JP4008341B2 - Error diffusion gradation generation method, error diffusion gradation image display method, and gradation generation encoder - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a system for displaying a digital image, and more particularly to a system using error diffusion gradation generation for displaying a digital image.
[0002]
[Prior art]
A digital image is usually represented as a pixel array, or pixels at several points during its recording and processing. Pixels are sequentially represented by values that quantify the brightness of one or more chromatic color components of the image at the spatial coordinates represented by the pixel. The luminance is usually expressed as a value of a gradation level that develops between the extremes of black and white “gradations”. In the case of a color image, the image is usually broken down into multiple color levels. Each color level represents a tone image that represents the luminance of a pixel for one of three color components (eg, red, green, and blue) or four colors (eg, cyan, magenta, yellow, and black) in the image. Contains. A color image is generated by superimposing a color level on the image.
[0003]
In order to visually appeal the realism of the image, the luminance of the pixel is expressed with a resolution of about 8 bits or 12 bits. The luminance is resolved to 256 levels with 8 bits and resolved to 4096 levels with 12 bits. On the other hand, a revealing mechanism such as a printer usually cannot reveal pixels at such a high level of resolution. Most printers are limited to binary resolution, that is, the choice of whether or not ink droplets adhere to pixel locations. Some printers can choose from limited dot sizes or light and dark inks of component colors, but their resolution is usually lower than the resolution of the input values for the pixels. In order to simulate the color resolution of an image, the printer is designed to exchange spatial resolution for color resolution in a process known as tone generation. In addition, the printer controls the adhesion of ink based on the ratio of the pixels in the corresponding portion of the printed image in accordance with the high resolution input value for the pixels in the image portion. When this control is performed correctly, the human visual system recognizes the ink dots arranged as a halftone or color area close to the corresponding halftone or color of the original image.
[0004]
One common tone generation method is “ordered dithering”. In the printer, when the input value of the pixel at a position on the print medium exceeds a threshold value, the control section instructs the printer mechanism section to deposit an ink droplet at that position. When the pixel input value is less than the threshold, no ink is deposited at that position. In the regular dithering method, the threshold matrix is conceptually overlaid on the output medium, and the different thresholds contained in the matrix are associated with various pixel locations on the medium. Regular dithering is not a prominent computational technique, but the characteristics that can occur in the process can cause unwanted defects under many conditions, so in certain applications a process that reduces the likelihood of these defects. It is desirable to apply.
[0005]
Error diffusion tone generation is a tone generation method made to improve the result generated by the regular dithering method. Also, in error diffusion tone generation, the high resolution input luminance value for a pixel is compared to a threshold value to determine whether ink adheres to the pixel location. However, the threshold is often set to approximately half of the maximum value of the pixel and generally does not change as a function of the pixel position. In order to obtain a halftone effect, the difference between the input value of the pixel and the value when it is revealed, ie the error is distributed to the non-printed pixels adjacent to that pixel, and one of those adjacent pixels is printed When done, the accumulated error is added to the input luminance to obtain an increased input luminance that is compared to the luminance threshold. For example, as shown in FIG. 1, when printing the reference pixel 20, the error is distributed to the adjacent pixels 22, 24, 26, and 28. In order to simplify the example, it is assumed that the pixel 20 has no accumulated error (Δ = 0), and the increased input value (A) is equal to the input value (V = 239) of the luminance of the pixel. Since the increased input luminance (A = 239) exceeds the threshold value (T = 127), the printer instructs whether to use a binary (B = 1) format for attaching ink to the pixel position. Pass. The difference between the increased input value (A) and the “when exposed” value of the pixel (B × 255 = 1 × 255) is the sum of the errors (δ = −16), which is the non-printed pixel adjacent to the pixel 20. Distributed. In this example, the errors are distributed according to the error filter weights originally proposed in Non-Patent Document 1 by Floyd and Steinberg. That is, 7/16 (−7) of the sum of errors is distributed to the pixel 22 immediately to the right of the pixel 20 to be printed. The less error part is 3/16, 5/16, 1/3 for the three other pixels, ie, pixels 28, 26, 24 in the column immediately following the column where the printed pixel 20 is located, respectively. 16 is distributed. Other error distribution algorithms distribute different portions of the error sum to other combinations of pixels around the print pixel.
