to a method and equipment intended to selectively position the boundaries between the active areas and the non-active areas of an image.



   Thermal printing is generally performed by moving a print medium incrementally or "step by step" in a y direction relative to a fixed thermal print head. Thermal print heads generally include several heating elements frequently placed in line in an x direction perpendicular to the direction of movement of the print medium. The step of progression is often chosen equal to the spacing in the direction x of the heating elements. For example, printers with 300 dots per inch (dpi) printheads, i.e. those with a pitch of 1/300 inch, often feed the print media next to the thermal head at 1/300 inch intervals.

   The area of 1/300 x 1/300 of a square inch covered by a dot provided by a heating element is generally called a picture element or "pixel".



   The heating elements are, in general, resistant elements in each of which passes a separate current. A printing control device selectively controls the various currents passing through the heating elements to activate the latter in the desired manner during each progression of the support. Different heating elements can be activated selectively for each step of the print medium, giving a two-dimensional image on the print medium.



   The heating elements act in such a way

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 joint either directly on a printing medium which reacts to heat, or indirectly, via a donor medium, with transfer to an ordinary printing medium. For the first case, which is called "direct thermal printing", the printing medium changes color at determined locations corresponding to the heating elements which are activated selectively. In the second case, which is called "donor printing" or "thermal transfer printing", determined heated locations of the donor medium transfer color to the printing medium.



  The donor support can be a ribbon covered with a wax or an ink which melts under the effect of the heat of the heating elements.



   There is a constant desire for thermal printers that provide clearer, more uniform and more regular images. In general, this desire has been met by reducing the spacing between the heating elements. However, the size of these heating elements and the distance between them is limited by current design and manufacturing considerations. Usually, each heating element must be individually connected by a cable to a control device which supplies the operating electrical energy to the heating element. When the spacing between the heating elements becomes smaller, it becomes more difficult and more expensive to house this equipment. In addition, thermal printers should have relatively low requirements for processors and memories.

   In general, the higher the requirements for processors and memories, the more expensive the thermal printing device is.



   A method and equipment for controlling a thermal print head for producing binary images having improved effective definition for details larger than a certain minimum is provided. The head control device

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 thermal print receives information about the original image and provides control energy levels to a thermal print head that has multiple heating elements. The heating elements produce images on a print medium which has active areas and non-active areas. The thermal print head control device provides several control energies with which thermal distribution is associated.

   The interaction between neighboring thermal distributions is used to selectively place the boundaries between active and non-active areas.



   According to another aspect of the invention, the time distribution of the application of control energy to selected heating elements is selectively controlled to produce a favorable interaction between thermal distributions, by selectively positioning, doing so, active area boundaries.



  Brief description of the drawings.



   Fig. 1 represents an image printed from a perfectly circular original image, in accordance with the state of the art; Fig. 2 represents an image printed from the original perfectly circular image with a progression rate equal to four times the step; Fig. 3 shows an image printed from the perfectly circular original image using the invention; Fig. 4 is a perspective view of part of a typical thermal printing head comprising several heating elements; Figs. 5,6 and 7 show thermal distributions, resulting binary images and the three different control pulses of the heating elements which form them;

   Figs. 8,9 and 10 represent the increase of the limit as a function of the order time on the element

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 heater A and on a neighboring heater B; Fig. 11 schematically represents the equipment of the invention used to control a thermal printhead to selectively position the limits of active areas; Fig. 12 schematically illustrates the method of the invention for controlling the thermal print head in order to selectively position the limits of active zones; Fig. 13 schematically illustrates the method of the invention used to assign temperatures to each grid cell in the image buffer; Fig. 14 is a row of image buffer values for specifying the original image data for the circular active area;

   Fig. 15 shows a series of temperatures produced from the series of image buffer values of FIG. 14 using the method and equipment of the invention; Fig. 16 shows a series of heating element control values formed from the temperature series of FIG. Using the method and equipment of the invention; Fig. 17 shows printed characters without selectively adjusting the limits of active areas; Fig. 18 represents the characters printed with adjustment of the limits of active zones in accordance with the invention; Fig. 19 is a graphical representation of neighboring control pulses according to the invention; Figs. 20 to 25 show thermal distributions in response to the control pulses of FIG. 19;

   Fig. 26 shows four vertically aligned dots placed on the print medium by repeating the control pulses of FIG. 19 to four

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 locations, and Fig. 27 is a schematic representation of the technique of controlling a thermal printer in accordance with another embodiment of the invention.



  Detailed description of preferred embodiments.



   Before discussing the equipment and process of the invention, it will be useful to define first of all several terms relating to the thermal printing process.



   Thermal printing occurs after applying heat to a medium that reacts to heat. For direct thermal printing, the print medium that reacts to heat can be generally thought of as changing color or becoming "active" if the temperature at any given location exceeds a certain threshold. for any length of time. Similarly, for donor printing, the donor medium generally transfers color if its temperature at any given location exceeds a certain threshold for any length of time. It is recognized that different heat-sensitive print media can have different temperature thresholds, as can different donor media.

   Color change most often involves placing black images on white or transparent print media, but thermal printing is commonly used to transfer other colors as well.



   Once the temperature threshold is reached, it is believed that the heat flux and / or the time necessary for coloring are negligible. Different print media or donor media may require different amounts of thermal energy to control color transfer, or may require different durations for this operation. The invention applies to thermal printers, regardless of the fact that the color change is controlled by a temperature threshold, by a

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 heat threshold, by a time threshold or by a combination of these, and it is not necessary to fully understand the thermal coloring and thermal transfer characteristics of the support to use this invention.



   Aside from the color change that occurs at a limit temperature, the print or donor media is damaged if excessive heat or temperature is applied. Therefore, there is a damage temperature which should not be exceeded during the printing process. Similar to what happens with color change, damage to the print media can be related to temperature, heat or time, or a combination of these factors. The invention is applicable to thermal printing with any of these modes of damage.



   Thermal printing can also be characterized by the type of image that is produced. We are dealing with a binary impression when any point of the image has one of the two possible colors. In binary printing, there are no shades or shadows. Areas where staining takes place are called active areas, and areas where no staining takes place are called inactive areas. Binary printing is often used for text and for linear graphic images. Continuous tone images that are made up of different tones or color levels are often printed by binary printing devices using halftone printing techniques. In typical cases, these techniques use a series of halftone cells to render information from the original image.

   By changing the proportion of colored and non-colored areas in the halftone cell, the observer perceives different levels of color, although the image is binary in nature when viewed more closely. Thermal printers can also be used to produce images at

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 continuous levels by varying the optical density of the support.



   When influencing the color change to occur at a temperature threshold, it is useful to consider the "thermal distributions" of different heating elements, regardless of the time and the heat applied to them. Thermal distributions are graphical representations of maximum temperatures at given x-y locations on the media during the printing process, regardless of when the maximum temperature is reached at each particular x-y location. If the thermal distribution curve is located above the limit temperature plane at a particular x-y location, the coloring of this location is performed. If the thermal distribution curve does not meet the temperature threshold, the location is not colored.



   The absolute or physical resolution power (sometimes called "specific resolution power") of a printer is the size of the smallest mark or smallest detail that the printer can accurately produce and place. In general, each of the heating elements places a "point" on the image, and it is generally desirable that all the points are uniform. The absolute resolving power or "specific resolving power" of a thermal printer in the x direction is based on the "pitch", or distance between neighboring heating elements. Although thermal printers can produce a mark smaller than the pitch, these printers cannot place these small marks in the x direction with greater precision than the pitch.

   In addition, thermal printers cannot place more than one point between neighboring heating elements. The absolute resolving power in the y direction is based on the size of the heating elements in the y direction and the step size. In both directions, the absolute resolution power is also based on the characteristics

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 for thermal transfer from the print head to the support. In all cases, the print media must be able to support the resolving power of the printer so that the printed image reflects the resolving power of the printer.



   Printers may also have apparent or actual resolution powers that are different from the absolute resolution power if the boundaries between active and non-active areas can be governed to a greater degree than absolute resolution powers. By choosing a step smaller than the absolute resolving power y of the print head, the apparent resolving power can be increased in the y direction. For example, if the printer is advancing at four steps per pixel, the apparent resolution power in the y direction is equal to four times the absolute resolution power.

   Although a mark cannot be produced and accurately placed on a location smaller than a pixel, the limit, in the y direction, between the active and non-active areas (i.e. the limit between neighboring locations) can be placed within a quarter of a pixel. Therefore, for active areas with a dimension equal to or greater than one pixel in the y direction, the limits in the y direction can be placed with a precision of a quarter of a pixel. Until the invention, it was not possible to obtain an apparent or real resolving power in the x direction which was greater than the absolute resolving power, and this, due to physical constraints concerning the head. impression.



   The fidelity of a printed image expresses how close this printed image or output image is to the original image information supplied to the printer. Often the original information that is provided perfectly describes the desired output image, such as for text instructions produced by a computer. The printer can then "render" or "quantize" the information of the ideal original image to give a

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 grid-like mapping. The grid size is usually chosen to be equivalent to the printer's absolute or specific resolution power. Since it has not been possible to improve the resolution in the x direction in printers previously used, the absolute resolution power of these printers is equal to the actual resolution power.

   Each pixel element formed by the print head defines an area which is called here specific area and which is roughly equal to an area defined by each part of the grid. Alternatively, the original image information may already be presented in the form of a grid-like mapping before being supplied to the printer; in this case, the ideal original image may not be known exactly.



  Different techniques exist to improve the quality of information in the original image if the ideal original image is not exactly known. In any case, the restitution process can be imagined as consisting in specifying zones in a grid of finite dimensions.



  Usually, this grid represents the real resolving power of the printing device.



   For binary images, each grid location is defined as either completely colored or completely colorless. In this text, the discussion of data manipulation assumes that the information in the original image is rendered to the degree necessary to support the real resolving power of the printer.



   The visual acuity of a printed image is the sharpness of the image as it is perceived by an observer. Because of the way the human eye receives and averages light from different locations in an image, visual acuity does not necessarily match the resolution of an output image.



   Fig. 1 shows an output image 12 produced by a conventional thermal printing head on the support

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 14. The outline of an ideal original image 15 is shown superimposed on the output image 12, for comparison purposes. The ideal original image 15 represents a perfect circle, as found in a point used for punctuation. For other forms, the original image could represent any part of an image without limitation, and, in particular, any text or any linear graphic image. The direction reference 16 indicates the direction x of the row of heating elements and the direction y of movement of the support 14 relative to the print head.



   The output image 12 includes an active part 18 and a non-active part 20, with a boundary 22 between them. An imaginary grid 24 of pixels is represented near the output image 12. Vertical lines 26, in the imaginary grid 24, represent the spacing or the pitch 28 between the heating elements. Horizontal lines 30 in the imaginary grid 24 represent the step of progression 32 for the movement of the support 14 relative to the print head. The active part 18 is constituted by a series of points or marks formed by individual heating elements, each point approximately filling a box 33 of the grid. This output image 12 is a binary image, the active area 18 being entirely hatched.



