JPH09300660A - Picture quality optimizing method for ink jet printer - Google Patents

Abstract

PROBLEM TO BE SOLVED: To determine the disposition of droplets in a pixel based on adjoining pixels for the purpose of optimizing the picture quality. SOLUTION: The selective droplet dispersion technology is used for optimizing the picture quality out of a continuous ink jet printer. First, a pixel to be printed is specified, and then the disposition of droplets 38 in the pixel to be printed is selected so as to optimize the picture quality. The selection can be adjusted unconditionally. In that case, the disposition of droplets 38 is independent of adjoining pixels. In the selection, first or last droplet 38 is printed, or centering is carried out for the droplet 38 in the pixel to be printed.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION This invention relates to continuous tone inkjet images, and more particularly to optimizing image quality.
It relates to a method of arranging a droplet within a pixel based on neighboring pixels.

[0002]

2. Description of the Prior Art In continuous ink jet printing, high pressure ink is supplied to a manifold region from which it is distributed to a plurality of orifices in a linear array. The ink from the orifice is discharged as a filament, and the filament is divided into a stream of droplets. Printing with a droplet stream selectively deflects certain droplets from their normal trajectory. Graphing is done by selectively charging and deflecting droplets from the droplet stream, depositing at least some of the droplets on the receiving medium, the remaining droplets on the droplet capture device. Be recovered. Continuous stream inkjet printing methods are disclosed, for example, in U.S. Pat. Nos. 4,255,754, 4,598,123 and 4,751,517.

Continuous tone inkjet images are obtained by varying the number of printed droplets within a single pixel location. In one example, there are several degrees of freedom in placing individual droplets within a pixel. Biasing the print drops based on the weighting of the density of adjacent pixels improves the overall print quality. This is particularly important at the boundaries of areas where the contrast is distinctly changed, such as transitions from dense printed areas to non-printed areas along the edges of characters and other image data.

Traditionally, anti-aliasing (anti-
Aliasing is known to be an effective technique for improving the quality of low resolution dot matrix text printing. This technique intentionally places gray dots along the boundary between white paper and dense black (jet black) text to create a smooth transition area between two contrasting densities. The human eye has a tendency to group light and dark dots together to make the edges of low resolution character and image data smoother. This is a "ladder step effect" by using gray droplets at pixel locations along the step.
Is most effective for diagonal lines that minimize

There are several ways to produce the gray scale needed to obtain anti-aliased text. The simplest method is to use a printing technique in which dots of the same size with various optical densities can be selectively arranged at each pixel position. Thermal dye sublimation patterns have the ability to form images in this manner.

Another method of producing a gray scale is to modulate the size of a dot of constant density ink within a particular pixel location. The amount of inked area relative to the blank area in a particular area of paper determines the level of gray that can be discerned by human sensory tissue. 1 inch (25.4m
For about 200 dots per m), the human eye summarizes the degree of change in density of individual pixels as a whole and perceives it as a single individual, such as text or image data.

The number of gray grade levels available for a particular printing technology dictates the ability to accurately represent the formation of text or image data. In general, the smaller the minimum dot size, the better the anti-aliasing technique works. Depending on the point size of the font, a few pixels within the width of the line are used to mix the transition from jet black to white, improving the overall image quality.

If the printing method can place spots with a more accurate accuracy than the nominal number of spots per inch (25.4 mm), another method of mixing the transition from jet black to white is possible. For example, the printing method is 1 inch (25.4
When printing with a spatial resolution of 200 dots per mm), a distinct ladder step phenomenon or staircase edge occurs in the diagonal lines. If the printing method can place small spots at various locations within the pixel, the appearance of the lines can be significantly improved. Such a method is disclosed in U.S. Pat. No. 4,620,196, in which the number of drops that can be placed in a pixel varies from zero to a certain maximum number.
In this type of method, a range of optical densities can be obtained by placing a selected number of drops within a pixel.

[0009]

However, in order to optimize the image quality in a printing apparatus capable of remarkably large spatial dot access from one direction with respect to the oblique direction within the printing surface, it can be used at a specific pixel position. There is a need for a technique for selectively identifying the optimum print droplet from a group of droplets.

