JP4229018B2 - Image processing apparatus and image processing program using diffusion gamma table - Google Patents

Description

  The present invention relates to an image processing apparatus and an image processing program that use a diffusion gamma table, and more particularly, to an image processing apparatus and an image processing program that correct a diffusion gamma table during calibration.

  An image generation apparatus such as a printer or a copier has an image processing apparatus that converts gradation input data into image reproduction output data that specifies a dot size in a pixel. The pixel gradation input data output from the host computer or scanner is, for example, 8-bit data representing the gradation value in the pixel. In order to express a halftone with such a gradation, a multi-value dither method is used in which dots of a predetermined size are formed in a pixel by a print engine and a desired gradation is expressed in a plane. The half-tone processing unit based on the multi-value dither method converts 8-bit 256 gradation input data into 8-bit 256 gradation image reproduction output data that specifies the dot size formed in the pixel, and performs pulse width modulation. The unit generates a drive pulse signal from the image reproduction output data, and the print engine forms dots in the pixels based on the drive pulse signal. According to the electrophotographic technique of a print engine, a laser is driven by a driving pulse, and an electrostatic latent image is formed on a photosensitive drum by the driven laser beam. Then, a developer of a desired color is attached to the formed electrostatic latent image to form dots.

  The image processing apparatus as described above is disclosed in, for example, Patent Documents 1 and 2.

In the conventional halftone processing, for example, 8-bit gradation input data is converted into output data of 8 bits or less, and the dot size in the pixel is determined by the output data. Therefore, the dot size in the pixel reproduced as an image has the number of gradations corresponding to the number of bits of output data, and it is desirable to increase the number of bits of output data in order to increase the number of gradations.
JP 2000-85187 A JP 2000-228728 A

  In recent years, due to the demand for cost reduction, the configuration of a pulse width modulation unit that generates a drive pulse signal from the output data by reducing the number of bits of output data generated by the halftone process, for example, 4 bits, is reduced. It is desirable to simplify. This is because the pulse width modulation unit has a simpler circuit configuration and a lower cost as the number of input data bits is smaller.

  However, if the number of bits of the output data is reduced, the number of gradations of the dot size that can be expressed by the output data is reduced, and high gradation characteristics and accurate density characteristics can be given to the dots in the reproduced pixels. Disappear. For example, if the output data is 8 bits, 256 gradations can be expressed, but if the output data is 4 bits, only 16 gradations can be expressed, the gradation characteristics are lowered, and the desired density is also obtained in the density characteristics. Cannot be reproduced.

  Accordingly, an object of the present invention is to provide high gradation characteristics and accurate density characteristics even when the number of gradations of output data for image reproduction generated by halftone processing is small, and further to calibrate image processing characteristics. It is an object to provide an image processing apparatus and an image processing program that enable the above.

  In order to achieve the above object, according to a first aspect of the present invention, in an image processing apparatus that generates image reproduction output data from gradation input data, the gradation input data corresponds to a dot size of a pixel. A plurality of halftone processing units for converting to the image reproduction output data, and a plurality of gamma tables having image reproduction output data corresponding to the gradation input data, corresponding to the pixel positions of the gradation input data, And a lookup table referenced by the halftone processing unit. In the lookup table, a plurality of diffusion gamma tables distributed in a plane in the same index pixel group are allocated to the same index pixel group arranged on a predetermined displacement vector, The plurality of diffusion gamma tables have discrete image reproduction output data that differs at least in part corresponding to the gradation input data, and an average value of the image reproduction output data of the plurality of diffusion gamma tables is It almost coincides with the output gradation value of the virtual representative gamma table of the same index pixel group. Further, the image processing apparatus generates a corrected representative gamma table by correcting the output tone value of the representative gamma table generated from the plurality of diffusion gamma tables based on the corrected gamma table during the calibration process. Calibration means for generating a plurality of corrected diffusion gamma tables from the corrected representative gamma table.

  According to the modification of the first aspect, a representative gamma table is assigned to each of the same index pixel groups arranged on a predetermined displacement vector, and each representative gamma table has the same corresponding index. The plurality of diffusion gamma tables are distributed in a plane within the pixel group, and the plurality of diffusion gamma tables are in the vicinity of the output gradation value of the representative gamma table corresponding to the gradation input data. One of the discrete image reproduction output data is included.