[0006]
FIG. 2 shows the error accumulation for pixel 40 and the effect of the pixel accumulated on the threshold determined when printing the pixel. The image is usually printed in raster scan order, and the diffused error is when the adjacent pixels 42, 44, 46 in the column preceding the column containing the target pixel 40 and the pixel 48 immediately to the left of the target pixel 40 are printed. Are accumulated in the pixel 40.
[0007]
In other known algorithms, the errors are accumulated in different ways but in different proportions, forming different pixels around. When the target pixel 40 is to be printed, the input value (V = 230) is increased by adding a cumulative error (Δ = 25), thereby increasing the input value (T = 127) compared with the threshold value (T = 127). A = V + Δ = 230 + 25 = 255). At this time, since the increased input value (increased input luminance) exceeds the threshold value, the ink adheres to the target pixel 40, but the increased input value (A) is the value “when revealed” (B × 255 = 1 × 255). ), There is no error (δ = 0) distributed to adjacent pixels. Since the binary pixel “when exposed” value is either the minimum or maximum value of the pixel, sufficient positive and negative errors are accumulated. The accumulated error is sufficient to print or not print a pixel even if the input luminance value naturally results in the opposite.
[0008]
[Patent Document 1]
JP-A-5-153398 (publication date: June 18, 1993)
[0009]
[Non-Patent Document 1]
ADAPTIVE ALGORITHM FOR SPATIAL GRAY SCALE, Sid. Int. Sym. Digest of Tech. Papers, 1975 ”
[0010]
[Problems to be solved by the invention]
In general, the size of the threshold used for error diffusion tone generation depends on the position, and error diffusion is more accurate than the regular dithering method. However, error diffusion, such as regular dithering, is a quantization process and produces defects in certain areas of the image. The error diffusion gradation generation generates a pattern that looks like a line consisting of “worms” or dots, particularly in a bright region. In addition, error diffusion to adjacent pixels delays printing of bright portions by generating a greater amount of patterns for dots in later pixels of the tone portion. In the halftone region, error diffusion results in a checkerboard effect that visually hinders a smooth transition between parts of the image. Error diffusion also causes dots of different colors to adhere to the same pixel location. For example, magenta dots on cyan dots generate dots close to black that make the image look rough.
[0011]
To reduce defects generated by error diffusion using the four errors of the aforementioned Floyd and Steinberg algorithm, it has been proposed that an error filter with additional weights further diffuse the error. An error filter with additional weights requires more computation processing, highlighting certain defects and degenerating other parts. In addition, error diffusion combined with perturbation and dithering does not require significant computation, but adds errors generated by error diffusion to dithering defects. A third method capable of reducing defects is to perform a spatial experiment using the original input pixel value and to change an error diffusion tone threshold value that makes it easy or difficult to attach ink to the pixel. The results of this experiment are used. Since this method requires an experiment in addition to the gradation generation process, it requires a lot of arithmetic processing.
[0012]
Accordingly, an object of the present invention is to provide an improved gradation generation method that reduces defects generated by the error diffusion gradation generation method and necessary operations.
[0013]
[Means for Solving the Problems]
In order to solve the above problems, an error diffusion tone generation method according to the present invention includes: (a) a target pixel and the error diffusion tone generation method for selecting a luminance threshold value for image tone generation; A step of selecting the first luminance threshold when at least one error of one adjacent pixel exceeds a first error threshold; and (b) the target pixel at a position away from the adjacent pixel. Selecting a second luminance threshold when an error of a neighboring pixel adjacent to the pixel exceeds a second error threshold, and (c) not selecting the first luminance threshold; and (c) the first and first A step of selecting a third luminance threshold when no two luminance threshold is selected.
[0014]
In the above method, by controlling the distribution of ink dots that generate gradation, it is possible to reduce gradation defects and to maintain edge sharpness and image details. In addition, the threshold value is selected and used, and the accumulated error of the pixels adjacent to the target pixel is used to control the distribution of the ink dots, so that the processing can be performed at high speed even by the conventional computer calculation.
[0015]
In the error diffusion gradation generation method, it is preferable that at least one of the first and second error threshold values is substantially zero error.