   The boundary 22 includes steps or irregularities 34. The irregularities 34 appear due to the resolving power of the printer. As irregularities 34 show, the conventional thermal printer has the same resolving power in the vertical and horizontal directions, and the actual resolving power is equal to the absolute resolving power. The irregularities 34 represent a poor fidelity of the output image 12 to the original image 15, contrasting with the slightly wavy circular outline of the ideal original image 15. The irregularities 34 cause a visual acuity which leaves to desire when we look

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 Fig. 1 at a certain distance.

   The way in which the thermal printing head of known technology is used limits the possibility, for the thermal irrimant, of approximating more closely the curvature of the ideal original image 14.



   Fig. 2 represents an output image 36 formed by the same conventional thermal printing head and based on the same ideal original image 14. As shown by the horizontal lines 38, the size 40 of the progression step used to print the image output 36 is equal to a quarter of the progression step 32 used to print the output image 12 of FIG. 1. The reduced progression step 40 increases the real resolving power in the y direction of the output image 36 since sections, in the y direction, of the boundary 22, can be placed in any of four locations for each grid space 33. The irregularities 34 which are associated with vertical edges are the same as in FIG. 1.

   However, the irregularities 34 which are associated with horizontal edges are smaller than the irregularities 34 in FIG. 1. While this gives a significant improvement for the almost horizontal edges, the improvement is weak or nil for the almost vertical edges, and the overall fidelity as well as the visual acuity of the output image 36 are only slightly increased .



   During the processing of the image 36 having this reduced progression step 40, locations located inside the active area 18 are heated and cooled in four heating cycles. With the temperature threshold and the damage model due to temperature which have been described above, it is assumed that these multiple thermal cycles do not affect the quality of the active zone 18.



   Fig. 3 shows an output image 42 formed by the same conventional thermal printer, based on the same ideal original image 14 and using the

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 not even a progression 40 as in FIG. 2. However, the output image 42 has been printed in accordance with the neighborhood effect of the invention as will be described later. We see that the output image 42 shows a significant increase in the real resolution power. An observer recognizes a clear uniform edge of the active area instead of the irregularities 34 corresponding to the known technique, and the visual acuity of the output image 42 is much greater than that of the image 12 of the known technique, shown in FIG. 1. The fidelity of image 42 to the original image 14, which is perfectly circular, is also very high.



   In each of these Figs. 1 to 3, limit 22 marks the extension towards the outside of the series of printed dots. The corners of the boundary 22 are shown relatively square so as to illustrate more clearly the effects of the resolution powers x and y on the overall image. The corners actually produced correspond to the shape of the individual dots produced by the print head and they may be more rounded than in FIGS. 1 to 3.



   Fig. 4 is a greatly enlarged view of part of a thermal printing head 50.



  Fig. 4 is not drawn to scale, in particular with regard to the thickness and dimension of the different layers, in order to better contrast the different elements.



   The thermal print head 50 is formed on a conventional support 52 such as alumina or ceramic. A layer of varnish 54 is placed on the support 52 with resistant heating elements 56 placed on the layer of varnish 54. An electrically conductive layer 58 is then placed on each heating element 56 to close a separate electrical circuit passing through each heating element 56. The conductive layer 58 is a conventional electrical conductor, used in the production of thick or thin layers,

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 such as gold. The heating elements 56 are conventional resistors with a thin or thick layer.

   The step 28 between adjacent heating elements 56a, 56b, 56c, in the print head of the preferred embodiment, is 1/600 of an inch (i.e. 600 dpi). The heating elements 56 are substantially square and they generally produce a point, on the support 14, which fills a space of 1/600 x 1/600 of a grid square in inch 33.



   It is known that there are other types of thermal printheads different from that of FIG. 4. The dimensions, shape and orientation of the various constituent elements shown can be modified, as desired, by specialists. A varnish can be applied to the entire line of heating elements 56. The heating elements 56 can be arranged not in a linear fashion, but rather with an offset y between the neighboring heating elements 56a, 56b, 56c. The row of heating elements 56 can be arranged at an angle relative to the direction of movement of the support 14. The thermal printing head 50 can be moved in a direction y relative to the fixed printing support 14.

   Therefore, either the thermal print head or the print medium 14 can be moved in an x direction between steps of the print medium 14, so as to create a non-rectangular output pattern. Other modifications can be made to the thermal printhead 50. The invention applies to all the thermal printheads, whatever their shape, their pitch, their thermal characteristics or their configuration.



   The thin film thermal print head 50 can be modeled as several different first order systems, each having a different thermal time constant. The thermal time constant is the time required to reach approximately 63% of a final temperature increase, in response to an input signal of one step across a heating element 56. The

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 head support which is fixed to the substrate 52, with its thermal capacity and its effects of dispersion of heat in the atmosphere, has a thermal time constant of the order of several minutes. The substrate 52 itself has a thermal time constant of the order of several seconds. Finally, the heating elements 56 have a thermal time constant of the order of one millisecond.

   Thermal analysis of thin film thermal print heads is discussed in an article entitled "Design of Thermal Print Head or High Speed and High Resolution Printing", by S. Shibata and T. Kanamori, published in Electronics and Communications in Japan , Part II, volume 75, no 6,1992.



   The thermal reaction of the print head 50 generally conforms to the description which follows. At the start of a heating cycle, the entire head 50 is substantially at a uniform temperature, room temperature. A voltage is applied to the heating element 56 between the conductors 58a and 58b, which produces heat in the heating element 56. The thermal capacity of the heating element 56 is very low compared to the thermal energy applied. , and, consequently, the temperature of this heating element 56 increases sharply and rapidly (ie of the order of 500 ° C. in a millisecond in response to the voltage applied in the preferred system).



  As the temperature of the heating element 56 increases, heat transfer occurs from the element 56 in all directions, through the conductors 58, through the varnish 54, along the resistive layer 56 and, most importantly, in a z direction from the heating element 56.



   When the print head 50 is printing, the support (not shown in Fig. 4) is kept in contact with each of the heating elements 56a, 56b and 56c by an elastic biasing means, like a plate made of an elastomer. Heat is transferred from a resistive element 56 under tension, in

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 a positive z direction, and thus arrives in the printing medium which is located opposite the heating element 56. Heat can also be transferred from neighboring locations onto conductive layers 58 as well as between neighboring heating elements 56a, 56b, 56c. The heat transfer inside the support can take place both in the x direction and in the y direction as well as in a z direction.

   The heat transfer in the z direction inside the support can be important when using a donor support to control the transfer of wax or ink from the donor support to the printing support located opposite. Whichever particular mode of heat transfer is employed, the suitably arranged system will see the heat produced by the heating elements 56 transferred to the print medium, thereby forming corresponding dots or marks on the print medium.



   Since the thermal time constants of the effects of heat dispersion in the atmosphere and in the substrate 52 are very large compared to the effects of dispersion from the elements 56 and the support, the atmosphere and the substrate 52 can be modeled as radiators at constant room temperature when modeling the transient behavior of the print head 50. However, in particular during a heating cycle, the layer of varnish 54 and other parts of the print head 50 immediately adjacent to a heating element 56 do not behave like room temperature radiators.

   Thermal interaction takes place in all directions, including the transfer of heat in the x direction between the heating elements 56a, 56b, 56c and between corresponding x locations on the print medium. The particular characteristics of this thermal interaction depend on the physical characteristics of the device and the support. In general, the exact thermal interaction will be different for devices with different physical characteristics and

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 using different media, but it always takes place to some degree. The invention uses the thermal interaction between the heating elements, unlike the apparatuses of the prior art, in which it is sought to eliminate or compensate for the thermal interactions.

   Unlike the devices corresponding to the state of the art, the invention recognizes this thermal interaction and uses it.



   Fig. 5 shows three thermal distributions 66, 68 and 70 formed by three control pulses 65, 67 and 69 of FIG. 7. The control pulses 65,67 and 69 have different energies, as evidenced by their different durations, and they are applied to heating elements 56b, 56d and 56g, respectively.



  Fig. 6 represents binary images formed on the support 14 sensitive to heat by the application of three control energies 65,67 and 69.



   The y axis 60 of FIG. 5 represents the temperature of support 14 sensitive to heat and the axis x 62 represents the distance in the direction x along the support 14. The heating elements 56a to 56g are shown diagrammatically at their respective locations x and the thermal distributions 66, 68 and 70 are taken along a median axis 71 of heating elements 56 for the printed line L. A temperature threshold 64 represents a temperature plane above which the support 14 becomes active.



   Curves 66, 68 and 70 are theoretical representations of thermal distributions from heating elements 56b, 56d and 56g, respectively, during the heating cycle for line L. In the case where the thermal head 50 acts directly in concert with a printing medium such as heat-sensitive paper, a location situated on the median axis 71 of the support 24 of FIG. 6 changes color if the peak temperature at this location exceeds the temperature threshold 64. In the case where the thermal printing head 50 acts indirectly in concert with a printing medium, as in transfer printing

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 with thermal wax, the thermal head 50 is used to transfer pigments or wax from a transfer medium or from a donor medium such as a colored ribbon on the print medium.

   Pigments are transferred if a location of the donor support reaches the temperature threshold 64. The color of the active area depends on the color of the pigments. Often the pigments are cyan, yellow, magenta or black (CYMK).



   Fig. 6 shows the printing medium 14 on which the imaginary grid 24 is superimposed. The printing medium 14 shown shows a current printing line, designated L, a row or a previous printing line, designated L1, and the next printing line, designated L + 1. The current printing line L is produced by activating the thermal printing head 50 in order to create the thermal distributions 66, 68 and 70 shown in FIG. 5.



   Heat from the heating element 56b produces the active part 72. The active part 72 has a limit 74 which corresponds to the intersection of the temperature threshold 64 and the thermal distribution 66. The active part 72 represents a "point "of an image. The active part 72 is large enough to entirely cover a grid box 33b, and it extends to a limited extent (about 10 to 20%) in the neighboring grid boxes, 33a, 33c. The application of control energy 65, here called pixel specific control energy, to the heating element 56b, produces the active part 72 whose area is equal to that of the specific area. With this point dimension, multiple active parts provide uniform coverage without gaps when neighboring pixels are active.

   With printers known in the art, it is generally preferred that activated heating elements uniformly produce dots of this size.



   The thermal distribution 68 never reaches the temperature threshold 64, which is why the heat

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 from the heating element 56d does not produce a corresponding active part.



   By supplying energy to the heating element 56g for a longer time than to the heating element 56b, the heating element 56g is caused to produce an active part 76. The active part 76 has a limit 78 which corresponds at the intersection between the temperature threshold 64 and the thermal distribution 70. The active part 76 is larger than the active part 72, and the limit 78 extends considerably (from approximately 30 to 40%) in the neighboring cell Grid 33f. Unlike the devices of the known technique, the invention uses the selective establishment of limits of active zones based on selected control energies of the heating elements.

   The active parts 72 and 76 illustrate the fact that active parts of different dimensions can be formed by the application of different control energies, each of them forming a corresponding thermal distribution which exceeds the threshold 64.