[0010]

This demand can be met by the present invention which discloses a selective droplet dispersion technique. The present invention determines droplet placement within a pixel based on neighboring pixels to optimize image quality.

In accordance with one aspect of the present invention, selective drop distribution is used to optimize image quality from a continuous ink jet printer. First, identify the pixels to print, then
The placement of the drops within the pixels to be printed is chosen to optimize image quality. The selection can be unconditional, in which case the droplet placement does not depend on adjacent pixels. Such an option prints the first or last available drop, or centers the drop within the pixel to be printed. The selection can also be conditional, in which case the drop placement is determined based on the surrounding pixels. In such a selection, at least 1 for the identified pixel
Data is obtained from one adjacent pixel and the adjacent pixel data is used to select the placement of the droplet within the identified pixel.

It is an object of the present invention to determine the placement of drops within a pixel based on neighboring pixels. Another object of the present invention is to selectively identify the optimal print drop among the drop groups available for a particular pixel location.
An advantage of the present invention is that it allows the image quality to be optimized for a printing device having a significantly large spatial dot access in one direction with respect to the orthogonal direction in the print plane.

[0013]

According to the present invention, in order to optimize the image quality in a printing apparatus capable of significantly large spatial dot access from one direction with respect to the orthogonal direction in the printing surface,
Techniques are available for selectively identifying the optimal print drop from a group of drops available at a particular pixel location.

In a continuous ink jet printer, the orifice spacing within the (orifice) array is physically constrained, thus limiting resolution. Full-color graphic printing by such continuous ink jet device is because of the huge amount of image information existing in the main color data plane,
Provide acceptable image quality. A slight color shift between colors within a pixel element softens the edges of the pixels, minimizing the effect of individual pixels on the image quality due to low spatial resolution. According to the present invention, the image quality of single color text and line art is improved.

The present invention uses anti-aliasing.
iasing). After determining the gray level of a particular pixel, a method of depositing the correct amount of ink at that pixel location must be employed. Determine gray scale using multiple drops per pixel. Due to the movement of the web (printing paper), it is virtually impossible to place successive drops on top of each other, thus selecting the drops to be printed within a particular pixel location. The present invention provides a selection method for selecting drops to be printed within a particular pixel location. The present invention primarily uses high dot access capability or resolution in the direction of web travel.

As is known in the art, the use of anti-aliasing methods can improve the text quality of the characters formed by the dot matrix. Referring to the drawings, FIG. 1A shows a character 10A imaged by conventional dot matrix technology and FIG. 1B shows a similar character 10B imaged by conventional anti-aliasing technology. As can be seen by comparing FIGS. 1A and 1B, the small dots 12 formed along the shaded portion 14 of the character 10B can reduce the apparent "ladder step" effect that occurs in the character 10A due to the dot matrix pattern. it can. Conventionally, it has been recognized that characters formed by dot matrix patterns at a resolution of 400 dots per inch (25.4 mm) do not require anti-aliasing. Modern laser printers can image at 600 dots per inch (25.4 mm), while typical continuous inkjet printers produce 30 dots per inch (25.4 mm).
Limited to 0 dots or less.

FIG. 2 illustrates an example of a method of printing the anti-aliased characters of FIG. 1B using drop selection according to the present invention. According to one embodiment of the present invention, by droplet selection, applicant's US patent application Ser. No. 08 / 379,93.
The anti-aliased character of FIG. 1B can be printed according to the technique described in specification No. 6 (filed January 27, 1995). In web printing using an orifice array in which the orifices are spaced along a direction orthogonal to the direction of movement of the web (printing paper), the dot spacing in the direction of movement of the web can be increased. This can be easily accomplished by producing a large number of available print droplets within the time that the web moves one pixel distance below the printhead. Any number of available drops can be deposited (ie printed) inside a rectangular pixel grid.

