AU605131B2 - Method and information-processing system for the production of digital images - Google Patents

Abstract

A method and information processing system for generating digital images by means of a laser printer (10) on an image carrier (7) is characterised by the fact that for the purpose of creating grey tone in the picture elements (4a to 4e), built up by pixels (5), of an image in each case one or several lines or bars (6, 6a, 6b, 8, 8a, 8b, 9, 9a, 9b, 9c, 11) are generated, the lines or bars extend in parallel with the direction of transportation (Y) of the image carrier (7), the bars or lines of all picture elements are flush with one another in the direction of transportation (Y) of the image carrier (7), the lines or bars have in the direction of printing lines of the laser printer (10) a total width which corresponds to the grey value of the picture element, and that the total width of the lines or bars is changed in steps which correspond to a fraction of the pixel width. The lines or bars of picture elements which are adjacent in the direction of transportation (Y) can also adjoin one another. <IMAGE>

Description

COMMONWEATH OF AS I13 C O M M O N W E A*L T H OF A U-S T R A L I A PATENT ACT 1952 COMPLETE SPECIFICATION (Original) FOR OFFICE USE Class Int. Class Application Number: Lodged: Z, g 0 O 7 Complete Specification Lodged: Accepted: Published: Priority: Related Art: This document contains the amendments made under Section 49 and is correct for printing.

Name of Applicant: Address of Applicant: MAURER ELECTRONICS GMBH AND LEIGH-MARDON PTY. LIMITED Nymphenburger Strasse 154, D-8000 Munich 19, FEDERAL REPUBLIC OF GERMANY; and 71-79 Macquarie Street, Sydney, New South Wales 2000, AUSTRALIA, respectively Thomas MAURER DAVIES COLLISON, Patent Attorneys, 1 Little Collins Street, Melbourne, 3000.

Actual Inventor(s): Address for Service: Complete Specification for the invention entitled: "METHOD AND INFORMATION-PROCESSING SYSTEM FOR THE PRODUCTION OF DIGITAL IMAGES" The following statement is a full description of this invention, including the best method of performing it known to us L EI.Ma rrrrr~- -il-II LI-- I 1 -2- METHOD AND INFORMATION-PROCESSING SYSTEM FOR THE PRODUCTION OF DIGITAL IMAGES 1 a The invention relates to the production of digital images by means of a "10 laser printer.

There is a known method of producing such images in which square image i 5 areas of an image stored in an image memory are reproduced in each case by pixel-containing image elements, the image is produced on a photo-sensitive cylinder in the form of a latent image, toner is applied electrostatically to the latent o 15 image and a toner image produced in this way is transferred to an image-carrier which is transported at right-angles to the direction of line printing by the laser printer, and in which grey tones of the image elements are determined by greyness values assigned to them and produced by different blackening of the image o elements. Correspondingly, there is known an information-processing system with 20 a computer, a data memory and a laser printer, for use in such methods.

The method of producing digital images by means of a laser printer in which square image areas of an image stored in an image memory are reproduced in each case by pixel-containing image elements, the image is produced as a latent image on a photo-sensitive cylinder, toner is applied electrostatically to the latent image and a toner image produced in this way is transferred to an image-carrier 11 which is transported at right-angles to the direction of the lines of printing of the i ^laser printer, and in which grey tones of the image elements are determined by greyness values associated with them and are produced by a different blackening of the image elements is generally known. Similarly, a laser printer for the application of this conventional process is generally known.

In the conventional process an image area of the image stored in a memory is in each case reproduced by an image element or elements which is, or are each, U 901008,el dsp.9,68004spe,2 NT O -3built up of a plurality of pixels. The pixels themselves may be depicted only as black or white areas and represent the smallest image unit. The ratio of black to white pixels is adjusted according to the greyness value of the image area to be reproduced within the associated image element. A further subdivision of the pixels themselves into white and black areas is not made in this case.

In order to obtain as high a definition as possible, the pixels may be made relatively small, which, however, especially in the case of a high number of greyness values of the image elements or image areas, but also in the case of a low oleonumber of greyness values, may lead to defects in the image. In this case in Q/ 10 particular severe fogging phenomena are observed. In addition, only a coarse setting of the greyness values which is difficult to control is possible in the case of 4 e 4. a small pixels.