   The active parts 72 and 76 are described as being circular. The shape of these active parts depends somewhat on the shape of the heating elements 56 and it can vary for different printers. The imaginary grid boxes 33 are described as being squares to represent the typical extent of a single active area 72 giving complete coverage of the grid box 33b on the support 14. For other forms of active areas, the imaginary grid 24 chosen may consist of rectangles, triangles or other figures which effectively represent the extent of a typical active area.



   Much of the prior art has attempted to improve the quality of the output image by making the dot size uniform and constant. In general, a number of factors can be offset to give a more constant point size, such as the residual heat from previous heating cycles. On the contrary, the invention creates points of

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 non-uniform sizes to form active areas which are more in line with an ideal original image or otherwise give better output. In the preferred embodiment, active points or parts are selectively extended in the x direction by the selective application of control energy to a neighboring heating element.



   Controlling or setting the limits between active zones and non-active zones in the direction x ensures an apparent or real increase in the resolution power in this direction. Because an active area can be developed or formed to extend to a selected extent in the x direction by applying heat to a neighboring heating element, the actual resolving power in the x direction is not limited only by the possibility that one has of governing the control energy which is applied both to a heating element and to the neighboring heating element.

   Although the active parts can be selectively extended in the x direction, there is still an active area of minimum size which can be formed and placed with precision, and there is therefore no increase in power. absolute resolution of the print head.



   Figs. 8 and 9 represent data obtained during tests using the method of increasing the active area of the invention. Fig. 8 gives operating times for neighboring heating elements A and B of a 400 dpi thermal head model, nO KWT-601-16MPJ8, manufactured by Kyocera Corporation of Kyoto, Japan. The control voltage applied to the heating elements was 15.5 volts instead of 24 volts, the normal control voltage. The reduced control voltage requires a longer duration of the control pulses for a given control energy in order to promote thermal interaction between the heating elements. The effects of the duration of the control pulses on the thermal interaction will be studied in more detail later.

   Based on

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 control time of FIG. 8, FIG. 9 shows the corresponding location of the boundary of the active area inside the neighboring grid box, given as a proportion of the total width of the neighboring grid box. Fig. 10 graphically represents the data of FIG. 9. Twelve different data points are shown; they allow you to selectively place the limit which goes from the beginning to the opposite edge of the neighboring grid box.



   As can be seen by studying this data, the first heating element must be operated for 5.2 milliseconds before its active zone limit reaches the neighboring grid box. When the operating time of heating element A is increased from 5.2 to 20 milliseconds, the limit selectively extends, passing through positions 1 to 5, until it reaches approximately 36% of the path going in grid box B. If the heating element A is used for periods exceeding 20 milliseconds, this tends to damage the support. With the size of the point and the duration of the operating time of position 5, it is estimated that the thermal distribution from the heating element A has reached a maximum practical limit.

   The control energy above which the print medium is damaged or active parts are not properly formed is referred to herein as maximum control energy. The control energies supplied to the heating elements which are greater than the specific control energy, but less than the maximum control energy are called, in this specification, over-control energies.



   While an increased control energy applied to the heating element A cannot extend the limit without damaging the support, the limit can be extended further in the grid box B without damaging the support by applying the energy to the heating element B. After heating element A has operated for

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 20 milliseconds, the fact of supplying control energy to the heating element B from 0.35 milliseconds to 2.25 milliseconds extends the limit of positions 6 to 12, over 95% of the path through box B grid. It should be noted that this supply of energy to the heating element B does not produce its own point, but rather extends the active area produced by the heating element A.

   It is estimated that the two heating elements contribute to the overall thermal distribution, and this thermal interaction allows the selective growth of the limit which is observed. The control energies supplied to the heating elements and which are lower than the specific control energies are called, in the present specification, subcommand energies.



   Using this method, the boundary can be placed selectively as desired inside the neighboring grid box. The selective positioning of the limit allows a considerable improvement in the real horizontal resolution power of the binary image obtained. Visual acuity and fidelity of the output image compared to the original image can be greatly improved.



   Fig. 11 shows the apparatus which is used with the method of the invention to control a thermal head in order to provide an improved result. As has been discussed in connection with Figs. 8 to 10, this improved result results from an ability to form active areas which can be extended or enlarged to a selected extent and which then form a binary image which is more consistent with the original image.



   One aspect of the invention consists of a thermal printing apparatus 100 which receives the information relating to the original image from a host unit 102. The thermal printing apparatus 100 comprises a head control unit 103 which includes a storage device 104, a processor 106 and a look-up table 108. The printing apparatus 100 further comprises a

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 print head control circuit 112 and a thermal print head 50 for forming images on the print medium.



   The host unit 102 is a computer or processor having one or more software programs executed therein to produce information relating to the original image. Alternatively, the host unit 102 may be a conventional image production apparatus such as a video camera or a scanner. Image information from the host unit 102 is sent to the thermal printing apparatus 100 for visual presentation.



   The image information from the host unit 102 can be provided in a number of different forms such as a bitmap representation or a standard page description language. When it receives the information relating to the original image, the printing apparatus 100 produces a cover map of an original image which is stored in the storage device 104. The rendering by the cover map of the original image represents information relating to the positioning of the active and non-active zones inside the output image. This coverage map can be presented in many different formats such as a multibit binary number representing the active area that is inside each grid cell associated with the output image.

   Alternatively, the coverage map can be a bit map or a set of binary values indicating which of the corresponding parts of the output image are active or inactive. Coverage maps that are in a bitmap format can have different resolution powers depending on the size of the corresponding part of the output image associated with each binary value in the set.



   The printing apparatus 100 may include an interpreter (not shown) for converting the image

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 origin to a cover map which is stored in the storage device 104. Interpreters for converting the original image information to give a cover map are known. Alternatively, the coverage information may be provided directly by the host device in a form usable by a processor. In this case, the storage device 104 can be deleted.



   An important aspect of the invention is the method and apparatus with which the print controller 103 uses the coverage information stored by the storage device 104. The print controller 103 uses information to generate control energies or control levels for each heating element in the thermal print head 116. These control levels are supplied to the print head control circuit 112. The control circuit for print head 112 converts these control levels into control voltage pulses which are applied to each of the heating elements.

   The print head 50, as described above, converts the control pulses into thermal distributions which, in concert with the print medium, act to form images either directly or indirectly.



   The storage device 104 can be any conventional digital storage device such as a magnetic storage device or a semiconductor storage device, to name but two. In a preferred embodiment, the storage device 104 is a storage device with a rotating magnetic medium, such as a disk storage device, which receives the coverage information by a network.



   In a preferred embodiment, the processor 106 is a programmable processor such as a microprocessor which operates under the control of software. It is also possible that the processor 106 is

  <Desc / Clms Page number 24>

 used in any application of conventional hardware such as logic devices, programmable logic circuits or control devices, pre-scattered arrays or some form of custom integrated circuit. The look-up table 108 can be any conventional storage device such as a magnetic or semiconductor storage device.



  In a preferred embodiment, the look-up table 108 is a dynamic random access memory (RAM). Although Fig. 11 shows the print control device 103, the control circuit 112 and the thermal print head 50 as separate functional blocks, one or more of these functional blocks can be physically joined. For example, the control circuit 112 or the print control device 103 can be physically mounted on the print head 50.



   Fig. 12 shows the method of the invention for converting the cover map in the storage device 104 into control levels for operating the thermal print head 50. As shown in step 120, temperature values are assigned to each grid cell that corresponds to the coverage information from the host device. The assigned temperatures form a temperature map to provide active areas that correspond to the coverage map.



   As shown in step 122, the control energies or the control levels are assigned to each grid cell taking as a basis the temperature map formed in step 120. The allocation of control energies accounts for the importance of the control energies supplied to the surrounding heating elements. In this way, the thermal distributions produced by these surrounding heating elements are taken into account. In a preferred embodiment, the neighboring heating elements as well as the neighboring heating elements of the neighboring heating elements are taken

  <Desc / Clms Page number 25>

 into consideration in assigning order levels.



   It is generally a difficult problem to calculate control energies which will combine with the heat produced by neighboring heating elements to arrive at the desired temperature of the support. This is why the invention makes use of a look-up table 108 to determine the control energies. In a preferred embodiment, the control levels are provided by the look-up table based on the temperatures of the grid box as well as by the four neighboring grid boxes which are closest. In this preferred embodiment, each of the grid cell temperatures is an 8-bit number representing temperatures between 0 and 1150C above ambient temperature.

   This is why all possible combinations of these 8-bit temperatures require a lookup table of 240 entries. In order to reduce the dimensions of this look-up table, the preferred embodiment approximates the temperatures in the grid cells with varying precision. With this preferred embodiment, the 6 most significant bits of the 8-bit grid cell temperature, the 4 most significant bits of the neighboring 8-bit grid cell values and the 3 most significant bits of the pair closest to 8-bit grid cell temperatures are used to select a command level in lookup table 108.

   Approximating temperatures in the grid boxes reduces the lookup table to 220 different temperature combinations that are used to select a corresponding control level. The use of a look-up table 108 in the invention makes it possible to quickly take into account these thermal contributions with only a slight increase in the equipment.



   Step 122 of the method, in FIG. 12, used to assign control energies to each temperature of

  <Desc / Clms Page number 26>

 grid box, uses the look-up table 108 which is described below. Step 122 of the process takes into consideration not only the temperature that the four closest heating elements will reach, but also the proximity of each of these heating elements with respect to the selected heating element. Lookup table 108 could be extended to include temperatures for more than four heating elements present. However, the contribution of the heating elements beyond the nearest four has only a small effect on the chosen thermal distribution, and it can therefore be neglected.



   To produce data in lookup table 108, one method is to use a mathematical model of the thermal properties of the thermal head 50. Techniques for modeling thermal print heads are known. See, for example, "Design of Thermal Print Head or High Speed and High Resolution Printing", by S. Shibata and T. Kanamori, published in Electronics and Communications in Japan, Part II, volume 75, no 6,1992. Simpler thermal models can also be used to speed up calculations. Since such a thermal model calculates peak temperatures as a function of control levels, an iterative technique can be used to calculate the set of control levels necessary to obtain a given set of peak temperatures.



   As shown in Figs. 11 and 12, step 120 is preferably executed by processor 106, and step 122 is preferably performed by processor 106 in conjunction with lookup table 108. When step 122 is completed, the assigned control energies can be stored in a buffer memory for use by the control circuit 112.



   As shown in step 124, the assigned control energies are then converted into control pulses and applied to the thermal print head.

  <Desc / Clms Page number 27>

 by the print head control circuit 112. The print head control circuit 112 can also include a resistance compensation means. Since it is now important to closely control the amount of heat produced by each heating element, any possible difference in the resistance of the heating elements is detrimental. The resistance compensation means adjusts the control energies applied based on the particular resistance of each heating element, thereby producing the desired thermal distribution.



   The control energies are supplied to the heating elements 56 to constitute a line of active zones on the printing medium. The print medium is moved relative to the thermal head 50, and a next set of control energies is applied to the thermal elements 56 to produce the next print line. In this way, a binary image is formed by advancing step by step or by moving the print medium relative to the print head 50.