The internal area of rectangle 18 of FIG. 1B is shown enlarged in FIG. Each jet black pixel 20 is formed from seven droplets 22. The seven droplets 22 surrounded by the dashed circle 24 spread together on the printing paper when attached to it, forming one large spot similar to the spot 26 shown in FIG. 1B.

The small dot 12 shown in FIG. 1B is formed by three droplets 28 surrounded by a dashed circle 30 in FIG. During printing, these droplets 28 spread together to form a solid spot 12a as shown in FIG. 1B. Note that in the example shown in FIG. 2, the first available printed drop within a pixel is used to form a character.

In one embodiment of the invention, by unconditionally adjusting the placement of droplets within the pixel to be printed,
Image quality is optimized. Without recognizing (considering) the advantages of preferential drop selection within a pixel, the first drop available to fill a particular pixel is the most inevitable choice. However, in addition to the first available print drop, the present invention also considers the benefits of selecting a drop for printing at the fiducial. If the dot resolution is high enough to eliminate smearing problems with printing, then the example of FIG. 2 printing the first available printed drop can easily achieve good results. Similarly, the last available print drop can be printed with good results.

FIG. 3 shows the shape of the resulting character when selecting the first available drop for a pixel. At the bottom of the character, small prints are located close to the body of the character, resulting in good print quality. The small droplets located above the diagonal are away from the body of the letter,
Therefore, the image quality along the upper side of the diagonal line deteriorates. The small dot 34 above the diagonal is displaced by a fraction of the pixel to bring it closer to the body of the character. Conversely, the small dot 36 below the diagonal is located close to the body of the character. The visual quality of characters formed in this manner is less than optimal. Those skilled in the art will appreciate that the amount of image quality degradation due to this asymmetrical placement of spots is a function of print speed, dot size and overall character size.

Referring to FIG. 4, character 10B is shown. By unconditionally adjusting the arrangement of the droplets, that is, by selecting the arrangement of the droplets without considering the positions of surrounding pixels, the image quality of the characters in FIG. It has been improved to be excellent. In FIG. 4, the center drop of the pixel is always selected for printing. In this case, the upper side and the lower side of the straight line look the same. The small droplet above the diagonal is closer to the character body than the droplet above the diagonal of the character of FIG. That is, image quality can be improved by always centering a group of print droplets 38 within a pixel location. In this case, the group of print drops is the drop selected for printing among the drops available for a particular pixel location. For example, if you want to print 3 drops out of the 8 drops available for a particular pixel, the first 3
, The last three, the middle three or any three drops can be printed to form a gray grade pixel.

In order to obtain the characters shown in FIG. 4, the printer must have sufficient intelligence to handle each pixel weight separately. Here, pixel weighting refers to the number of droplets available for a pixel to be printed. For example, if there are seven drops available to print a particular pixel location, then there are eight unique weights from zero to seven.
In order to center the print drop within a pixel, it is necessary to store a unique print / capture pattern for each pixel weight. For example, if two central drops are to be printed, the printer must capture the first three drops, print the next two drops, and capture the next three drops. I won't. If four central drops are to be printed, the printer must capture the first two drops, print the next four drops, and the next two drops. Similarly, if one of the seven available drops is to be printed, the printer (the electronics of) will capture the first three drops of the pixel and the fourth one. The drops have to be printed and the fifth to seventh drops have to be captured. If the first available drop is always printed, then (the electronics of) the printer need not have the same capabilities as above. Upon receiving the print signal, the printer continues to print drops until the proper number of drops have been printed. The printer then performs a drop capture operation and waits for the next print signal.

By the drop selection embodiment of the present invention described above, ie by printing the first or last available drop,
Alternatively, an embodiment in which the placement of the drops is unconditionally adjusted by easily centering the print drops within the pixel location is easy to implement. The printing of the first or last available drop is a simple embodiment of the invention. This simplest implementation is valid, but does not always provide good image quality. Second centering the droplet within the pixel
Although somewhat more complex, this embodiment can provide improved image quality. The third embodiment of the present invention conditionally adjusts the placement of droplets within a pixel to be printed. In this embodiment, the selection of droplets to print is based on adjacent or surrounding pixels. This places all the small droplets closer to the body of the character. Therefore, due to this pixel formation, the diagonal line (in human vision) appears to be a smooth straight line.