[0016]
In the error diffusion gradation generation method, the luminance of the first luminance threshold is larger than the luminance of the second luminance threshold, and the luminance of the second luminance threshold is the third luminance threshold. It is preferably greater than the value brightness.
[0017]
In the error diffusion gradation generation method, it is preferable that at least one of the errors of the target pixel, the adjacent pixel, and the separated adjacent pixel includes a component color error of the target pixel.
[0018]
In order to solve the above problems, the gradation image revealing method of the present invention includes: (a) determining the luminance of the target pixel in the image; and (b) increasing the luminance of the target pixel having a target pixel error. (C) when at least one of the target pixel error and an adjacent pixel error of an adjacent pixel adjacent to the target pixel is smaller than an error threshold, a first luminance threshold is selected; Sometimes selecting a second luminance threshold; and (d) revealing the target pixel at a first apparent luminance when the increased luminance of the target pixel exceeds the selected first or second luminance threshold In other cases, the step of revealing the target pixel at the second display brightness, and (e) an error between the first or second display brightness and the increased brightness of the target pixel is adjacent to the target pixel. Out of pixels It is characterized in that it contains a step of assigning one even without.
[0019]
In this method, as in the error diffusion gradation generation method, gradation defects can be reduced and high-speed processing can be performed by conventional computer computation.
[0020]
In the gradation image revealing method, the error threshold is preferably substantially zero error.
[0021]
In the gradation image revealing method, it is preferable that the first reveal brightness includes a maximum brightness and the second reveal brightness includes a minimum brightness.
[0022]
In the gradation image revealing method, it is preferable that the luminance of the target pixel includes a luminance of a color component of the target pixel.
[0023]
In the gradation image revealing method, the luminance of the first luminance threshold is preferably larger than the luminance of the second luminance threshold.
[0024]
In the gradation image revealing method, when the increased brightness of the target pixel exceeds a third brightness threshold value that is greater than the brightness of the first brightness threshold value, the target image is displayed at the first reveal brightness level. Preferably, the method further includes revealing the pixel.
[0025]
The gradation image revealing method. Preferably, at least one of the target pixel error and the adjacent pixel error includes a component color error.
[0026]
The error diffusion gradation image revealing method includes (a) determining the luminance of the target pixel in the image, (b) increasing the luminance of the target pixel having the target pixel error, and (c) ) Selecting a first luminance threshold when at least one of the target pixel error and an adjacent pixel error of an adjacent pixel adjacent to the target pixel is smaller than a first error threshold; and (d) the adjacent The second luminance threshold is selected when the separated proximity pixel error of the separated proximity pixel that is close to the target pixel at a position away from the pixel is smaller than the second error threshold, and the first error threshold is not selected. And (e) selecting a third luminance threshold when the separated proximity pixel error is smaller than a third error threshold, and not selecting one of the first and second error thresholds. When (F) when one of the first, second and third luminance thresholds is not selected, selecting a fourth luminance threshold; and (g) the increased luminance of the target pixel is selected. Revealing the target pixel with a first display brightness when the first, second, third or fourth brightness threshold is exceeded; otherwise, revealing the target pixel with a second display brightness; And (h) assigning an error between the first or second apparent luminance and the increased luminance of the target pixel to at least one of the pixels adjacent to the target pixel. .
[0027]
In this method, as in the error diffusion gradation generation method, gradation defects can be reduced and high-speed processing can also be performed by conventional computer calculation, as in the error diffusion gradation generation method.
[0028]
In the error diffusion gradation image revealing method, it is preferable that the first, second, and third error thresholds are substantially zero errors.
[0029]
In the error diffusion gradation image display method, it is preferable that the first display brightness includes a maximum brightness and the second display brightness includes a minimum brightness.
[0030]
In the error diffusion gradation image revealing method, it is preferable that the luminance of the target pixel includes a luminance of a color component of the target pixel.
[0031]
In the error diffusion gradation image revealing method, the luminance of the first luminance threshold is larger than the luminance of the second luminance threshold, and the luminance of the second luminance threshold is the third luminance threshold. It is preferable that the brightness of the third brightness threshold is greater than the brightness of the fourth brightness threshold.