   In each interval between applications of control energy, some time may be allowed to allow the system to cool. For example, when applying control energies with a maximum duration of 4 or 5 milliseconds, the heat can be allowed to dissipate for 9 or 10 milliseconds before the next heating cycle. With this cooling cycle, the latent heat from a previous heating cycle is negligible, and no compensation is necessary for the previous heating cycle. This simplification of the thermal cycles allows a simpler calculation and less costly requirements with regard to the control device.



   It should be recognized that the support need not actually be stopped during each heating cycle and that a "step size", as used herein, is not limited to displacement

  <Desc / Clms Page number 28>

 incremental. In fact, the support can be moved continuously. Whether for incremental movement or for continuous movement, the "step size" is equivalent to the distance the support travels during a complete heating and cooling cycle.

   Because the time during which the coloring of the support occurs is relatively short compared to the duration of a complete heating and cooling cycle, and the fact that the step size can be relatively small compared to the absolute resolving power in the y direction, the continuous displacement of the support should not strongly affect the quality of the image obtained.



   To improve the real resolving power in the y direction, the print media can be advanced in increments smaller than the absolute resolving power of the thermal print head. For example, advancing the print head 50 to 600 dpi, four times for each pixel or for each point, we obtain a real resolution power of 2400 lines per inch in the y direction. It should be noted, however, that the minimum size of a detail, or the absolute resolving power, is limited to the physical characteristic of the print head 50 and therefore is 1/600 of an inch .



   Fig. 13 shows the method of the invention for producing a temperature map. A coverage value is chosen in the storage device 104, as shown in step 130. If the coverage value indicates that the pixel is 100% active, a temperature is assigned to this grid cell to provide coverage of 100%. The temperature is selected to constitute an active area large enough to cover the entire grid box 33 without damaging the print medium which is sensitive to heat.



   If the cover value indicates that the grid box is 100% inactive, a temperature of 0 is assigned to this grid box cover, as the

  <Desc / Clms Page number 29>

 shows step 132. In the preferred embodiment, a 4-bit coverage value, a value of 1111 (binary) or 15 (decimal) indicates that the grid cell is 100% active, and a value of 0000 (binary) or 0 (decimal) indicates that the grid cell is active at 0%.



   As shown in step 134, if the coverage value indicates that the corresponding grid cell is both active and non-active [i.e. if the value is between 0000 and 1111 (binary) or between 0 and 15 (decimal)], there is a boundary condition. A boundary condition simply means that a boundary or an edge between an active area and a non-active area is present inside the corresponding grid output box. A boundary condition temperature is determined based on the amount of coverage of the grid box and the thermal interaction characteristics of the thermal head 50 as shown in Figs. 8.9 and 10.



   If all the cover values which are in the storage device 104 have not been selected, the process is continued until all these values have been selected, as shown in step 138.



   The method of the invention implicitly includes the fact that each limit or edge grid cell is close to an active grid cell. Therefore, the temperatures of the limit grid box that are assigned extend the limit from the active grid box to provide adequate coverage of the limit grid box. The only time a boundary grid box is not next to an active grid box is when there is a detail in the original image that is smaller than two grid boxes. This condition can be treated separately or simply overlooked.



   Figs. 14 to 16 show an example of the mappings used by the invention in printing from the ideal circular original image 15 of FIG. 3.

  <Desc / Clms Page number 30>

 



  Fig. 14 shows the coverage map of this original image 15 in the storage device 104. The coverage values of FIG. 14 are decimal values ranging from 0 to 15, and each represents one of the possible values of the corresponding 4-bit binary value which is stored in the storage device 104. The data are shown separated in boxes 150 which correspond to boxes 33 of imaginary grid 24 of FIGS. 1 to 3. Four values are indicated in each box 150, representing the coverage values for four vertical parts of each grid box 33.



   Results similar to those of Figs. 1 and 2 can be produced from the data in FIG. 14 by simple fixing of a threshold. For example, each box 150 having an average value of 7 or more can activate a heating element. For image 36 with higher real resolution power y of FIG. 2, a heating element can be activated for each value of coverage less than 7. The data in FIGS. 1 and 2 can also be modified so as to give symmetrical output images 12 and 36.



   Fig. 15 shows the temperature map produced from the data of the coverage data map of FIG. 14. The maximum temperature is equal to 114, the normal temperature is equal to 84 and the minimum temperature is equal to 0. We see that the maximum temperature is generally close to each of the active area limits, and the interior temperatures generally have have been reduced to the normal temperature of 84. The vertical edges or boundaries between the active and non-active areas have temperatures assigned to them to selectively extend the limit from the neighboring heating element. It should be noted that the invention can be used with many modifications of this temperature map.



   Fig. 16 shows the control map or control levels that are applied to the thermal head

  <Desc / Clms Page number 31>

 50 to print the output image 42 of FIG. 3. The control levels are selected by means of the look-up table 108, taking into account not only the desired temperature given by the corresponding location in FIG. 15, but also of the temperature given by the four neighboring boxes 152 of FIG. 15. In this way, a control level is selected to provide a thermal distribution which, when combined with the four neighboring distributions, produces the desired temperature of the print medium. The output image 42 of FIG. 3 is formed by applying to the thermal head 50 the control levels indicated in FIG. 16.



   Figs. 14 to 16 show maps used in a particular preferred embodiment to suitably produce the control levels for the different heating elements. While the map is shown to illustrate the general process which is followed, the data shown is dependent on the particular techniques used. Those skilled in the art will realize that various other methods and techniques can be used to convert the original information into control levels, each of which can use the invention to control the boundaries of the active areas.



   It should, moreover, be realized that the present process can theoretically allow an infinite real power of resolution. Now the actual resolution power is not hindered by the print head. Rather, actual resolving power may now be hindered either by the size of the progression steps, or by discrete levels of control energy available, or by the desired printing speed, or by the size of memory. of the printer. Printing according to the invention allows limits to be placed at selected locations along a continuum, the selected locations being determined by these other barriers.



   Figs. 17 and 18 give an illustration

  <Desc / Clms Page number 32>

 spectacular improvement in text images obtained by using the technique of the invention. Fig. 17 shows an output image 170 plotted from photographs taken using an enlargement of
 EMI32.1
 text characters "0", "U", "V", rendered using a thermal printer with a resolution of 600 dpi in the horizontal or x direction and 2400 dpi in the vertical or y direction. The x and y directions are indicated by the directional reference 16. The characters "0" and "V" are represented fragmentarily. The dashed lines 172a, 172b and 172c represent the outlines of the original ideal images for the characters "0", "U" and "V", respectively.

   Each of these characters undergoes errors which give a aliasing effect due to the limitation of the resolving power in the horizontal or x direction. On the contrary, Fig. 18 shows an output image 174 plotted from photographs taken using an enlargement of the same text characters as those in FIG. 17. This image is formed using the technique described above of the invention to selectively extend the limits of active areas in the x direction in order to improve the resolution power and the visual acuity of the output image. . It can be seen that the characters in FIG. 18 are clear and sharp and do not exhibit the irregularities which result from the limitation of the power of resolution in the x or horizontal direction, as is the case for the characters of FIG. 16.



   Figs. 19 to 24 show the method of the invention for controlling the position of the boundary between the active areas and the non-active areas of the binary image. Fig. 19 is a representation of the control voltages applied to neighboring heating elements 56a and 56b during a heating cycle. The control pulse 180 supplied to the heating element 56a begins at time t, and ends at time t. The control pulse 182 supplied to the "neighboring" heating element 56b

  <Desc / Clms Page number 33>

 begins at time t3 and ends at time te. Fig. 19 therefore represents the duration as well as the order of succession of the control pulses 74 and 78. The heating element 56c is not controlled.



   Figs. 20 to 24 are representations, along the dimension x, of the temperature distributions resulting, in the printing medium, from the application of the control energy to neighboring heating elements 56a and 56b at different times. Fig. 20 is immediately shown in FIG. 21 is at time t, FIG. 22 is at time t, FIG. 23 is at time t and FIG. 24 is at time t6. In each of these Figs. 20 to 24, curve 184 represents the temperature in the print medium resulting simply from the control pulse 180. Curve 186 represents the temperature in the print medium resulting simply from the control pulse 182.

   Curve 188 represents the combined temperature in the print medium resulting from the interaction of heat from the heating elements 56a and 56b.



  Each of these curves 184, 186, 188 is mapped in the x direction, and the x orientations of the heating elements 56a, 56b, 56c are shown.



   As shown in Fig. 20, the temperature of the print medium at time to is room temperature, with negligible residual heat from the previous heating cycle. The temperature of the support remains at room temperature until time t, since neither the heating element 56a nor the 56b has been supplied.



   The heating element 56a is supplied at the instant t1 and, immediately after, it begins to transmit heat to the support. As shown in Fig. 21, at time t, the print medium is heated around the heating element 56a, so that it just begins to cross the temperature threshold 190 and to color. As shown by curve 186 of FIG. 21, the heating element 56b has not yet been supplied at time t. ,, and the curve 186 remains,

  <Desc / Clms Page number 34>

 therefore, at room temperature. The heating elements 56b and 56c remain unpowered until time t3 and, at time t3, the curve 188 increases, the curve 184 being due to the sole effect of the heat coming from the heating element 56a.



     At time t4 in FIG. 22, the heating element 56b provides a portion of the heat giving the resulting curve 188. The combined heat from the heating elements 56a and 56b caused the point produced by the heating element 56a to extend towards the heating element 56b .



  This asymmetric growth of the point is shown by the distance 192 between the curve 188 and the curve 184 when they cross the temperature threshold 190. However, the curve 186 has not yet reached the temperature threshold 190 which allows it to produce independently coloring the print medium.



   The instant tg of FIG. 23 represents the maximum heat production of the two heating elements 56a and 56b during the heating cycle. The heating element 56c, because it is outside the desired active area, is not supplied. The asymmetric growth of the point is substantially completed, and the distance 194 indicates the growth achieved by the heating element 56b in combination with the heating element 56a. It should be noted that the coloring represented by the distance 194 would not occur under the effect of the heating element 56a or the heating element 56b independently of one another and that it did not instead due to the thermal interaction between the two heating elements 56a, 56b.



   The instant t6 of FIG. 24 indicates the cooling of the system which has occurred and which can be used with a step-by-step progression of the print medium at time to in a following heating cycle.



   While Figs. 20 to 24 show temperature curves in the x direction only, it is understood that the printing takes place in more space

  <Desc / Clms Page number 35>

 complex, three-dimensional, with varying time. Figs. 20 to 24 show the temperatures at a location y centered on the print head 50 and at a location z located on the surface of the print medium. Corresponding temperature curves could be produced for different locations y and z and would be related to the temperature curves represented by heat transfer in the y and z directions.



   Fig. 25 shows the thermal distributions corresponding to the control pulses shown in FIG. 19. The thermal distribution 184a represents the maximum temperature, over time, obtained in the printing medium only following the command pulse 180. The thermal distribution 186a represents the maximum temperature, over time, obtained in the printing medium printing only following the control pulse 182. The thermal distribution 188a represents the resulting thermal distribution produced by the interaction of heat from the heating elements 56a and 56b. The part of the thermal distribution 188a which exceeds the temperature threshold 190 colors the printing medium.