In an embodiment of the invention in which the droplets are conditionally selected by taking into account surrounding pixels, printing is done so as to take into account the pixel weights of the preceding and following pixels printed by the same inkjet. By pre-processing the data, the image quality is further improved. The characters obtained as a result of performing the droplet selection method according to this embodiment of the present invention are shown in FIG. Droplets 40 are similarly adjacent to the character body above and below the diagonal. Above the diagonal, the last drop of the drop group available for a pixel is printed close to the body of the character. Below the diagonal,
The first available drop of pixels is printed in close proximity to the character body.

To take into account adjacent pixels, the printer must be capable of maintaining the trajectories of previous and next pixels. In accordance with a preferred embodiment of the present invention, the data is preprocessed using a lookup table that determines the placement of dots within a pixel before printing. This preprocessing is performed based on the data from the previous pixel and the next pixel, and a block diagram of this preprocessing is shown in FIG. The multi-bit / pixel data from block 42 is provided to the lookup table shown in block 44. The look-up table can be tailored to a particular application or can be based on a generic algorithm.

In FIG. 6, the digital image to be printed is accepted in one of many standard formats. Regardless of the file format selected, these files represent image data as the primary colors of cyan, magenta, yellow and black. Alternatively, the image data can be represented in red, green and blue. There are numerous known methods for transforming data from one color coordinate system to another. In each case, each pixel is represented by a density value (typically from zero to 255) for each primary color. An image processing device, referred to as a "font end" processor, processes the image data. Such devices are used to edit pictures, superimpose text, and generally prepare a desired image on the screen.

This comprehensive image file is sent to a specific output device. The purpose of the output device is to accurately reproduce the screen image as prepared by the "font end" processor. Each output device has its own way of handling the same standard input data depending on the technology used by the printer. The multi-bit / pixel data in block 42 represents standard image data to be sent to the printer, and lookup table 44 can be incorporated as part of the printer and method of reducing the data to a printable format.
Of course, it goes without saying that a substantial amount of data processing is performed in the printer in order to accurately represent the original image.

The final step in preparing the image for printing is to reduce the file to a pure binary form as shown in binary data file block 46. This final output from block 46 goes directly to the inkjet drive electronics that control the printing of each inkjet as it passes through the printer. The binary file of 0s and 1s is shown in FIG.

A portion of the typical lookup table of FIG. 6 is shown in FIG.
Shown in In FIG. 7, ◯ 48 represents print droplets from the group of eight available droplet pixels. The "middle" pixel is the pixel determined by the "first" pixel value and the "final" pixel value. As one of ordinary skill in the art will appreciate, there are numerous ways to determine which drop to print. FIG. 7 shows a portion of a look-up table that considers the weighting of pixels above and below a particular pixel to determine how to distribute printed drops within the pixel. In this table, the dots represent the available drops and the circles represent the printed drops. The printing of drops in the middle pixels is determined based on the data in the first and last pixels.

Although the invention has been described with reference to text characters, this is not a limitation of the invention. It will be apparent to those skilled in the art that the droplet selection concept of the present invention is equally applicable to image data.

It will be clear that the number of dot selections within a pixel can be derived from a number of different algorithms. In the case of an image rather than text or line art, it is advantageous to evenly distribute the print droplets within the pixel to soften the edges and minimize print misregistration between colors. As another example, the preferred print drop within a pixel can be selected as a function of adjacent pixels in the two-dimensional direction.
In such a case, the printer considers all pixel values around a particular pixel under evaluation to determine which drop to print. This utilizes pixels on either side and in front of and behind a particular pixel to optimize the selection of droplet printing within the particular pixel. Optimization is obtained by deflecting the droplets within a particular pixel towards the edge (of the pixel) adjacent to the adjacent pixel with the higher weighting. In general, if adjacent pixels have the same weight, the print drops under evaluation will be evenly distributed within the pixel.