In the case of large pixels the image defects mentioned can indeed be avoided and the setting of the greyness values better controlled, but the disadvantage of only a low definition (coarse image grid) with a relatively low contrast range (number of blackening stages) then arises. As the number of pixels per image element becomes smaller a poorer linearization is also obtained, which means that the prescribed blackening values within a stored blackening process can no longer be precisely recovered.

In addition to this, there is the fact that in the conventional method, on account of unavoidable variations during the transport of the image-carrier or paper to be printed in the Y direction, lines transverse to the direction of transport, that is, in the X direction or in the direction of line-scanning, are produced which Shave an interfering effect in the image, since variations in the distance between the 25 image lines appear to the human eye as variations in grey tones.

The invention is based on the problem of further developing a method and an information-processing system of the type aforementioned so that stored images can be reproduced more precisely and with a better control of the distribution of greyness values.

It should be noted that the terms "pixel" and "sub-element" are used interchangeably in this specification, both indicating a sub-element of an image element.

9 -4- According to a first aspect of the present invention, there is provided a method of producing digital images by means of a laser printer, in which image areas of an image to be reproduced are stored in an image memory, each area being represented in the image memory by a plurality of image elements each of which contains one or an integral number of sub-elements of the digital image to be produced, said sub-elements all being of equal size, said printer being of a form wherein an image is produced on a photosensitive cylinder in the form of a latent image, toner is applied electrostatically to the latent image and a toner image produced in this way is transferred to an image-carrier which is transported at right-angles to the direction of line printing by the printer, 4 1 oi. by use of which printer an image can be represented by a plurality of subelements which are differently printed so as together to provide said digital image, the printer being controlled in relation to each sub-element by a control signal °il 15 which may be a print control signal in response to which the printer will wholly 4 4: print that sub-element, a grey tone of one or more image elements being determined by a greyness value stored in relation to that image element, said grey tone being represented in the digital image by selective printing of the subelements of that image element in accordance with the associated greyness value, wherein, in said method, said selective printing can be provided in relation to a sub-element of an image element by the combination of a print control signal, in response to which the printer will start to wholly print the sub-element, and a subsequent print termination signal, in response to which the printer will terminate printing the sub-element, said print termination signal being output by variable timing means during printing of the sub-element,such that at least one line or bar can be printed across each sub-element of the image element, extending transverse Sto the direction of line printing, being parallel or substantially parallel to the direction of transport of the image-carrier, the total width of the lines or bars in said direction of transport corresponding to the greyness value stored in relation to that image element.

In contrasf to known methods, in a method according to the invention the Lj ,1 4a blackening area of an image element in the direction of the printed lines is not altered in stages of one pixel (or sub-element) width, but in smaller steps which correspond to a fraction of the pixel width. In this way images with higher contrast at the same time as good definition and with better linearization are produced. With the same size of image element it is possible to produce essentially more greyness stages than in the conventional method, these being able also to be reproduced better by means of a stored blackening process (linearization).

A method of plotting grey tones of different brightness on a plot-carrier with an electro-sensitive coating, in which, in order to modulate the areas of individual grid points, the duration of electrical pulses which are applied to marking electrodes by means of which the grid points are burned out of the electro-sensitive coating and evaporated is modulated, is known from German Patent 30 40 153. This method, however, differs fundamentally from that according to the invention and cannot be applied by means of a laser printer.

S 901008,cldspe.009,68004.spe,5 According to a very advantageous further d'evelopment of the invention, the lines or bars of image elements adjacent to each other in the direction of transport of the image-carrier adjoin each other. Since the lines or bars between different pixels of an image element or between different image elements adjoin each other in the direction of transport of the imagecarrier and are aligned in relation to each other, continuous lines or bars in the direction of transport of the image-carrier are obtained during the process of applying the toner, even if chance variations in the line advance occur. Hence no lines running horizontally or in the direction of the line printing (at right-angles to the direction of transport) inside the reproduced image are now produced, as is the case in the state of the art, so that the image quality is still further improved.

According to one advantageous embodiment of the invention compo~ew the image elements may in each case be bui-!t-up of only one pixel. Through this images with an even higher definition and very good contrast can be obtained.

The lines or bars in each case may conveniently lie adjacent to the edge of the pixel first appearing in the direction of line scanning, so that the beginning of one line or bar width can be adjusted very simply and in agreement with the pixel period or image element period.