   It is clear that, by extending the thermal distribution 188a shown in FIG. 25 in the x and y directions indicated by the directional reference 16, the total area for which the thermal distribution 188a exceeds the temperature threshold plane can be projected by the printer, as a binary image, onto the printing medium.



   These Figs. 19 to 25 represent a theoretical underpinning of what happens, it is believed, when using the invention, and they are not based on tests or data. It is clear that this theoretical support can be modified later to allow a better understanding of the phenomena involved in the coloring of the printing medium. In particular, the temperature threshold may not be as accurate as that of the modeling. Color transfer is linked at least in some systems to the amount of heat

  <Desc / Clms Page number 36>

 and the duration of application of the energy, and it is therefore possible that it is not modeled in an exact manner by a simple temperature threshold.

   In addition, it is possible that the color transfer takes place in a range of temperatures rather than at a particular temperature. Such a range of temperature thresholds is used to produce a phase change of a wax-based donor support in which the molecular structure, which is not homogeneous, requires for its fusion a range of temperatures.



   Fig. 26 shows an area of an image 195 created by four applications of the control pulses 180,182 shown in FIG. 19. Each application of the control pulses 180,182 has created a point 196. In the intervals between these four applications, the printing medium 14 has progressed by a linear distance 32 which is approximately equal to step 28. The image 196 is divided into pixel grid cells 33. The size of these grid cells 33 is roughly equivalent to the size of a dot formed by a printing element of the printer and, therefore, it represents the absolute resolution power of the printer.



  Other smaller linear distances 32 can be advanced step by step 14D to increase the real resolution power in the y direction for part 195 of the output image.



   The coloring of the printing medium can be easily observed, and it is therefore known that the coloring shown in FIG. 26 occurs in response to the control pulses shown in FIG. 19, without any theoretical support being necessary. The active area 196, which has the limit 198, has been colored by heat from the heating elements 56a and 56b. The limit 200, shown in dashed lines, is the limit of an active area of normal size (such as the active area 72 of FIG. 6) which would have been produced only by the pulse 180.



  As seen in Fig. 26, the right edge of each limit 198 has been effectively offset by an extension distance d, under the effect of the control energy 182

  <Desc / Clms Page number 37>

 applied to the neighboring heating element 56b.



   We can imagine each of the dots printed as having a "dimension ratio" defined as being the ratio between its largest dimension x and its largest dimension y. Powering a single heating element produces a symmetrical point with a given initial aspect ratio which, if the point is close to square or circular (i.e. rather than rectangular) is 1: 1 With existing printers, it was desirable that all of the printed dots have the same aspect ratio. Rather than the preferred symmetric dots for existing printers, the dots 196 created by the invention are ovoid in shape and have a combined aspect ratio which is very different from the original aspect ratio.

   The importance of the "neighborhood" growth or the combined aspect ratio of the point created by the two heating elements 56a and 56b can be selectively adjusted by varying the voltages, durations and / or the succession of the pulses 180,182.



  Applying large amounts of control energy to the neighboring heating element 56b can be used to further extend the right edge of each boundary 198 by a distance greater than d, which creates points with more dimensional ratios. great. Conversely, a lesser amount of control energy supplied to the neighboring heating element 56b can be used to move the right edge of each limit 198 by a smaller distance d from the limit 200, which creates points having smaller dimension ratios. The selection of the control energy supplied to the element 56a can also be used to selectively control the combined dimension ratios, which selectively positions the right edge of the limit 198.



   By governing a series of neighborhood growths, one can selectively adjust the exact location of the boundary for part 195 of the output image to the right or left in the column of grid cells

  <Desc / Clms Page number 38>

 neighbor 202. Similarly, the exact location of the right edge of the limit 198 can be selectively adjusted for each advancement of the support and the part of the output image can therefore curve or tilt in the column of boxes of grid 202, as desired. The pulses applied to neighboring heating elements 56a, 56b therefore allow a real resolution power in the x direction which is clearly greater or finer than the absolute resolution power in the x direction.



   An important aspect of the invention is that the time sequence of the control pulses applied to adjacent heating elements 56a and 56b can be selected to provide the most advantageous resulting distribution 188a. It is believed that the thermal distributions 184 and 186 have their greatest surface extension at or near the end of the pulse. At the end of the pulse, the heat created by the pulse is fully generated in the heating element 56b and has not had sufficient time to dissipate to a large extent.



   Neighborhood control pulses that start at the same time are "left justified" or "justified at the start". It is believed, for commands with equal voltages as a function of steps that command pulses justified at the start create individual thermal distributions which increase simultaneously and in an equivalent manner. It is desirable that a neighborhood control pulse is cut off before its thermal distribution crosses the temperature threshold so as not to create, independently, a point in the neighboring grid square.

   However, a justified control pulse at the start to the neighboring heating element 56b would be cut off significantly before the heat created by the heating element 56a reaches its maximum, and would therefore not be as effective in creating a combined thermal distribution 188a.



   In the preferred embodiment, both

  <Desc / Clms Page number 39>

 control pulses 180 and 182 terminate or have rear flanks at time t. Control pulses 180 and 182 are "right justified" or "justified at the end" because these two pulses, regardless of when they start, end at the same time at time ts. To prevent the thermal distribution 188a from crossing the temperature threshold 190 and thus not to independently create a point at the heating element 56b, the start of the control pulse 182 is delayed relative to the start of the control pulse 180.

   Since it is the resulting distribution 188a which controls the coloring of the support, it is believed that the justification at the end of the pulses 180.182 guarantees the maximum interaction between the thermal distributions 184 and 186. The justification at the end ensures stability as well that the consistency and predictability of the positioning of the limits 198.



   It is believed that another time sequence of applications of control pulses can be used to govern the characteristics of the resulting thermal distribution 188a. In addition to the successions justified at the beginning and justified at the end, respective control pulses 180,182 could succeed one another with an offset such that one or the other of the pulses begins, that one or the other of the pulses ends, that one or the other or both pulses are modulated in pulse width, etc.



   In particular, it may be advantageous to delay the neighboring pulse 182, so that it ends soon after the pulse 180, thus offering additional time to allow the thermal distribution 184 to move away from the heating element 56a. This type of succession over time may, moreover, be necessary if damage temperatures are reached by the resulting thermal distribution 188a with justification at the end. It may even be desirable to delay the start of one pulse until the other

  <Desc / Clms Page number 40>

 impulse ended. It is believed that the resulting thermal distribution for this delay will be similar to a wave and that the maximum temperature will be reached at different times at different locations x.

   In any case, it is important to recognize that it is the resulting distribution 188a which controls the coloring of the support and that the pulses 180, 182 can be selected so as to each contribute to the desired resulting distribution 188a.



   In order for the boundary 198 which is formed by the resulting distribution 188a to be well defined, it is believed that the resulting distribution 188a should have a high "gradient threshold" 204. The gradient threshold 204 is shown in FIG. 26 by a triangle having the same slope as the thermal distribution 188a at the temperature threshold 190. The resulting distribution 188a should decrease significantly when it crosses the temperature threshold 190 to create a high gradient threshold 204. As shown in Fig. 26, it is believed that the thermal distribution 188a has a higher gradient threshold at its left edge than at its right edge.



   It is believed that a high gradient threshold 204 produces a distinct boundary 198 between an active area and a non-active area. Conversely, a low gradient threshold 204 can give either a limit 198 having various gray levels, or an uncertain location of this limit 198. If the combined thermal distribution 188a does not decrease with a sufficient gradient 204, the threshold 190 of formation may not be crossed at a defined and predictable location. The gradient threshold 204 necessary to form a well-defined limit 198 generally depends on the characteristics of optical density or of variations in optical density with respect to the temperature of the printing medium 14.



   It is believed that, in general, the justification at the end gives a maximum gradient threshold 204. The maximum gradient threshold 204 also depends on the importance of the

  <Desc / Clms Page number 41>

 thermal distribution 184 supplied to the primary heating element 56a. In order to be certain that the gradient threshold 204 of the resulting thermal distribution 188a is high, the gradient originating from the thermal distribution 184a must dominate. This is why the thermal distribution 184a must be relatively large and form a relatively wide energy distribution, so that this results in a relatively large gradient threshold 204 in the combined thermal distribution 188a.



   The slope of the gradient threshold 204 also depends on the location x of the limit 198. If the desired limit 198 penetrates less than 50% in the neighboring column 202 of grid cells, the gradient given by the thermal distribution 186a tends to lower the gradient threshold 204 of the general thermal distribution 188a. If the desired limit 198 penetrates more than 50% into the neighboring column 202 of grid cells, the gradient given by the thermal distribution 186a tends to increase the gradient threshold 204 of the general thermal distribution 188a. Once the exact location x that is desired for the limit 198 is known, the control energies 180,182 can be selected to bring the gradient threshold 204 to the maximum location x.

   In general, many combinations of control energies 180,182 may exist to give a sufficient gradient threshold 204.



   A suitable control of the growth in the neighboring pixel also depends on the step 28 of the thermal printing head 50. All other things being equal, a thermal distribution extends further in the neighboring grid box for the printing heads. Thermal printing 50 having a smaller pitch 28. While the heat generation and heat transfer of the print head having a smaller pitch are assumed to be equal, the distance or spacing between the heating elements is smaller. Because the heat disperses in three dimensions, a small change in the pitch 28 of the print head 50 can

  <Desc / Clms Page number 42>

 have a significant effect on the contributions provided by neighboring thermal distributions.

   The technique of the invention is therefore better and easier to control for thermal print heads 50 having a high resolution power.



   Thermal print heads 50 which have a larger pitch 28 may require that the heating elements 56a, 56b be energized for a longer period of time to allow thermal interaction to occur between neighboring heating elements 56a, 56b. For example, a wider thermal distribution can be obtained by increasing the supply time and reducing the control voltage. This increase in the feeding time results in increasing the possibility of thermal interaction between the heating elements, so that the technique of the invention can be used.



  Adjustments may also be necessary for thermal print heads and / or for media with poor heat transfer, especially if the latter does not readily occur in the x direction.



   Another aspect of the invention is that the voltage or magnitude of the control pulses 180,182 applied to the neighboring heating elements 56 and 56a can be selected or modified to give the most beneficial resulting distribution 188a. The control pulses 180, 182 are both represented as step functions, each having a magnitude V,. The step function is preferred to simplify the processing requirements for controlling the heating elements, since the only variable is present when the pulse begins. It is not necessary that the two voltages have this magnitude or that the two voltages have the same magnitude. Furthermore, it is not necessary either that one or the other of the voltages is a step function.

   It may turn out that the control voltages which vary over time create beneficial distributions 188a, both in terms of

  <Desc / Clms Page number 43>

 growth in neighboring boxes as the gradient thresholds 204.



   So far, we have described limit control by selective feeding of the heating elements in the x direction only. However, it is possible to act on the limits in the y direction as well as in the x direction. The real resolving power in the y direction can be increased by selectively controlling the on-board heating elements with different energies, thus forming active areas of different sizes. For example, the real resolution power in the y direction, of 2400 dpi or more, can be obtained by advancing at 1200 steps per inch and by growing dots in the y direction in previous and following grid boxes.