[0032]
Further, the error diffusion gradation image revealing method may be configured such that when the increased luminance of the target pixel exceeds a fifth luminance threshold value that is higher than the first luminance threshold value, the target pixel is set at the maximum exposed luminance. Preferably, the method further includes a revealing step.
[0033]
In the error diffusion gradation image display method, it is preferable that at least one of the target pixel, the adjacent pixel error, and the separated adjacent pixel error includes a component color error, and further, the component color error is a component of the target pixel. It is preferable that errors of component colors other than colors are included.
[0034]
In order to solve the above-described problem, the gradation generation encoder of the present invention includes a selection threshold unit that compares the input luminance of the target pixel with the selection threshold luminance, and the adjacent adjacent to the target pixel and the target pixel. A threshold selection unit that selects one of a plurality of threshold luminances as the selection threshold luminance for the selection threshold unit in response to an error of at least one of the pixels. It is characterized by that.
[0035]
In this gradation generation encoder, as in the error diffusion gradation generation method, gradation defects can be reduced and high-speed processing can also be performed by conventional computer computation, as in the error diffusion gradation generation method. it can.
[0036]
Moreover, it is preferable that the gradation generation encoder further includes the input luminance of the target pixel with respect to an initial threshold luminance larger than the selected threshold luminance.
[0037]
In addition, the gradation generation encoder includes an error filter for detecting errors generated by printing the target pixel for a plurality of pixels adjacent to the target pixel, and an error for accumulating the errors distributed to the pixels. And a buffer.
[0038]
DETAILED DESCRIPTION OF THE INVENTION
One embodiment of the present invention will be described with reference to FIGS. 3 to 5 as follows.
[0039]
FIG. 3 illustrates a typical personal computer system 50 that includes a personal computer 52 that transmits instructions to a display device such as an inkjet printer 54. The indication usually defines as a signal that an individual pixel of the image receives an ink dot and the color of the ink containing the dot. The printer 54 receives these signals via the appropriate communication channel 54.
[0040]
There are many configurations of a computer system using a printer to which the present invention can be applied. In a typical computer system 50, a central processing unit (hereinafter referred to as CPU) 58 transfers data to the printer 54 via an internal bus 60 connected to a communication channel 56 by an input / output adapter 62. The CPU 58 retrieves data and instructions from various signal sources including a read-only memory (hereinafter referred to as ROM) 64 and a read-write random access memory (hereinafter referred to as RAM).
[0041]
The personal computer system 50 also includes a keyboard 68 that is connected to the internal bus 60 of the computer 52 by an interface adapter 72. The keyboard 68 includes a mechanism for user input of data and instructions to the computer 52. The computer 52 also includes a display 70 such as a CRT or liquid crystal display monitor connected to the internal bus 60 by a display adapter 74. The present invention is typically implemented as part of one printer driver of several components of personal computer operating system software stored in the disk drive 76 of the computer 52.
[0042]
The computer 52 normally performs reading and operation in response to a print command input by the user from the user interface of the application program in response to a printer driver command. The application program responds to the print command using a system call that requests an image to be printed. The printer driver responds with a series of operations necessary to convert the image content into low-level printer instructions. The low-level printer instructions cause the printer 54 to properly place the ink dots on the print medium that represents the image as accurately as possible.
[0043]
For simplicity of explanation, the printer 54 is an ink jet printer capable of outputting binary (ink or non-ink) of four color components (cyan, magenta, yellow and black) at each pixel position. Assume that there is. However, the method of the present invention is equally applicable to other types of printers and display devices such as color laser printers and monitors where the resolution of the visible image is inferior to that of the original image. For example, the method is applicable to printers that can change the dot size or apply one or more shades of ink to a color component.
[0044]
An image is usually broken down into multiple color levels. The color level is a gradation image expressing the luminance of the color component at each pixel position in the image. A color image is constructed by superimposing a color level on each of the three or more color components of the image. The input luminance value of the pixel is generally expressed by 8 bits or 12 bits depending on the high resolution (256 level or 4096 level, respectively) of the pixel luminance. The printer driver performs a tone generation operation to simulate the effect of the high resolution input values of the pixels in the visible image reproduced at a low resolution, such as a normal binary resolution that can be realized by a printer or other display device. Apply.