  According to the first aspect and the modification thereof, even if the number of gradations of the image reproduction output data generated by the halftone processing unit is reduced, the image quality is improved by reproducing the high gradation and the high-precision output density. Further, at the time of calibration, the diffusion gamma table can be corrected without impairing the characteristics of high gradation and high accuracy output density.

  In order to achieve the above object, according to a second aspect of the present invention, an image processing program stores a gamma table having image reproduction output data corresponding to gradation input data, a pixel of gradation input data. With reference to a plurality of look-up tables corresponding to positions, the computer is caused to execute a halftone processing procedure for converting pixel gradation input data into image reproduction output data corresponding to the pixel dot size. The look-up table is the same as that of the first aspect. Further, the image merchant processing program generates a corrected representative gamma table by correcting the output tone value of the representative gamma table generated from the plurality of diffusion gamma tables based on the corrected gamma table, and generates the corrected representative gamma table from the corrected representative gamma table. Causes a computer to execute a calibration procedure for generating a plurality of corrected diffusion gamma tables.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. However, the technical scope of the present invention is not limited to these embodiments, but extends to the matters described in the claims and equivalents thereof.

[Diffusion Gamma Table]
FIG. 1 is a configuration diagram of an image processing apparatus according to the present embodiment. The image processing apparatus 100 includes a halftone processing unit 110 that converts gradation input data IN of a pixel into image reproduction output data OUT corresponding to the dot size of the pixel, and image reproduction output data OUT corresponding to the gradation input data IN. And a lookup table 122 having a plurality of diffusion gamma tables 124 corresponding to the pixel positions (X, Y) of the gradation input data. The diffusion gamma table will be described in detail later. The lookup table 122 is composed of, for example, a high-speed static RAM, and includes an index table 120 having an index Index that specifies a gamma table to be referred to corresponding to the pixel position (X, Y), and a plurality of diffusion gamma tables 124. And have. The halftone processing unit 110 obtains the pixel position (x, y) of the index table 120 to be referred to according to the pixel position (X, Y), and refers to the index table 120 based on the pixel position (x, y). To do. Then, the image reproduction output data OUT is acquired with reference to the diffusion gamma table 124 corresponding to the index number Index of the referenced index table 120. Then, the halftone processing unit 110 supplies the acquired image reproduction output data OUT to the pulse width modulation unit 130, and the pulse width modulation unit 130 generates a drive pulse width signal PW and supplies it to a print engine (not shown). In the print engine, the photosensitive drum is irradiated with a beam based on the drive pulse width signal PW to form an electrostatic latent image on the photosensitive drum. Then, toner is attached to the electrostatic latent image, developed into dots, and transferred onto a recording medium such as recording paper. Accordingly, the image reproduction output data OUT is data for reproducing a print image relating to the dot size of the pixel. As will be described later, this image reproduction output data OUT includes information L / R indicating whether the position of a dot to be generated in a pixel is right-justified or left-justified. In the following embodiment, whether the position is right-justified or left-justified Information L / R is output from the index table 120.

  The image processing apparatus 100 further includes a processor 128 that manages a lookup table and a ROM 126 that stores a normal diffusion gamma table that is optimally designed in advance. The processor 128 downloads the normal diffusion version table in the ROM to the SRAM via the CPU bus C-BUS when the power is turned on or otherwise requested, and constructs the lookup table 122. Furthermore, the processor 128 executes a calibration process in response to a request from the print engine when the photosensitive drum is replaced or when the accumulated usage time exceeds a predetermined time. In the calibration process, a normal diffusion gamma table is used to form a calibration pattern on the photosensitive drum, the density of the pattern is detected by a density sensor, deviation from the density characteristics at the time of design is detected, and normal diffusion is performed. The gamma table is corrected, and the corrected diffusion gamma table is stored in the lookup table 122. By the calibration process, it is possible to prevent the deterioration of the image quality in conformity with the secular change of the print engine and the characteristic change of the photosensitive drum.