.o The width of the stages in which the width of the lines S#a or bars can be adjusted preferably corresponds to 1/32 of the pixel width. This stage width can readily be obtained with the currently usual operating speed of laser printers of the $lower prices range with electronic components available on the market.

According to one very advantageous embodiment of the invention, a count value of a counter operating at constant frequency is set for the production of the widths of the lines or bars, the count value corresponding to the required line or

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-74 IVT Oe(J~ roJ -6bar width and the lines or bars being widened until the counter has reached a predetermined count value such as zero.

On increasing the size of an image element by a factor the width of the lines or bars is also increased by this factor so that an even greater contrast is obtained. The geometrical definition is in this case naturally reduced.

According to a second aspect of the present invention, there is provided a 0. printing apparatus, for producing a digital image comprising a plurality of subelements, some of which can be printed differently from others so as together to provide said image, the apparatus comprising a laser printer, an image memory, and grey tone determining means, in which image areas of the image to be produced are stored in the image memory, each area being represented in the image 0 4 I memory by a plurality of image elements each of which contains one or an integral number of said sub-elements, said printer being of a form wherein an image is produced on a photo- 15 sensitive cylinder in the form of a latent image, toner is applied electrostatically to Sthe latent image and a toner image produced in this way is transferred to an image-carrier which is transported at right-angles to the direction of line printing by the printer, said grey tone determining means comprising means for storing a greyness value such that, in use 6f the apparatus, a grev tone for an image element can be determined by a greyness value stored in relation to that image element, said grey tone being represented in the digital image by selective printing of the subelements of the image element in accordance with the associated greyness value, wherein said grey tone determining means further comprises; means for providing a print control signal, and variable timing means for outputting a print termination signal, the printer responding to a print control signal alone, in relation to a sub-element, by wholly printing the sub-element, and Sresponding to a print control signal followed by a print termination signal by ceasing to print a sub-element on output of the print termination signal, such that said selective printing for the production of a grey tone in an image element can be produced by the printing of at least one line or bar over each sub-element of an image element, extending transverse to the direction of line S"-8,Idsp.9,6pc6 L S-7printing, being parallel or substantially parallel to the direction of transport of the image-carrier, the total width of the lines or bars in said direction of transport corresponding to the greyness value stored in relation to that image element.

Said means for controlling printing may comprise a counter, the count value of the counter for successive image element data along a printing line being in each case set anew to a value which corresponds to the greyness value in question Sfor an image element, the counter emitting a counter output signal until the count value has reached a predetermined value, such as zero, the width of said lines or bars being controlled by said output signal. Said means may further comprise an OR gate through which the counter output signal or a text signal of the laser printer can be transferred for the printing line in question to a lasing element of the laser printer.

The counter is to advantage reset always when a period adjustment counter has passed through a predetermined number of counting steps through which the size of an image element in the direction of a printing line is determined. If, for example, an image element is built up in square form of four pixels (or subelements), the image element period is two pixel widths. In the case of image elements with only one pixel, the image element period is one pixel width. The counter is then set at the beginning of a period to a count value which corresponds o 20 to the blackening value of the image area which is to be reproduced by the image element.

According to the size of the image element, the image element data of a printing line must be read out so often that the number of pixels in the Y direction (direction of transport of the image-carrier) corresponds to the number of pixels in the X direction (direction of the printing line). The period counter may obviously also be set so that its period in the case of image elements with only one pixel is less than the pixel period, or that in the case of image elements with more than one pixel it is less than the image element period. The count value of the counter for the line or bar width must then be changed accordingly (Fig. right-hand half).

According to one very advantageous embodiment of the invention, a table memory is disposed at the input of the counter, in which table memory the relationship between the greyness value determined by the image element data and I 1 901o08,eldspe9 s 1 'T O 7a the counter value for the production of this value is stored, which is passed to the counter on the input of the image element data. The necessary counter value can be very rapidly determined by this table memory.

According to a further very advantageous embodiment of the invention, the means for storing a greyness value comprises two line memories, each for one printed line of the laser printer, which are charged alternately with image element data and which then deliver their image data to the counter when the other line memory in each case is being charged.