[0045]
Gradation generation is a quantization process that converts a pixel's high resolution input luminance value into a low resolution output value by comparing the pixel's input luminance value with a threshold luminance value. The binary resolution printer 54 operates to deposit ink at the pixel position when the input value exceeds the threshold value, but does not deposit ink when the input value is smaller than the threshold value. In the error diffusion gradation generation, the input luminance of the pixel increased due to the error accumulated from various pixels, the luminance value at the time of display (“0” in the case of binary output, or the maximum value of the pixel) The difference (error) is distributed to non-printed pixels adjacent to the pixel. Since the error is positive or negative, increasing the input brightness of a pixel with accumulated error increases or decreases the likelihood that ink will adhere to the pixel.
[0046]
While error diffusion tone generation generally yields good results, the printed output may include artificial tone generation. For example, light areas include dots or “worm” patterned lines. Error diffusion, on the other hand, produces a checkerboard effect that visually hinders a smooth transition between portions of the image and the distribution of errors to later printed pixels that causes dark areas in the image. Furthermore, dots of different color inks can be deposited at the same position. A magenta dot and a cyan dot at the same location produce a near black dot that can give an image in which the particles appear rough. Yellow ink usually does not cause such problems, and black ink used in a printer for four color components is generally controlled by a different mechanism than other color inks.
[0047]
As a result of the non-uniform distribution of the color ink dots in the tone portion of the image, halftone artifacts are obtained. Prior attempts to improve dot distribution non-uniformity are attempted on the portion of the original image surrounding pixels to determine how ink dots should be distributed in the display image. Such a method is intensively executed by computation by a computer. The inventor of the present application can adjust the gradation generation threshold based on an error accumulated in a pixel adjacent to the pixel so as to increase / decrease the ink adhesion at the pixel, thereby reducing the scale with a small number of operations. A conclusion was reached that tonal defects could be reduced.
[0048]
FIG. 4 is a flowchart showing the procedure of the error diffusion tone generation method of the present invention. In this error diffusion tone generation method, the brightness of the halftone level of the image or halftone image in the spatial coordinates corresponding to the pixel to be printed (hereinafter referred to as “target” pixel) is converted into an individual value. The First, starting from step 120, when the target pixel to be processed is white or black (step 122), an appropriate signal is transmitted to the printer 54 by another process without executing the gradation generation method. In step 120, the next pixel is selected. If the target pixel is not black or white, the luminance of the input pixel 92 is input to the gradation encoder 90 as shown in FIG.
[0049]
If it is determined in step 122 that the pixel is not white or black, the adder 94 detects an error (e _0,0 ) For input luminance (I _P ) To increase brightness (I _A ) Is obtained (step 126). Error of target pixel (e _0,0 ) Is obtained from an error buffer 96 where the error generated by printing the previous pixel is accumulated against the number of pixels.
[0050]
For convenience, the error of the target print pixel is the column “0” (e _0,0 ) In the position designated by the index position “0”. A typical tone encoder 90 uses an error filter 98 with filter weights first proposed by Floyd and Steinberg in “ADAPTIVE ALGORITHM FOR SPATIAL GRAY SCALE, Sid. Int. Sym. Digest of Tech. Papers, 1975”. ing. As shown, the error filter 98 distributes the error generated by printing the target pixel 100 to the non-printing pixel immediately to the right of the target pixel 100 and the three adjacent pixels in the next pixel column. To do.
[0051]
A typical error buffer 96 depends on the target pixel 100 (in column “0”) 106 and the next column (in column “1”) 108 and a number of other pixels (index positions relative to the index position of the target pixel). Cumulative error (e _0,0 ) 104. Other error diffusion algorithms require an error buffer that is suitable to distribute errors across more or less spaced columns of pixels in the target pixel column and store the accumulated error for the affected pixels. To do. The error for each color level is stored separately in error buffer 96, or stored in a single buffer as interlaced data. Furthermore, since the threshold selection unit 112 determines the threshold by the sign of the accumulated error, the size of the error buffer can be reduced by storing only the sign bit for the previously printed pixel. it can.