   With proper adjustment of the control levels to set the limits in the y direction, the step size 40 and the step-by-step speed can be increased, resulting in much higher print speeds without loss for the real resolving power.



   It is further contemplated that the look-up table 108 of FIG. It can be used to compensate for residual heat from previous heating cycles. This residual heat appears if the time between the heating cycles is small enough that the residual heat from previous heating cycles no longer has a negligible effect on the thermal distributions associated with the current heating cycle. This application requires an increase in the size of the look-up table 108 to take account of the residual heat in a manner similar to the method with which the neighboring heat is taken into account, as explained above.



   In addition, while it is particularly envisaged to use the invention with rows of heating elements of only one dimension, it is realized that it is possible that, in the future, devices for

  <Desc / Clms Page number 44>

 two-dimensional heaters are manufactured which could use the invention. Heating elements located immediately above or below the location primarily for pixels could similarly be energized above the temperature threshold to create identical neighborhood effects in a two-dimensional heating arrangement . Increasing the number of heating elements in a two-dimensional arrangement would greatly increase the printing speed.

   The printing speed could be further increased not only because the time required for a cooling cycle could be reduced or eliminated, but also because it would not be necessary to progress in increments smaller than the absolute resolving power. of the printer.



   In addition, while the use of the invention is envisaged for thermal printers, other types of printing can be imagined, for which the interaction effects between neighboring printing elements make it possible to obtain an improvement. the actual resolution power of the output image.



   It is currently contemplated that the progression increments used may be much smaller than a pixel in order to increase the real resolving power in the y direction. Advancing in smaller increments allows more choices to start a y end of a character line and to end the y boundary between the active layer and the non-active layer.



   The foregoing considerations relate to the control of the energy applied to neighboring heating elements to give improved fidelity and visual acuity of the output image, for text and for original images belonging to linear graphics. The Applicant has realized that its technique is well suited for precisely controlling the proportion of colored and non-colored zones in the formation of a cell.

  <Desc / Clms Page number 45>

 halftone which is used for original images with gradients.



   The original images with gradients consist of a series of regularly spaced samples, the tones ranging from the absence of coloration to coloration passing through intermediate shades of color which can be rendered in the form of a binary image using different halftone rendering techniques. These halftone rendering techniques use the arrangement of binary elements of an image or of dots which give the illusion of a gradient image. Some of these techniques for obtaining gradients are described in "Digital Halftoning", by R. Ulichney, MIT Press, Cambridge, Mass., (1987).



   Often a halftone cell is used; it represents an area in the output image or in the printed image. Because of the selective modification of the proportion of the active zone compared to the non-active zone in this cell, the printed image, when it is seen at a certain distance, can give the impression of gradient. Often, the halftone cell is made up of multiple dots or areas that are selectively activated to give increasing levels of coloration. For example, a 4 x 4 halftone cell consisting of 16 points or areas is capable of representing 16 different levels of gradient in the output image.



   One method of increasing the number of shades that can be represented by a halftone cell of a given size is to reduce the size of the step of progression of the print medium relative to the print head. The reduction in the size of the progression step has been discussed above with respect to increasing the real resolving power in the y direction. This reduction in the size of the progression step effectively increases the number of gradations or shades of gray that can be represented by a given halftone cell. This reduction in step size

  <Desc / Clms Page number 46>

 allows controlling the proportion of the active area compared to the non-active area, inside the halftone cell, in smaller increments, in the range from 0% to 100% for the active area.

   For example, a 4 x 4 halftone cell that progresses four times per pixel is capable of representing 4 x 16, or 64 different levels of gradient.



   One aspect of the invention is to control the energy that is applied to neighboring heating elements to selectively extend the active area along the x axis, to control the proportion of the active area to the inside the halftone cell.



  By selectively extending the active area of a neighboring active area, the proportion of the active area in the halftone cell can be formed as it were at any value between 0 and 100%, depending on the control of the supply energy which is available. Furthermore, this method can be used in conjunction with a reduction in the size of the step of progression of the print medium which is moved in front of the print head, a reduction shown in FIG. 2, as well as a modification of the control energy applied to a single point or to a single zone in order to modify the size of the active zone in a symmetrical manner, as shown in FIGS. 3 and 4.



   By using this technique to increase the number of shades that are represented in the printed image as well as the technique to improve the fidelity and visual acuity of the output image for text and graphics, the Applicant has believes that the thermal printing device is suitable for certain applications that were not previously believed possible.



   Another aspect of the invention consists in using the techniques described above with regard to improving the resolution power as well as the visual acuity of text and images in graphics and the number of tones which can be represented in images

  <Desc / Clms Page number 47>

 in gradient, to achieve color control. The color control operation is a method of knowing what the appearance of the seam image will look like when it has been printed by a printing press. The control operation constitutes an attempt to correct the output image before the costs of manufacturing the plates and printing are incurred.



   Printing presses which are used for mass production require, in typical cases, the manufacture of an intermediate image carrier, often called a "plate", which is used in combination with the printing press, to form an output image. The image on the printing plate represents a single color in the output image formed by the printing press. Therefore, for a four-color output image such as cyan, yellow, magenta and black, four different plates are used to print each color, thereby forming a four-color output image.

   It is typical that each of these colors, in the output image, has some relation to each other, such as a frame offset or orientation, so that these colors have an appropriate interaction with each other. One method for manufacturing these printing plates consists in forming an image representative of the contribution of a single color such as cyan, magenta, yellow or black, on a transparent film. This film is then used, in a photomechanical process, to expose a photosensitive layer which is on the printing plate. The exposed parts of the layer become insoluble in water or other solutions. On the contrary, the unexposed part dissolves, leaving the exposed part in the form of an image or a stencil to be used in the printing process.



   An important aspect of the invention resides in the use of the application of controlled amounts of energy to neighboring heating elements to extend the zones.

  <Desc / Clms Page number 48>

 selectively active in the x direction, to improve horizontal resolution for text and line graphics and the number of tones that can be represented between 0% and 100% tone for half images shades. These techniques of the invention allow a more robust color quality as well as a higher quality and a greater visual acuity in the output image, which is suitable for forming an image on a film to be used in a photomechanical process for manufacturing printing plates.



   Another aspect of the invention resides in the use of the energy control technique which is applied to the heating elements both to improve the fidelity or visual acuity of the text and of the linear graphic images and to improve the number of tones that can be reproduced. This technique allows the indirect creation of individual color images on transparent films, sometimes called "color separations" or "progressive". In a preferred embodiment, these color separations are performed using a thermal wax transfer process.

   They are used to check colors, taken in isolation, to detect possible defects before printing, and they can be superimposed to allow checking what the final printed image might look like on the press.



   There are several possible embodiments of the invention. In one of them, the over-order energy is produced in response to a determination based on the values assigned to each grid cell or pixel element 230 in the coverage map 228. In the example of FIG. 27, an ideal contour 248, at the right edge of the image to be printed 236, cuts printing rows or scanning lines 232. For each grid cell or pixel element 230 that the ideal contour 248 crosses, a determination is performed to establish what is the proportion of this pixel element 230 which is at

  <Desc / Clms Page number 49>

 inside the ideal contour 248. For example, the proportion of the pixel element 230x which is inside the ideal contour is approximately 30%.

   Therefore, the pixel element 230y is provided with a pixel value of 1.3 which, when applied to a controller 226, produces over-control energy which inflates the active area boundary or the edge of pixel 242 for the pixel image 238x to the right, causing the pixel image 238x to overlap the pixel image 238y and occupy about 30% of it. A similar phenomenon occurs for the pixel elements 238m and 238n and for the images of pixels 238n and 238m in the next image line 240. For the last line of images 240, both an over-order energy and a subcommand energy to create the pixel images 244 and 246 as described above.



  In this case, the pixel elements 230i and 230j are provided with pixel values of 1.4 to 0.6 respectively to produce the over-order energy and the sub-control energy. By applying both the over-order energy and the sub-control energy, the size of the pixel 244 is increased relative to the absolute or specific resolution power, and it is formed adjacent to the pixel 246, the size of which is reduced compared to the specific resolving power. The contiguous pixel image 244-246 given by this overlap occupies 100% of the area of the pixel element 230i and about 60% of the area of the pixel element 230j.



   It is preferable that a decision to use the combined and overlapping over-order energy and subcommand energy is made if a proportion of an area of a pixel image 246 to be printed is greater than one. first percentage of the specific surface and less than a second percentage of the specific surface, the second percentage being greater than the first. The values of the first and second percentages vary depending on the particular characteristics of the thermal printer and the support.

  <Desc / Clms Page number 50>

 14. Typical values for the first percentage could range from 20 to 50% and for the second percentage they could range from 80 to 100%.

   In another embodiment, the proportion of the area of the pixel element 230 is compared with more percentages than the first and only the second, and the over-order and sub-order energies are chosen from several energy levels. of pre-order and sub-order defined in advance. In yet another embodiment, the over-control energy and the sub-control energy are determined as a pair of values based on the proportion of the area of the heating element 222 relative to the pixel image 246 .

   In this embodiment, a known look-up table procedure can be used, for example to compare the proportion of the area of the heating element 222 to the area of the pixel image 246 and provide a selected pair of values for the over-order energy and the sub-control energy.



   In another possible alternative embodiment, a control device or a processor (not shown) receives the representation of pixels 228 of the image. The controller smooths the binary pixel images 238 which are to be printed on the print medium 14 by performing a pattern matching operation for each pixel element 230 rather than an ideal contour comparison as it has was described above. For a subset of pixel elements 230, inside the representation of pixels 232, which are above the pixel element 230 being processed, a determination is made as to whether the pixel values of the pixel elements 230 of the sub-assembly correspond to any one of a plurality of model shapes defined in advance.

   The type of template shapes that can be used to smooth the output image 236 is known in the art of smoothing output images for dot-matrix printers, for example as indicated

  <Desc / Clms Page number 51>

 in U.S. Patent No. 4,847,641 issued to Tung. If there is no correspondence with a model, and if the pixel value of the pixel element 230 which is being processed is zero, a control energy is applied to the heating element 222 corresponding to the pixel element 230 which is being processed, and it will not produce a pixel binary image 238.

   If there is no correspondence with a model, and if the pixel value of the pixel element 230 which is being processed is one, control energy is applied to the heating element 222 corresponding to the pixel element 230 which is being processed, and it is equivalent to the control energy of the specific pixel. Finally, if there is a correspondence with a model, an over-control energy is applied to the heating element 222 corresponding to a pixel element 230 adjacent to the pixel element 230 which is being processed; it is greater than the control energy of the specific pixel, but less than a maximum transfer energy for the thermal printer.

   As for the option allowing to produce the control energies directly and to produce the control energies starting from a comparison between an ideal contour and a representation of the image by pixels, the option allowing to compare the representation by pixels with several smoothing models defined in advance can be combined with the embodiment involving a subcommand.



   Although the invention has been described with reference to preferred embodiments, those skilled in the art will appreciate that it is possible to make modifications to the form and details without departing from the spirit nor depart from the scope of the invention.