The drawings show, in addition to the state of the art, examples of embodiments of the invention. Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 shows a section of a data field stored in an image memory for an Figure 2 Figures 3(a) image with individual image areas, shows an image element with several pixels for the reproduction of an image area of the said data field according to the conventional method, to 3(c) show image elements for the reproduction in each case of an image area of the data field S s -55 901OO8,cdse.009,6800 4.s c8 iru corresponding to the method according to the invention Figures 4(a) to 4(c) show graphical representations to explain the adjustment of the width of a line within an image element corresponding to a given blackening value, Figure 5 shows the relationship between the blackening and a count value of a counter for line widening, Figure 6 shows a modular mimic display of an informationprocessing system according to the invention, and Figure 7 shows various signal process graphs to explain the operation of the period-setting counter and of the counter for the widening of the lines within the image elements.

0 An image memory 1 in which a data field 2 with several image B areas 3 of an image is stored is shown schematically in Fig. 1 OI Image element data which give a greyness value or blackening Svalue S for the image area 3 in question are associated with each image area 3.

According to Fig. 2, in the conventional method each image area 3 of the data field 2 is reproduced by an image elem- 00 ent 4 with the associated greyness value or blackening value S.

I The image element 4 contains for this purpose a plurality of pixels 5 and is square in construction. In each case a pixel is represented in either black or white, so that the corresponding blackening value S of the image element 4 is produced by the ratio of black to white pixels Figs. 3(a) to 3(c) show image elements such as are produced by the method according to the invention for the reproduction of the digital image. As is shown in Fig. an image element 4 consists in each case of one pixel 5, the size of the pixel coinciding, for example, with that from Fig. 2. In the pixel 5 shown on the left there is at the left-hand margin a line 6, the longitudinal direction of which lies in the direction of transport of the image-carrier 7 (cf. Fig. i.e. in -9the present case in the Y direction of the image-carrier 7.

The width of the line 6, however, extends in the direction of line-printing of the laser printer 10, i.e. in the X direction at right-angles to the Y direction. This width of the line 6 present within the pixel 5 is set to coincide with the blackening value S which is associated with the image area 3 that is to be produced by means of the pixel 5. The line width in this case can be adjusted in stages, the width of which is in each case 1/32 of the pixel width. Thus in all 32 1 different greyness values can be produced within the pixel Inside the pixel 5 shown on the right in Fig. 3(a) two lines 6a and 6b are present by means of which a greyness value S can also be produced such as that obtained by means of the pixel 5 on the left in Fig. 3(a).

oi If the image areas 3 of the data field 2 are reproduced ooby such pixels 5, the lines 6 or 6a, 6b lie in the Y direction, in each case aligned in relation to each other and one after of the other and may, in addition, have their faces adjoining each other. On the application of toner to the latent image built up by the pixels 5, practically no transitions between the pixels adjacent in the Y direction are obtained in the S, toner image in this case, so that in the reproduced there are o0 now no interfering lines running in the X direction. Each line 6 of printing may, however, also beshifted in the direction of the printing line in relation to the preceding one in order to obtain other requried image grids.

Definition, linearity and contrast or extent of blacken- 6" ing are relatively high, since the size of the pixel is small, S and within one pixel 5 the blackening value S can be precisely set on account of the fine subdivisibility of the line width.

Fig. 3(b) shwws two image elements 4a, 4b which are in each case square and made up of four pixels 5. The image element 4a contains at its left-hand margin a line 8 which extends longitudinally over practically two lines of printing of the laser printer 10. In the image element 4b two lines 8a, 8b are present which also extend over two lines of printing in the Y direction. In this case an image area 3 is reproduced either by the image element 4a or the image element 4b. Here also the width or total width of the lines 8 or 8a, 8b corresponds to the required blackening value of the image area 3. The definition is naturally reduced as compared with the first embodiment example on account of the larger image elements 4a, 4b. Linearity and contrast, however, are improved.

Altogether twice as many greyness stages can be set within the image elements 4a, 4b as in the image elements or pixels shown in each case above them.

Further examples of image elements 4c, 4d are shown in Fig. the image element 4c containing nine pixels 5 and one line 9 at the left-hand margin, while the image element 4d contains nine pixels 5 and three lines 9a, 9b, 9c which extend 09.. in the longitudinal or Y direction over three printing lines of the laser orinter 10. In this case also ar image area 3 is reproduced in each cse by the image elements 4a and 4b respectively.