  FIG. 2 is a diagram illustrating a relationship between a general index table and a gamma table. The index table 120 shown here stores gamma table numbers 1 to 6 to be referred to in a 12 × 12 pixel matrix. The index table 120 is a repetition of a 4 × 6 minimum matrix unit surrounded by a thick frame, and within this minimum matrix unit, there are two gamma tables 1, four gamma tables 2, two gamma tables 3, There are 8 gamma tables 4, 4 gamma tables 5, and 4 gamma tables 6. By repeating this minimum matrix unit, the gamma table 1 is repeatedly arranged at the positions of the displacement vectors (+2, +3), (+2, −3), (−2, +3), and (−2, −3). The However, the upper right position of the index table 120 in FIG. 2 is the origin of coordinates (X, Y). The gamma table 2 is adjacent in the X direction, and is repeatedly arranged at the positions of the displacement vectors (+2, +3), (+2, −3), (−2, +3), and (−2, −3). Other gamma tables are similarly arranged on unique displacement vectors.

  Thus, the same index number is given to the pixel groups arranged on the common displacement vector, and the gamma table corresponding to the index number is referred to. Therefore, the pixel group arranged on the same displacement vector in this way is referred to as the same index pixel group.

  FIG. 2 shows six types of gamma tables 124. The gamma table stores an output pulse width value that is an image reproduction output corresponding to an input gradation value. In the figure, the gamma table 1 is a minimum input value in a low input gradation value region. It has a growing output pulse width value. As shown in the figure, the gamma tables 2, 3, and 4 have output pulse width values that grow from the minimum value to the maximum value in the intermediate input gradation value region, and the gamma tables 5 and 6 are relatively high. It has an output pulse width value that grows from the minimum value to the maximum value in the input gradation value region.

  FIG. 3 is a diagram illustrating the growth of the dot of the pixel when the input gradation value is increased in the example of FIG. According to the gamma table and the index table 120 as described above, in the region where the input gradation value is low (input gradation values 10, 40, 90) as the input gradation value increases, the pixel of index 1 is assigned. A dot is formed (input gradation value 10), a dot is subsequently formed at the pixel at index 2 (input gradation value 40), and a dot is formed at the pixel at index 3 in parallel with the pixel at index 2 (Input gradation value 90). That is, in an area where the input gradation value is low, dots grow sequentially on the pixels of indexes 1, 2, and 3 in the index table 20. Then, in a region where the input tone value is high (input tone values 150, 220, 250), a dot grows on the pixel with index 4 (input tone value 150), and subsequently, the pixels with index 5, 6 Dots grow (input gradation values 220 and 250).

  Therefore, a screen line is formed on the line connecting the pixels with the index 1 as indicated by the growth dots having the input gradation value of 90 or 150. In other words, the displacement vector at index 1 identifies the screen line.

  The gamma table 124 of FIG. 2 has an 8-bit output pulse width for an 8-bit input gradation value. Therefore, it has an output of 256 gradations (0 to 255) for an input of 256 gradations. Therefore, the output gradation value of the gamma table can be changed as necessary in accordance with the change in the gradation of the input gradation value. However, when the output pulse width data is 6 bits, the output can take only 64 gradations (0 to 63), and the output gradation value of the gamma table is set in accordance with the change in gradation of the input gradation value. Sometimes it cannot be changed. In addition, when the output pulse width data is 4 bits, the output is only 16 gradations (0 to 15), and the output gradation value cannot be changed corresponding to the gradation change of the input gradation value. Will further increase. Furthermore, when the number of bits of output data decreases, the gradation becomes more discrete, and the gradation resolution of the dot size that can be reproduced decreases.

  FIG. 4 is a diagram illustrating the change amount of the output tone value with respect to the change of the input tone value of the gamma table. For the gamma table shown in FIG. 2, when the output data is 8 bits, (B) the output data is 6 bits, (C) the output data is 4 bits, the input gradation value is the first floor. The difference (change amount) of the total gradation values of the output data of the six gamma tables when the tone changes is shown. 4A, 4B, and 4C, the upper row shows the gamma table, and the lower row shows the difference between the total output gradation values.