    

Claims (61)

 EMI52.1   CLAIMS CLAIMS 1.-A method of thermal printing on a support (14), characterized in that it comprises: the excitation of a first heating element (56a) by means of a first control pulse (180) which is sufficient to independently produce an active area on the support (14), and the excitation of a second heating element (56b) adjacent to the first heating element (56a), by means of a second control pulse (182 ), this second control pulse (182) being insufficient to independently produce an active area on the support (14).    2. - A method of thermal printing on a support (14), characterized in that it comprises: the excitation of a first heating element (56a) by means of a first control pulse (180) sufficient to produce independently an active area on the support (14), and the excitation of a second heating element (56b) adjacent to the first heating element (56a), by means of a second control pulse (182), to extend a limit of the active zone, the first and second control pulses not being identical.  3.-Method according to claim 2, characterized in that the excitation of the second heating element (56b) selectively extends the location of the limit along a continuum.  4.-A method according to one and / or the other of claims 1 and 2, characterized in that it further comprises the steps consisting in: determining a desired extent of the limit from original data , and producing the first and second control pulses (180,182) corresponding to the desired extent of the limit.    5.-Method of printing an active area on a  <Desc / Clms Page number 53>  print medium (14), characterized in that it comprises: energizing a first heating element (56a) with a first control pulse (180) to create an initial active area on the print medium ( 14) around the first heating element (56a), the initial active zone having a width in a direction x and a height in a direction y and thus defining an initial dimension ratio of the width to the height, and the excitation of a second heating element (56b) with a second control pulse (182), combining this with the heat of the first heating element (56a) to increase the initial active zone so as to make it a combined active zone,  the combined active area having a width in an x direction and a height in a y direction and thereby defining a combined aspect ratio, the second heating element being energized so that the combined aspect ratio differs significantly from the initial aspect ratio.  6.-A method according to claim 5, characterized in that the initial active area is circular and that the combined active area is ovoid.  7.-Method according to one and / or the other of claims 5 and 6, characterized in that an axis x is defined by a line passing through the first heating element (56a) and the second heating element (56b) , a y axis being defined by a line passing through the first heating element and perpendicular to the line x, and in that the initial active area is symmetrical with respect to the y axis and the combined active area is not symmetrical with respect to the y axis.  8.-A method according to any one of claims 2 to 7, characterized in that the second control pulse (182) is not enough to independently produce an active area on the support (14).  9.-A method according to any one of claims 1 to 8, characterized in that the first and  <Desc / Clms Page number 54>  the second control pulse (180,182) starts at different times.    10.-Method according to any one of claims 1 to 9, characterized in that the first and the second control pulses (180,182) are justified at the end.    11.-Method according to any one of claims 1 to 10, characterized in that the first and the second control pulses (180,182) have voltages of different amplitudes.    12.-A method according to any one of claims 1 to 11, characterized in that it further comprises the steps consisting in: allowing the heat to dissipate substantially from the first and second heating elements (56a , 56b), and to move the support (14) relative to the heating elements (56).  13.-A method according to any one of claims 1 to 11, characterized in that it further comprises the step of: advancing the support (14) step by step relative to the heating elements (56) .    14.-A method according to claim 13, characterized in that the steps of excitation of the first heating element (56a), excitation of the second heating element (56b) and step-by-step progression of the support (14), are repeated until an image has been printed on the support (14).    15.-A method of thermal printing on a support (14) for locally adjusting an edge of an image, characterized in that it comprises the steps consisting in: creating an active area on the support (14) opposite a first heating element (56a) by activating the first heating element (56a), the active area constituting a part of the image; and grow the active area to an element  <Desc / Clms Page number 55>  neighboring heating element (56b) by activating the neighboring heating element (56b) during the activation of the first heating element (56a), thereby locally adjusting the edge of the image.    16.-A method according to claim 15, characterized in that the active area is further enlarged towards the neighboring heating element (56b) by increasing a level of control of the neighboring heating element (56b).    17. - A thermal printing process used to form images having a high visual acuity on a printing medium (14), characterized in that it comprises: the supply of a thermal printing head (50) having a plurality of heating elements (56) arranged substantially in a first direction; determining control energies for the plurality of heating elements (56), based on the information on the original images to selectively set the boundaries of active areas along a continuum in the first direction on the print medium (14); and applying the determined control energies to the plurality of heating elements (56).    18. - A thermal printing method for forming images having a high visual acuity on the printing medium (14), characterized in that it comprises: the supply of a thermal printing head (50) equipped with 'a plurality of heating elements (56) arranged substantially in a first direction and thus defining a spacing (28) between neighboring heating elements (56);  determining control energies for the plurality of heating elements (56), based on information relating to the original image, to selectively place boundaries of active areas with precision, in the first direction, greater than the spacing between neighboring heating elements (56), and  <Desc / Clms Page number 56>  applying the determined control energies to the plurality of heating elements (56).    19.-A thermal printing process according to either of claims 17 and 18, characterized in that the control energies determined for the heating elements (56) outside the limits of the active area are zero .    20. - A thermal printing process for forming active zones in order to fill ideal contours with original images, characterized in that it comprises: determining the control energies for selected heating elements (56) arranged in an x direction to form active parts within ideal contours of original images; determining control energies for at least one second heating element (56) adjacent to the selected heating elements (56) in order to extend the active parts towards ideal contours in the x direction, and applying the determined control energies to the corresponding heating elements (56).    21.-Binary thermal printing process for the formation of active areas on a support (14) by means of heating elements (56) in order to fill the ideal contours of original images, the process being characterized in that it includes: determining the boundary heating elements (56) associated with the ideal contours; selecting the control energies for the limit heaters (56) based on the ideal contours; and applying the selected control energies to the boundary heating elements (56), to form active zones which better fill the ideal contours.    22. - Binary thermal printing method according to claim 21, characterized in that the limit heating elements (56) comprise at least one  <Desc / Clms Page number 57>  a heating element (56) located entirely within the ideal contours, at least one heating element (56) located at the level of the ideal contours, and at least one heating element (56) located entirely outside the ideal contours.    23.-A method according to any one of claims 15 to 19, 21 and 22, characterized in that it further comprises the step consisting in: advancing the support (14) step by step relative to the elements heaters (56).    24. - Binary thermal printing method according to one and / or the other of claims 21 and 22, characterized in that it further comprises the steps consisting in: waiting for a certain period of time in order to allowing the heat to dissipate notably from the heating elements (56); advancing the support (14) relative to the heating elements (56), and repeating the steps of determining, selecting, applying, waiting and advancing until an image is completed.    25.-In a thermal printer having a thermal print head (50) provided with a plurality of heating elements (56) spaced generally along a line to define an absolute resolution power, method of setting using the thermal print head (50) to print a detail having a size along the line greater than the absolute resolving power, characterized in that the heating elements (56) are operated so as to obtain a real resolution power much finer than the absolute resolution power.  26.-In a thermal printer comprising a thermal print head (50) for heating the print medium (14) in order to create a binary image and comprising a transport system for moving the medium  <Desc / Clms Page number 58>  print (14) in steps of a step size relative to the thermal print head (50), method of operating the thermal print head (50) to print binary limits with precision greater than the step size.  27.-A method according to claim 26, characterized in that the displacement of the print medium (14) relative to the thermal print head (50) defines a direction y and in that the binary limits printed with precision larger than the step size are boundaries between adjacent locations thereon on the print medium (14).    28.-A method according to claim 26, used with a thermal print head (50) comprising a plurality of heating elements (56) spaced substantially along a line to define a direction x, characterized in that the limits binaries printed with greater precision than step size are boundaries between neighboring x locations on the print medium (14).    29.-A method for controlling a thermal print head (50) comprising a plurality of heating elements (56), characterized in that it comprises: determining the desired thermal distribution from the information on the 'original image; determining control energies for each of the plurality of heating elements (56) to provide the desired heat distribution; and applying the determined control energies to each of the plurality of heating elements (56) to form binary images on the print medium (14).    30.-A method of controlling a thermal print head (50) according to claim 29, characterized in that the step of determining the control energies comprises: the selection of control energies for each  <Desc / Clms Page number 59>  element of the plurality of heating elements (56), in a table (108) containing a plurality of temperatures and corresponding control energies.    31.-A method of controlling a thermal print head (50) according to one and / or the other of claims 29 and 30, characterized in that the step of determining the control energies further comprises : adjusting the corresponding control energies to take account of the heat produced by at least neighboring heating elements (56).    32.-A method of controlling a thermal print head (50) according to any one of claims 29 to 31, characterized in that the step of determining the control energies comprises: the selection of control energies for bring a maximum of a gradient threshold (204) of the thermal distribution to a desired limit of the binary image.    33.-A method of controlling a thermal print head (50) comprising a plurality of heating elements (56) to give a binary image resulting from information relating to the original image, characterized in that it comprises: selecting a control energy for each element of the plurality of heating elements (56) according to information on the original image; and supplying the selected control energy to each of the respective heating elements (56) of the thermal print head (50), such that neighboring heating elements (56) heat together portions of the print medium thermal (14) to form parts of binary images of different sizes.  34.-A method according to any one of claims 1 to 19, to 24 and 26 to 33, characterized in that the support (14) is heat sensitive.  35.-A method according to any one of claims 1 to 19, 21 to 24 and 26 to 33, characterized in  <Desc / Clms Page number 60>  that the support (14) receives its coloring from a heat-sensitive donor support.    36.-Method for forming a half-tone cell with a view to rendering information of original degraded images, characterized in that it comprises: the determination of control energies for heating elements (56) selected on the base of the desired proportion of active area; and applying the determined control energies such that the thermal interactions between at least two neighboring heating elements (56) form an enlarged active area.  EMI60.1   37.-Thermal printer for printing on a print medium (14), characterized in that it comprises: a thermal print head (50) comprising a plurality of heating elements (56) extending from generally in an x direction, the plurality of heating elements (56) defining an absolute resolving power in the x direction; a transport system for relative movement in a y direction between the print medium (14) and the thermal print head (50), the y direction forming an angle with the x direction; it being understood that the real resolution power in the x direction is finer than the absolute resolution power in the x direction.  38.-Printer according to claim 37, characterized in that the direction y is perpendicular to the direction x.  39.-Thermal printer for printing on a print medium (14), characterized in that it comprises: a thermal print head (50) comprising a plurality of heating elements (56); a transport system for relative movement between the print medium (14) and the head  <Desc / Clms Page number 61>  thermal printing (50), the relative movement taking place in steps of a determined size between heating cycles and defining a direction y; it being understood that the real resolution power in the direction is finer there than the step size.  40.-Printer according to any one of claims 37 to 39, characterized in that the transport system transports the print medium (14).  41.-A thermal print head control device (103) for controlling the control energy supplied to a thermal print head, the thermal print head (50) comprising a plurality of heating elements (56 ) intended to produce images on a printing medium (14), the control device (103) being characterized in that it comprises: an input intended to receive original image information;  a control circuit (112) for selecting control energies for each of the heating elements (56), the control energies being selected to form binary images comprising active and inactive zones in reaction to the original image information, the control energies being, in addition, selected to produce interactive thermal distributions intended to control the establishment of limits between active and inactive zones; and an output for applying the selected control energies to each of the plurality of heating elements (56).  