[0052]
In the gradation encoder 90, the increased luminance (I _A ) Is compared to one or more threshold values in threshold unit 110. In order to improve edge sharpness and fine detail in the visible pixels, the initial threshold unit 109 increases brightness (I _A ) Is compared with an initial threshold value (threshold value “0”) (step 128). The present invention improves the non-uniformity of the ink distribution in order to reduce the possibility of an increase in gradation defects. However, a more uniform distribution of ink impairs edge sharpness and fine detail in the printed image. For this reason, the initial threshold value is higher than other gradation generation threshold values used in the present method (for example, T ₀ = 224) Although set, the adhering ink at the pixels with high increased brightness will increase the sharpness of the image revealed in the appropriate part and reduce the processing time. Increased luminance (I _A ) Exceeds the initial threshold value, a binary signal (B = 1) is output (step 130), and the printer 54 is instructed to reveal or attach ink at the pixel (step 132). Then, the pixel is assigned to a maximum display luminance or a luminance value (for example, B × 255) at the time of display (step 155), and error dispersion is performed (step 156).
[0053]
In step 128, the increased luminance (I _A ) Does not exceed the initial threshold value, the increased luminance (I _A ) Is compared to the threshold selected by the threshold selection unit 112. In the threshold selection unit 112, the accumulated error for the target pixel and its neighboring pixels is used to select the optimal threshold luminance for comparison. The inventors of the present invention have found that gradation defects are reduced by improving the non-uniformity of the ink dot distribution, and the cumulative error indicates the density of the ink dots in the vicinity of the target print pixel. As shown in FIG. 1, when a pixel 20 is printed, that pixel 20 is assigned a “on display” luminance value equal to the maximum luminance result for the exposed pixel, and the binary value is output to the printer. . When ink is deposited, the “in-progress” luminance value of the pixel is maximized, and a negative error (δ) is distributed to adjacent pixels of the pixel. Therefore, a negative error indicates the presence of ink dots in the vicinity of the target pixel. The inventor of the present invention has concluded that the error for neighboring pixels can be used to select an appropriate tone generation threshold for uniformly spacing ink dots.
[0054]
In the threshold selection unit 112, the cumulative error (e _0,0 ) Is negative (step 134). Where cumulative error (e _0,0 ) Is not negative, the accumulated error (e) of two pixels immediately adjacent to the target pixel in the target pixel column 106 is also displayed. _{0, -1} ) And (e _0,1 ) Is negative (step 136). If it is determined in steps 134 and 136 that even one of the errors of each pixel is negative, the first threshold value (Ta) is selected (step 138), and the threshold value is input to the threshold unit 110. input.
[0055]
If it is determined in steps 134 and 136 that each accumulated error for the target pixel and the adjacent pixel is positive, the accumulated error (e _{0, -2} ) And (e _0,2 ) Is also negative (step 140). If the accumulated error for these pixels is negative, the second threshold (Tb) is selected (step 142) and the threshold is input to the threshold unit 110.
[0056]
If it is determined in step 142 that the accumulated error of the pixels further away from the target pixel is positive, the accumulated error (e _{0, -3} ) And (e _0,3 ) Is also determined as to whether it is negative (step 144). If the accumulated error for these pixels is negative, a third threshold (Tc) is selected (step 146) and the threshold is input to the threshold unit 110. If it is determined in step 144 that the accumulated error of the pixels further away from the target pixel is positive, (Td) is selected as the default threshold value (step 148), and the threshold value is set in the threshold unit 110. input.
[0057]
Each of the threshold values (Ta) to (Td) is a value that decreases sequentially, and is a value obtained in advance through an experiment. For example, in the conventional error diffusion method, the threshold value for the luminance of an 8-bit resolution pixel is usually approximately 127 (255/2). In order to more uniformly distribute the ink in the gradation portion of the image, a plurality of threshold values Ta = 200, Tb = 180, Tc = 160, Td = 120 were found, and good results were obtained by using these. .
[0058]
A negative cumulative error for the target pixel or the nearest neighboring pixel indicates that there is an ink dot near it. A higher threshold also reduces the likelihood that the target pixel will accept ink and push the ink dots away in the printed image. A negative cumulative error for a pixel that is further away from the target pixel indicates that there is an ink dot in the vicinity, but the visual effect is reduced because the dot is further away from the target pixel. The lower threshold adjusts the reduced visual effect of the more spaced ink dots. It has been found that a default threshold value (Td) slightly smaller than the conventional error diffusion threshold value is effective in balancing the spread of the dots generated by the method of the present embodiment.