Continuous tone inkjet images are obtained by varying the number of printed drops within a single pixel location. According to the present invention, selective drop distribution is utilized to determine the placement of individual drops within a pixel. Biasing the printed drops based on the density weighting of adjacent print pixels improves the overall print quality. This is especially noticeable at the boundaries of areas where the contrast is distinctly changed, such as the transition from dense printed areas to unprinted areas along the edges of the characters.

The present invention solves a previously unrecognized problem that determines how droplets are printed for optimal image quality. In most printing techniques, horizontal dot access and vertical dot access are equal and dots are printed above the dots within a pixel. The present invention has the ability to select droplets within a pixel location, which is particularly advantageous when horizontal dot access is not equal to vertical dot access.

[0035]

INDUSTRIAL APPLICABILITY AND EFFECTS The present invention is useful in the field of ink jet printing, and has an effect of improving the quality of ink jet printed images. The present invention also has the effect of controlling the placement of droplets within a pixel in order to optimize image quality. The invention is particularly useful in continuous tone inkjet printers along the boundaries of areas of sharply varying contrast, such as transitions from densely printed to unprinted areas along the edges of text or other image data. It is advantageous.

[Brief description of drawings]

FIG. 1A shows a typical character image formed by a conventional dot matrix technique, and FIG. 1B shows a typical character image formed by a conventional anti-aliasing technique.

FIG. 2 is a diagram when the dot selection according to the present invention is used.
It is a partially expanded view of a character similar to B.

FIG. 3 is a diagram showing an effect on a character similar to that of FIG. 1B when printed according to an embodiment of droplet selection of the present invention.

FIG. 4 is a diagram illustrating the effect on a character similar to that of FIG. 1B when printed according to another embodiment of droplet selection of the present invention.

FIG. 5 is a diagram showing an effect on a character similar to that of FIG. 1B when printed according to still another embodiment of the droplet selection of the present invention.

FIG. 6 is a block diagram illustrating data processing according to the present invention.

FIG. 7 is a diagram showing a part of the search table of FIG.

[Explanation of symbols]

 10B character 38 printing droplet

Claims (10)

[Claims] 1. A method for optimizing image quality from a continuous inkjet printer, the method comprising: identifying pixels to be printed; selecting to selectively place droplets within the pixels to be printed to optimize image quality. An image quality optimizing method comprising: an arranging step; 2. The method of claim 1, wherein the selective placement step includes the step of unconditionally adjusting the placement of the droplet within the pixel to be printed. 3. The method of claim 1, wherein the selective placement step comprises a conditional adjustment step of conditionally adjusting the placement of droplets within a pixel to be printed. 4. The conditional adjusting step obtains data from at least one surrounding pixel surrounding the identified pixel, and the data of the surrounding pixel is used to determine the droplets within the identified pixel. 4. The method of claim 3 including the step of selecting an arrangement. 5. A method for optimizing image quality from a continuous inkjet printer, the steps of: identifying pixels to be treated; obtaining data from at least one surrounding pixel surrounding the identified pixels; Using the data to select the placement of the droplet within the identified pixel. 6. The method of claim 5, wherein the data acquisition step comprises an adjacent pixel data acquisition step of obtaining data from adjacent pixels. 7. The method according to claim 6, wherein the adjacent pixel data obtaining step includes a step of obtaining data from a previous pixel and a step of obtaining data from a next pixel. 8. The adjacent pixel data obtaining step obtains data from an adjacent pixel on the left side, obtains data from an adjacent pixel on the right side, obtains data from a front pixel, and obtains a pixel on the rear side. 7. The method of claim 6 including the step of obtaining data from the. 9. A method of selectively dispensing droplets for optimizing image quality from a continuous inkjet printer, the method comprising: identifying pixels to be treated; within the pixels to be printed, independent of surrounding pixels. A selective arrangement step of selectively arranging the droplets; 10. The method of claim 9 wherein the selective placement step comprises the step of unconditionally adjusting the placement of the droplet within the pixel to be printed.