42.-thermal print head control device (103) according to claim 41, characterized in that the control circuit (112) selectively controls the succession over time of the control energies reaching each of the heating elements (56 ) to produce an interaction of thermal distributions for the purpose of setting up active zone limits.  43.-Printhead control device  <Desc / Clms Page number 62>  thermal (103) characterized in that the control circuit (112) comprises: a control energy compensation circuit which compensates for the thermal distributions produced by neighboring heating elements (56) at the limits of active zones.  44.-thermal print head control device (103) according to claim 43, characterized in that the control energy compensation circuit comprises: a table (108) comprising a plurality of compensated control levels, each compensated control level being associated with a combined thermal distribution of heating elements (56) and with a thermal distribution of neighboring heating elements (56); and a table controller for providing compensated control levels from the table (108).    45. - print head control device (103) according to any one of claims 41 to 44, characterized in that the control energies for heating elements (56), at the limit, are insufficient to produce independently an active area on the print medium (14).  46.-A thermal print head controller (103) for receiving original image information and supplying control energy to a thermal print head (50) having a plurality of heating elements (56 ) for producing binary images on a printing medium (14), the control device (103) being characterized in that it comprises: means supplying a plurality of control energies; and means responsive to the original image information for selectively supplying control energy to selected heating elements (56) to produce portions of binary images of different sizes, at the  <Desc / Clms Page number 63>  at least one of the binary image portions being formed by control energies provided by more than one heating element (56).  47.-A thermal print head control device (103) according to claim 46, characterized in that said means intended to supply a plurality of control energies is a control circuit (112).  48.-thermal print head control device (103) according to one and / or the other of claims 46 and 47, characterized in that said means intended to supply control energy comprises reacting means to the original image information for selecting a control energy from a range of control energies from zero to a maximum control energy at which thermal breakdown of the print medium (14) or the print head thermal printing (50) occurs.  49.-thermal print head control device (103) according to any one of claims 46 to 48, characterized in that said at least one binary image part is formed by adjacent heating elements (56).  50.-Thermal print head control device (103) for receiving original image information and supplying control energy to a thermal print head (50) having a plurality of heating elements (56) for producing binary images on a print medium (14), characterized in that the control device (103) comprises: a control circuit (112) connected to a thermal print head ( 50) to supply a plurality of different control energies thereto, each control energy of the plurality of different control energies being provided to produce a plurality of different heating energies;  and means responsive to the original image information for selectively providing energy from  <Desc / Clms Page number 64>  controlling selected heating elements (56) to produce binary image portions of different magnitude, at least one of the binary image portions being formed by several heating elements (56).  51.-Thermal print head control device (103) for receiving original image information and supplying control energy to a thermal print head (50) having a plurality of heating elements (56) for producing binary images on a print medium (14), characterized in that said control device (103) comprises: a control circuit (112) connected to a plurality of heating elements (56 ) to selectively provide first and second pixel drive energies having first and second bit image areas associated with the plurality of heaters (56), the first pixel drive energy being greater than the second energy pixel control;  control circuit control means responsive to the original image for selectively supplying first and second pixel control energies to the plurality of heating elements (56), the first and second pixel control energies producing together an enlarged binary image area, larger than the first and second binary image areas.  52.-Thermal print head control device (103) for receiving original image information and supplying control energy to a thermal print head (50) having a plurality of heating elements (56) for producing binary images on a print medium (14), characterized in that the control device (103) comprises: a control circuit adapted to be connected to a plurality of heating elements (56 ) to provide a plurality of different control energies, each of the different control energies of the plurality being provided to produce a plurality of heating energies  <Desc / Clms Page number 65>  different;  and means responsive to the original image information for selectively supplying control energy to selected heating elements (56) so that different heating energies produced by different heating elements (56) combine to produce parts of binary images of different sizes.    53.-Improved method for controlling a thermal printer which produces binary images on a print medium (14) by, selectively, heating or not heating a plurality of individual resistant heating elements (56) arranged along a line of printing, each heating element (56) being operable in response to a corresponding control energy produced by a control circuit (42) which is actively coupled to a pixel representation of the image, the thermal printer having resolving power specific on the printing line which corresponds to a center to center distance (28) between adjacent heating elements (56) and having a specific pixel control energy which is applied to a heating element (56)  in order to produce a binary pixel image comprising a specific area corresponding to the specific resolving power of the thermal printer, characterized in that the improved method comprises the step of: providing each heating element (56), from one or more selected heating elements, of an over-control energy which is greater than the specific pixel control energy, but less than a maximum control energy for the thermal printer, above which a thermal transfer of the image of binary pixel will not regularly produce a printed image of uniform quality such as the binary pixel image on the print medium (14) corresponding to each of the heating elements (56)  selected is increased in area beyond  <Desc / Clms Page number 66>  of the specific surface of the thermal printer, which allows the thermal printer to increase the real resolution of the binary image.    54.-An improved method according to claim 53, characterized in that the increase in the real resolution power of the binary image is obtained by using the over-order energy to control a relative position of a binary edge of the image. binary pixel resolution which is greater than the specific resolution power of the thermal printer.    55.-improved method according to one and / or the other of claims 53 and 54, characterized in that it further comprises the step consisting in: providing each heating element (56) among one or more elements selected adjacent heaters (56) which are adjacent to a corresponding selected heater (56), of a subcontrol energy which is less than the specific pixel control energy, so that the binary pixel image on the support (14) corresponding to each of the selected adjacent heating elements (56) is formed adjacent to the corresponding selected heating element (56) and is smaller in area than the specific surface of the thermal printer.    56.-Method for rendering and outputting a linear graphic image by means of a thermal printer which produces binary images (238) on a printing medium (14) by, selectively, heating or not heating a plurality of elements individual resistive heaters (56,222) arranged in a printing line, each heating element (56,222) operable in response to a corresponding control energy produced by a control circuit (112) which is actively coupled to a pixel representation (232 ) of the linear graphic image, the pixel representation (232) comprising a plurality of raster scan lines, each raster scan line comprising a  <Desc / Clms Page number 67>  plurality of pixel elements (230), each pixel element (230)  corresponding to a particular heating element (56, 222) in the printing line, the thermal printer having a specific resolution on the printing line which corresponds to a center-to-center distance (28) between adjacent heating elements ( 56, 222), and having a specific pixel control energy level which is applied to a heating element (56,222) to produce a binary pixel image (238) having a specific surface corresponding to the specific resolving power of the thermal printer, characterized in that the method comprises the steps of: providing a processor (106) with an ideal contour of the linear graphic image;  using the processor to convert the ideal outline of the line graph image to a point-to-point image to store the pixel representation (238) of the line graph image in a frame buffer, including performing the following steps to each raster scan line in the pixel representation (232); determining if there are contour limits for which the ideal contour intersects the raster scan line; for any pixel elements (230) in the raster scan line, which are completely outside of an edge boundary, store a pixel command value in the frame buffer corresponding to the pixel element (230) which gives a control energy which does not produce a binary pi el image (238);  for any pixel elements (230) that are located partly within an edge boundary, store a pixel command value in the frame buffer corresponding to an adjacent pixel element (230) located completely within inside the contour limit which gives an over-order energy which is  <Desc / Clms Page number 68>  greater than the specific pixel control energy, but less than a maximum control energy for the thermal printer above which thermal transfer of the binary pixel image (238) does not regularly produce an image uniform quality print;  and for all other pixel elements (230) which are completely within a contour boundary, store a pixel command value in the frame buffer corresponding to the pixel element (230) which gives a control energy which is equivalent to the specific pixel control energy; and communicating the pixel representation (232) stored in the frame buffer to the thermal printer, so that the thermal printer can print the smoothed linear graphic image.    57.-A method according to claim 56, characterized in that the smoothed linear graphic image is obtained by using the overcommand energy to control a relative position of a binary edge of the binary pixel image (238) at a resolving power which is greater than the specific resolving power of the thermal printer.  58.-A method according to either of claims 56 and 57, characterized in that the pixel control value stored for each adjacent pixel (230) partly inside a contour limit is selected from a plurality of predefined pixel control values.  59.-Method according to any one of claims 56 to 58, characterized in that the step consisting in storing a pixel control value for pixel elements (230) which are partly inside a contour boundary further comprises the steps of: determining a proportion of an area of the pixel element (230) which lies within the boundary of  <Desc / Clms Page number 69>  outline;  if the proportion of the area of the pixel element (230) is greater than a first percentage of the specific area and is less than a second percentage of the specific area which is greater than the first percentage, store a pixel command value in the frame buffer corresponding to this pixel element (230) which gives a subcommand energy which is less than the specific pixel command energy.    60.-A method according to any one of claims 53 to 59, characterized in that it further comprises the steps consisting in: decreasing a distance from which the print medium (14) is moved opposite the thermal printer print line at each of a plurality of print cycles up to a distance which is less than a distance from which the print medium (14) is moved at the specific resolving power of the thermal printer.    61.-Method for smoothing images printed by a thermal printer which produces binary images (238) on a printing medium (14) by, selectively, heating or not heating a plurality of individual resistant heating elements (56,222) arranged along a printing line, each heating element (56,222) capable of operating in response to a corresponding control energy produced by a control circuit (112) which is actively coupled to a pixel representation (232) of an image, the pixel representation (232) comprising a plurality of raster scan lines, each raster scan line having a plurality of pixel elements (230), each pixel element (230) corresponding to a single heating element (56,222) in the line printing,  the thermal printer having a specific resolving power on the printing line which corresponds to a center-to-center distance (28) between adjacent heating elements (56,222) and comprising a  <Desc / Clms Page number 70>  specific pixel control energy level which is applied to a heating element (56,222) to produce a binary pixel image having a specific area corresponding to the specific resolving power of the thermal printer, characterized in that the method comprises the steps of: providing a device for controlling the pixel representation (232) of the image, each pixel element (230) in the pixel representation (232) having a binary pixel value;  using the control device to smooth the binary image to be printed on the printing medium (14) by performing for each pixel element (232) the steps consisting in: for a subset of pixel elements (230) in the representation in pixels (232) that are around the pixel element (230) being processed, determining whether the pixel values of the pixel element subset (230) correspond to any of a plurality of pre-defined models; if there is no correspondence of the pixel values with a model and if the pixel value of the pixel element (230) being processed is zero, apply control energy to the heating element (56,222 ) corresponding to the pixel element (230) being processed which will not produce a binary pixel image;  if there is no correspondence of the pixel values with a model and if the pixel value of the pixel element (230) being processed is equal to one, apply a control energy to the heating element (56,222 ) corresponding to the pixel element (230) being processed which is equivalent to the specific pixel control energy;  and if there is a match of pixel values to a model, applying over-control energy to the heating element (56,222) corresponding to a pixel element (230) adjacent to the pixel element (230) being processed which  <Desc / Clms Page number 71>  is greater than the specific pixel control energy, but less than a maximum control energy for the thermal printer above which thermal transfer of the binary pixel image will not consistently produce a printed image of uniform quality.