[0059]
To reduce the likelihood of cyan and magenta dot deposition at the same location, negative errors in one color level can be taken into account when printing pixels at other color levels. If the error is stored in the error buffer 96 as a combined value, when selecting a tone threshold, a negative error for any component color is a negative error for the other component color. Is considered.
[0060]
In the threshold unit 110, the increased luminance (I _A ) Is compared with the selection threshold in the selection threshold unit 111 (step 150). Here, if the increased luminance exceeds the selection threshold value, a pixel reveal signal 114 (for example, binary “1” for a binary printer) as a binary output is output from the gradation encoder 90 ( Step 130) reveals the pixel (Step 132). On the other hand, if the increased luminance does not exceed the selection threshold, a pixel non-display signal (binary “0”) is output from the gradation encoder 90 as a binary output (step 154).
[0061]
The pixel reveal signal is also input to the adder 116. This adder 116 increases the luminance (I) for the target pixel assigned to the “as-displayed” luminance value in step 155 _A In order to obtain the “displayed” pixel luminance subtracted from (), the sum of the errors generated by the target pixel is determined by multiplying the maximum value of the pixel by the binary display signal. Then, the sum of errors is input to the error filter 98, and the sum is distributed to adjacent pixels according to the assignment by the filter weight (step 156). Thereafter, it is determined whether or not the target pixel is the last pixel in the image (step 158). If the target pixel is not the last pixel, the process returns to step 120, and the above-described series of processing is performed for the next pixel. . On the other hand, if the target pixel is the last pixel, the gradation generation process is finished (step 160).
[0062]
The gradation generation method of the present embodiment can reduce gradation defects and maintain edge sharpness and image details by controlling the distribution of ink dots that create a gradation image. In the method of the present embodiment, the threshold value is selected and used, and the accumulated error of the pixel adjacent to the pixel to be printed is used to control the ink dot distribution. Can also be processed at high speed.
[0063]
In the above description of the embodiment, specific details have been described in order to understand the present invention, but the present invention is not limited thereto. The terms and expressions used in the present embodiment are used only in the embodiment and do not limit the present invention.
[0064]
【The invention's effect】
As described above, the error diffusion tone generation method of the present invention according to the present invention is an error diffusion tone generation method for selecting a luminance threshold value for image tone generation in order to solve the above-described problem. Selecting the first brightness threshold when at least one error of the target pixel and one adjacent pixel adjacent to the target pixel exceeds a first error threshold; and at a position away from the adjacent pixel Selecting a second luminance threshold when an error of a neighboring pixel adjacent to the target pixel exceeds a second error threshold, and not selecting the first luminance threshold; and the first and second Selecting a third luminance threshold when no luminance threshold is selected.
[0065]
Thereby, gradation defects can be reduced by controlling the distribution of ink dots that generate gradation. In addition, the threshold value is selected and used, and the accumulated error of the pixels adjacent to the target pixel is used to control the distribution of the ink dots, so that the processing can be performed at high speed even by the conventional computer calculation.
[Brief description of the drawings]
FIG. 1 is a diagram showing error diffusion in a conventional error diffusion tone generation method.
FIG. 2 is a diagram illustrating an effect of an error diffused on a pixel in a halftone image by another conventional error diffusion tone generation method.
FIG. 3 is a block diagram showing a configuration of a computer system showing an embodiment of the present invention.
FIG. 4 is a flowchart illustrating a gradation generation processing procedure according to a gradation generation method according to the embodiment of this invention;
FIG. 5 is a block diagram showing a configuration of a gradation encoder used in the computer system.