  In the case of 8-bit output data in FIG. 4A, the gamma table 124 is the same as the gamma table 124 in FIG. In this case, the difference in the output pulse width when the input gradation value changes by one gradation is all other than 0 gradation, and the total output pulse width is also 255 for the 255 gradation changes of the input gradation value. The gradation changes over time. Therefore, an output pulse width value that changes faithfully with respect to a change in input gradation value can be generated as output data.

In the case of 6 (M = 6) -bit output data in FIG. 4B, the minimum output width of each of the gamma tables 1 to 6 is 255 / (2 ^M −1) = 4. According to 124, it can be seen that the interval between the discrete values of the output pulse width is rough compared to the case of FIG. Therefore, with respect to 255 gradation changes in the input gradation value, the total output pulse width becomes zero gradation change over 44 times, and the gradation change only 211 times.

Further, in the case of output data of 4 (M = 4) bits in FIG. 4C, the minimum output width of each gamma table 1 to 6 is 255 / (2 ^M −1) = 17. According to the gamma table 124, the interval between the discrete values of the output pulse width is further roughened compared to the cases of FIGS. 4 (A) and 4 (B). For this reason, the total output pulse width has changed only 82 times with respect to 255 gradation changes of the input gradation value, and the gradation change becomes zero over 173 times. Moreover, the difference between the output pulse widths is mostly 0 or 17, and the difference between the output pulse widths may be 34 in the high input gradation value region.

  That is, as a result of reducing the number of output data bits and decreasing the number of output data gradations, the number of times that the difference in output pulse width becomes zero increases, and even if it changes, the difference amount increases. This means that the number of gradations of the output pulse width is greatly reduced and the difference is greatly increased with respect to the number of gradations of 255 of the input gradation value, and high gradation characteristics and accurate density characteristics are obtained. It means being damaged.

  FIG. 5 is an enlarged view of the gamma table when the output gradation is 4 bits. In this figure, the solid line represents the input / output characteristics of the gamma table G, and the vertical axis represents the gradation value when the output pulse width is 8 bits (256 gradations). As described above, since the minimum width of the 4-bit output gradation is 17, the possible discrete points are 0, 17, 34, 51, 68, 85, 102, 8 bits (256 gradations). 119, 136, 153, 170, 187, 204, 221, 238, 255. The vertical axis in FIG. 5 shows some of the output discrete points 153, 170, and 187. If the gamma table G indicated by the solid line is 8 bits, the gradation values of the points indicated by the squares W, X, and Y are output as output data, but if it is 4 bits, 153, 170, and 187 discrete values are output. Since only points can be taken, the gradation values indicated by black circles are output as output data. For example, in the case of 8-bit output, the square X has an output gradation value 161, whereas in the case of 4 bits, the output gradation value 153 of a black circle in the vicinity thereof (1001 = 9 in 4-bit display). That is, the 4-bit output data becomes more discrete and cannot be matched with the gradation value of the 8-bit output data. Further, on the other hand, the output gradation values 145 and 161 of the squares W and X in the 8-bit output data become the same output gradation value 153 in the 4-bit output data, and the difference between the output gradation values is zero. However, on the other hand, the output gradation values 161 and 167 of the squares X and Y in the 8-bit output data become 153 and 170 in the 4-bit output data, and the difference between the output gradation values is 17. Become. Therefore, with a 4-bit output tone value, the difference may be zero with respect to the change in the input tone value, or different from the expected difference in the output tone value with respect to the change in the input tone value. It can be a big difference.

  FIG. 6 is a diagram for explaining the principle of the diffusion gamma table in the present embodiment. According to the principle of the diffusion gamma table, for a square X in the case of 8-bit output, 4-bit output discrete points X1 (output gradation value 153), X2 in the vicinity of the output gradation value 161 of the square X (Output tone value 170) is assigned to a plurality of diffusion gamma tables, the number of diffusion gamma tables having discrete points X1 and X2 is optimized, and their average output tone value is closest to 161 of square X Like that. For example, if the number of discrete points X1 is 5 and the number of discrete points X2 is 4, the average value of the output gradation values of these 9 discrete points is (153 × 5 + 170 × 4) /9=160.55, which is a square. It becomes a value close to the output gradation value 161 of X. In this way, even when only an output gradation value having a large discrete width can be taken as in the case of 4-bit output, by assigning different discrete values X1 and X2 to a plurality of pixels, output of the square X of the gamma table G A gradation value very close to the gradation value can be expressed. Here, the gamma table G is referred to as a representative gamma table, and a table having different discrete values is referred to as a diffusion gamma table.