Qo A more detailed explanation is given below by means of S Figs. 4 and 5 of how the blackening of an image area is set by line-widening. Fig. 4(a) shows an image element 4e with sixteen pixels 5 for the reproduction of an image area 3 with a blackening value S which is about 35% of the total blackening. On dividing the width of a pixel 5 into 32 stages the total r0 blackening of 100% is reached after 128 stages, while a blackoning of about 35% is already obtained after 64 stages (of.

ig. With this blackening of about 35% the image eler.,nt 4e is thus half-filled with a line 11 which extends in the Y direction over four printing lines of the laser printer If the laser beam of the laser printer 10 reaches the image element 4e along a printing line I, the count value Z of a counter 12 is first set to a value which corresponds to the i -i -11required blackening S, in agreement with the blackening curve shown in Fig. 5. The count value Z is thus in this case 64. The counter 12 in 6 and 7 is adjusted so that it reduces the count value set to zero. As long as it counts down it gives, in accordance with the count graph shown in Fig. 4(b), a counter output signal D corresponding to Fig. 4(c) which is used to operate a laser diode 13 inside a printing unit 14 of the lasor printer 10. Thus as long as the laser diode 13 is operated by the counter output signal D the width of the line 11 in Fig. 4(a) is widened in the direction of printing of the line I until the counter value zero is reached. Since there are altogether four pixels 5 in the direction of line printing, this process is repeated for for successive printing lines I to IV in order to obtain a square image element 4e. If a wider or narrower line 11 is to be produced in accordance with the blackening value of the image area 3 in each case, the counter 12 is initially set to a correspondingly greater or smaller counter value Z.

l In contrast to the conventional method, therefore, the size o" the smallest image unit for the production of a digital image does not correspond to the size of a pixel, but to the size of a line element within the pixel, the width of which amounts to only a fraction of the pixel width. A line with so small a width can be produced, for example, by only a minimal deflection of the laser beam.

An information-processing system according to the invention is shown in Fig. 6. This information-processing system has a conventional computer 15, a data memory 16 with the said image memory 1 and a text memory 17, as well as the laser printer which has already been mentioned. The laser printer comprises the printing unit 14 with the laser diode 13, a control 18, a text buffer 19 for one page and a symbol generator 20. Text from the text memory 17 is fed page by page into the text buffer 19 and passed by way of the symbol generator to the printing unit 14, in order to beprinted by means of the laser diode 13t before which an amplifier 21 is installed, on the imagecarrier 7. The printing process is controlled by the L -Y L -i_-l -12 computer 15 and the control system 18, which in turn transmits a line-synchronization signal and a side-synchronization signal to the computer 15 or the data memory 16.

N The information-processing system has an additional unit 22 which contains the already mentioned counter 12 for linei widening, a logical OR gate 23 and two line memories 24, O~e -/tio e with in each case a Iln4e?- memory content. The two line memories 24, 25 (line buffers) are connected on the input side i with the image memory 1 and on the output side with the counter S12, the output of which is in turn connected with an input of the OR gate 23. The other input of the OR gate 23 is connected to the symbol generator 20. In the output side the OR gate 23 i is connected through the amplifier 21 to the laser diode 13 of the printing unit 14. The line memories 24, 25 also receive the line-synchronization signal and the side-synchronization signal from the control system 18 of the laser printer As long as text is being printed the symbol generator delivers a text signal Te through the OR gate 23 and the amplifier 21 to the laser diode 13. As soon as an image is to be produced within a text on a page, which is recognized by means of an appropriate instruction (control identification mark in the prepared text), image element data from the image memory 1 for the next two printing lines of the laser printer are stored by lines in the line memories 24 and 21. The image element data from the line memory 24 are transmitted in sequence to the counter 12, the count value Z of which is in each case set afresh by the successive image element data I along a printing line to a value which corresponds to the data value or blackening value i~i question for an image element, and which emits a counter output signal D until such time as its count value z has reached the value zero. This counter output signal D is also transmitted through the OR gate 23 and the amplifier 21 to the laser diode 13 in order to print an image line on the image-carrier 7. In this way the lines are produced along the image line within the individual image -13elements with a width which corresponds in each case to the count value Z of the counter 12. The line width is thus controlled by the counter output signal D of the counter 12.

The text signal Te and the counter output signal D may obviously also be produced simultaneously so that text can be written within an image area.