[Explanation of symbols]
104 Target pixel
110 threshold unit (selection threshold unit)
112 Threshold selection unit
90 gradation encoder (gradation generation encoder)
96 Error buffer
98 Error filter

Claims (10)

In an error diffusion gradation generation method for selecting a luminance threshold value for image gradation generation,
(A) a step of previously distributing an error generated by printing the target pixel to a plurality of pixels adjacent to the target pixel and accumulating the error buffer for each pixel;
(B) selecting the first luminance threshold when at least one error is negative among the target pixel and one adjacent pixel adjacent thereto;
(C) selecting a second luminance threshold when an error of a neighboring pixel adjacent to the target pixel at a position away from the neighboring pixel is negative, and not selecting the first luminance threshold; ,
(D) when not selecting the first and second luminance threshold, it looks including the step of selecting the third luminance threshold,
The error diffusion characterized in that the brightness of the first brightness threshold is greater than the brightness of the second brightness threshold and the brightness of the second brightness threshold is greater than the brightness of the third brightness threshold. Gradation generation method.   The error diffusion gradation generation method according to claim 1, wherein at least one of the errors of the target pixel, the adjacent pixel, and the separated adjacent pixel includes a component color error of the target pixel. (A) a step of previously distributing an error generated by printing the target pixel to a plurality of pixels adjacent to the target pixel and accumulating the error buffer for each pixel;
(B) and increasing the Brightness of the target pixel with the newly entered object pixel error,
(C) when at least one of the target pixel error and an adjacent pixel error of an adjacent pixel adjacent to the target pixel is negative, selecting a first luminance threshold;
(D) The second luminance threshold value is selected when the separation proximity pixel error of the separation proximity pixel adjacent to the target pixel at a position away from the adjacent pixel is negative, and the first error threshold value is not selected. Steps,
(E) the steps of the closely spaced pixel error selects the third luminance threshold time is negative, do not select the first and second luminance threshold value,
(F) selecting a fourth luminance threshold when one of the first, second and third luminance thresholds is not selected;
(G) when the increased brightness of the target pixel exceeds the selected first, second, third or fourth brightness threshold, reveal the target pixel with a first reveal brightness, otherwise Revealing the target pixel with a second revealing brightness;
(H) viewing including the step of assigning at least one of said pixels the first or error between the second manifestation luminance with increased luminance of the target pixel adjacent to the target pixel,
The brightness of the first brightness threshold is greater than the brightness of the second brightness threshold, the brightness of the second brightness threshold is greater than the brightness of the third brightness threshold, and the third brightness threshold. A method for revealing an error diffusion gradation image, wherein a luminance of a value is larger than a luminance of the fourth luminance threshold value . The error diffusion gradation image display method according to claim 3 , wherein the first display brightness includes a maximum brightness and the second display brightness includes a minimum brightness. The error diffusion gradation image display method according to claim 3 , wherein the luminance of the target pixel includes luminance of a color component of the target pixel. The increased brightness of the target pixel, when exceeding the Brightness of the first luminance threshold, claim 3, characterized in that assigning the maximum manifestation luminance with respect to the target pixel further includes performing error variance The error diffusion gradation image display method described in 1. 4. The error diffusion gradation image display method according to claim 3 , wherein at least one of the target pixel, the adjacent pixel error, and the separated adjacent pixel error includes a component color error. 8. The error diffusion gradation image display method according to claim 7 , wherein the component color error includes an error of a component color other than the component color of the target pixel. An error filter that distributes an error generated by printing the target pixel to a plurality of pixels adjacent to the target pixel;
An error buffer for accumulating the error generated by the error filter for the target pixel;
An adder that adds the error detected in the error buffer to the target pixel that is not white or black, and obtains an increased brightness by adding the error detected by the error buffer to the target pixel.
A selection threshold unit that compares the increased luminance of the target pixel with a selection threshold luminance;
One of a plurality of threshold luminances is selected as the selection threshold luminance for the selection threshold unit according to an error of at least one of the target pixel and an adjacent pixel adjacent to the target pixel A threshold selection unit that selects the first luminance threshold when at least one error is negative among the target pixel and one adjacent pixel adjacent to the target pixel, and is separated from the adjacent pixel Selecting a second luminance threshold value that is smaller than the first luminance threshold value when an error of a neighboring pixel adjacent to the target pixel at a position is negative, and not selecting the first luminance threshold value; If you do not select the first and second luminance threshold, gradation, characterized in that it contains a threshold selection unit that selects the second luminance threshold value smaller than the third luminance threshold Formed encoder. Tone generation encoder according to claim 9, further comprising the input luminance of the target pixel with respect to a large initial threshold luminance than said selected threshold intensity.

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