The counter 12 is always set afresh when a period-setting counter 26 has passed through a predetermined number of counting stages by means of which the size of an image element in the line-printing direction is determined. The image element period P is thus determined by the period-setting counter 26. For this purpose the count value of the period-setting counter 26 is periodically set to a count value which is fed to it from an 8-bit register 27. The count value in the 8-bit register 27 is stored by the computer in accordance with the size of the image element. If, for example, the counter value 32 is in accordance with Fig. the image element period P is the pixel period. With a counter value of 64 for the period-setting counter 26 the image element period P is two pixel periods, V and so on. Coinciding with the beginning of a period P a tempo signal T depending on the time t is transmitted, which then causes the counter 12 to be set to its count value Z for the following image element period P. Both counters 26 and 12 are set to operate with the same constant frequency. In this case the count value of the counter 12 is always equal to or less Sthan the count value of the counter 26. According to the count duration of the counter 12, the counter output signal D already mentioned is produced for the switching on and off of the laser diode 13 of the laser printer 10, as can be seen from fig. 7.

A table memory 28 (look-up table) is disposed to advantage between the outputof the line memory 24, 25 and the input of the counter 12, the relationship between the greyness value determined by the image element data and the count value necessary for the production of this greyness value being stored in it, and the count value Z being transmitted to the counter -14- 12 on the input of the image element data into the table memory 28.

On account of the high deflection rate of the laser printer two line memories 24, 25 are provided. These are connected with the image memory 1 and the counter 12 in such a way that the image data are always output to the counter 12 from only one line memory 24), while the other one (e.g.

can be charged by the computer 15 with image element data from the image memory 1 independently of the fixed time behaviour of the laser printer 10. The computer 1 monitors the line memory 24, 25 from which the image data are output to the counter 12, and accordingly which line memory 24, 25 is just being charged.

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Claims (6)

1. Method of producing digital images by means of a laser printer, in which image areas of an image to be reproduced are stored in an image memory, each area being represented in the image memory by a plurality of image elements each of which contains one or an integral number of sub-elements of the digital image i to be produced, said sub-elements all being of equal size, said printer being of a form wherein an image is produced on a photo- sensitive cylinder in the form of a latent image, toner is applied electrostatically to the latent image and a toner image produced in this way is transferred to an i image-carrier which is transported at right-angles to the direction of line printing t' by the printer, by use of which printer an image can be represented by a plurality of sub- elements which are differently printed so as together to provide said digital image, o" 15 the printer being controlled in relation to each sub-element by a control signal 0't C 0 S which may be a print control signal in response to which the printer will wholly 114 0 print that sub-element, a grey tone of one or more image elements being o too. determined by a greyness value stored in relation to that image element, said grey Og tone being represented in the digital image by selective printing of the sub- o 0 020 elements of that image element in accordance with the associated greyness value, wherein, in said method, said selective printing can be provided in relation to a sub-element of an image element by the combination of a print control signal, in response to which the printer will start to wholly print the sub-element, and a subsequent print termination signal, in response to which the printer will terminate printing the sub-element, said print termination signal being output by variable timing means during printing of the sub-element, such that at least one line or bar can be printed across each sub-element of the image element, extending transverse to the direction of line printing, being parallel or substantially parallel to the direction of transport of the image-carrier, the total width of the lines or bars in said direction of transport corresponding to the greyness value stored in relation to that image element. i I-U II-- i; till-i_-

16- 2. Method according to claim 1, wherein each image element comprises a plurality of sub-elements, the lines or bars being aligned. 3. Method as in claim 2, characterised in that the lines or bars of image elements adjacent to each other in the direction of transport of the image carrier adjoin each other. o 4. Method as in claim 1 or 3, characterised in that each image element comprises only one sub-element. Method as in any one of claims 1 to 4 wherein the lines or bars are in each 0 1 case adjacent to the edge of the associated sub-element first appearing in the direction of line printing. 15 6. Method as in any one of claims 1 to 5, wherein the width of each of said lines or bars can be selected from any integral multiple of 1/32 of the width of the associated sub-element, such that the greyness value for each image element can be selected from 33 different values. 441 7. Method as in any one of claims 1 to 6, wherein to control the width of a line or bar in a sub-element, the variable timing means sets a count value of a counter operating at a constant frequency, which count value corresponds to the li required line or bar width, and that the printer continues to print while receiving an ,1 output from the counter such that each line or bar is widened until the counter has 25 reached a predetermined value, this providing said print termination signal. r; 8. Method as in claim 7 wherein there is only one predetermined value, said predetermined value being zero. 9. Method as in any one of claims 1 to 8, wherein in the case of an enlargement of an image element by a factor the width of the lines or bars is also increased by this factor. V^A, 901008,cldspc.009,68004.spc16 1 17 o 0o 0P I 1000 0 #0) o 0 I 0 0lB 1 0I 0 0 00 0) 00 Printing apparatus, for producing a digital image comprising a plurality of sub-elements, some of which can be printed differently from others so as together to provide said image, the apparatus comprising a laser printer, an image memory, and grey tone determining means, in which image areas of the image to be produced are stored in the image memory, each area being represented in the image memory by a plurality of image elements each of which contains one or an integral number of said sub-elements, said printer being of a form wherein an image is produced on a photo- sensitive cylinder in the form of a latent image, toner is applied electrostatically to 10 the latent image and a toner image produced in this way is transferred to an image-carrier which is transported at right-angles to the direction of line printing by the printer, said grey tone determining means comprising means for storing a greyness value such that, in use of the apparatus, a grey tone for an image element can be determined by a greyness value stored in relation to that image element, said grey tone being represented in the digital image by selective printing of the sub- elements of the image element in accordance with the associated greyness value, wherein said grey tone determining means further comprises; means for providing a print control signal, and variable timing means for 20 outputting a print termination signal, the printer responding to a print control signal alone, in relation to a sub-clement, by wholly printing the sub-element, and responding to a print control signal followed by a print termination signal by ceasing to print a sub-element on output of the print termination signal, such that said selective printing for the production of a grey tone in an image element can be produced by the printing of at least one line or bar over each sub-element of an image element, extending transverse to the direction of line printing, being parallel or substantially parallel to the direction of transport of the image-carrier, the total width of the lines or bars in said direction of transport corresponding to the greyness value stored in relation to that image element. 11. Printing apparatus according to claim 10, wherein each image element comprises a plurality of sub-elements, the lines or bars being aligned. S *'901W8,cldspc.009,68004.sp,17 ,A -18 12. Printing apparatus according to either one of claims 10 or 11 wherein said variable timing means comprises a counter, the count value of the counter for successive image elements along a printing line being in each case set to a value which corresponds to the greyness value in question for that image element, the counter emitting a counter output signa. until the count value has reached a predetermined value, the width of said lines or bars being determined by said output signal. 13. Printing apparatus according to claims 12, further comprising an OR gate through which the counter output signal or a text signal of the printer can be transferred for the printing line in question to a printing element of the printer. 14. Printing apparatus according to either one of claims 12 or 13 wherein said predetermined value is zero. Printing apparatus as in any one of claims 12 to 14, wherein a fresh setting of the counter is always made when a period-setting counter has completed a predetermined number of counting stages by means of which the size of an image element in the printing line direction is determined. o :o 16. Printing apparatus as in any one of claims 10 to 15, wherein image element data of each printing line representing the size of an image element can be repeatedly read out.

17. Printing apparatus as in any one of claims characterised in that at an input of the counter a-table memory is disposed in which the relationship between a greyness value determined by the image element data and the count value necessary for the production of this greyness value is stored and passed to the counter on input of image element data to the counter.

18. Printing apparatus as in any one of claims 10 to 17, wherein said means for storing a greyness value comprises two line memories, each line memory in use 901008,eldsp.009,6804.spe,18 11 -19- storing data concerning a printing line of the laser printer, which line memories are alternately charged with image element data and which then transmit their image element data to the variable timing means when the other line memory is being charged.

19. An information-processing system according to any one of claims 10 to 18 wherein a computer comprises the image memory. An information-processing system according to any one of claims 10 to 19 wherein a computer comprises the means for storing the greyness value.

21. A method of producing digital images substantially as hereinbefore described with reference to the drawings. 15 22. Printing apparatus substantially as hereinbefore described with reference to the drawings. DATED this 8th day of September, 1990 MAURER ELECTRONICS GmbH and i LEIGH-MARDON PTY. LIMITED D By their Patent Attorneys DAVIES COLLISON 1,00.8,cdp.009,68 4.sp19