  FIG. 7 is a diagram illustrating an example in which a plurality of diffusion gamma tables G1, G2, and G3 are assigned to the representative gamma table G. In this example, in order to simplify the display, three diffusion gamma tables G1, G2, and G3 are assigned to the representative gamma table G. The diffusion gamma table G1 takes white round discrete points (for example, X2) among 4-bit discrete points (for example, X1 and X2) located in the vicinity of the square (for example, X) of the representative gamma table G. It has an output value that changes up and down in the vicinity. Similarly, the diffusion gamma table G2 takes triangular discrete points among the neighboring 4-bit discrete points, and the diffusion gamma table G3 takes diamond-shaped discrete points among the neighboring 4-bit discrete points. As an example, the square X is diffused or dispersed into two discrete points X1 and one discrete point X2. In this case, the average value is (153 × 2 + 170 × 1) /3=158.7. Thus, by increasing the number of diffusion gamma tables to be diffused or dispersed, the average value of the output tone values can be made closer to the output tone value of the representative gamma table G.

  As shown in the index table of FIG. 2, since there are a plurality of pixels having the same representative gamma table G in a predetermined area of the pixel matrix, the representative gamma table G of the plurality of pixels is changed to a different diffusion gamma table. By assigning to, it is possible to express the output tone value of the representative ideal gamma table in a plane within the predetermined area. The arrangement of the plurality of diffusion gamma tables may be distributed, but is preferably distributed irregularly or randomly rather than regularly. Also, the discrete value taken by each diffusion gamma table may be any of the 4-bit discrete values in the vicinity of the output tone value of the representative gamma table G, but preferably any of the nearby 4-bit discrete values. Try randomly.

FIG. 17 is a flowchart for generating a diffusion gamma table from the representative gamma table in the present embodiment. The methods described with reference to FIGS. 6 and 7 are summarized as follows. For a representative gamma table G, an output pulse width X corresponding to the input gradation value IN is acquired (S12). In the example of FIG. 6, X = 161. Then, the adjacent discrete points xl = 170 and xs = 153 with respect to the output pulse width X = 161 as the target value are distributed to the diffusion number N = 3, and the distribution number L = 1 and NL = 2 of xl and xs. Ask for. The distribution number L of xl is, for example,
L = INT [{(X−xs) / (xl−xs)} × N + 0.5]
It can ask for. Here, INT [] indicates a truncation process. Applying the above example,
L = INT [{(161−153) / (170−153)} × 3 + 0.5] = 1
It becomes. For N = 3 diffusion gamma tables, a discrete value xl = 170 is randomly given to L = 1 and a discrete value xs = 153 is given randomly to NL = 2. The arrangement of N = 3 diffusion gamma tables is also determined randomly.

An average value x of L = 1 output value xl = 170 and NL = 2 output values xs = 153 of the diffused N = 3 diffusion gamma tables is:
x = {xl × L + xs × (N−L)} / N = 158.7
become. By increasing the diffusion number N, the accuracy of the average value x can be increased.

  FIG. 8 is a diagram showing the same index pixel group having a representative gamma table in the present embodiment. The index table 120 shown here is an example in which the index numbers 1 to 6 in the index table 120 in FIG. 2 are converted into new index numbers 0 to 11 as shown in the correspondence table 121A in FIG. The new index numbers of even numbers 0, 2, 4, 6, 8, and 10 left-justify the dots in the pixels, and the odd numbers 1, 3, 5, 7, 9, and 11 the pixels that right-justify the dots in the pixels. The even / odd information L / R of the index number is added to the image reproduction output data OUT to the pulse width modulation unit. Therefore, index numbers 0, 2, 3, 4, 6, 7, 8, 9, 10, and 11 are assigned in the upper left 4 × 6 matrix of the index table of FIG. 2 is the gamma table 1 of FIG. 2, the index numbers 2 and 3 are the gamma table 2, the index numbers 4 and 5 are the gamma table 3, and the following 6 and 7 are the gamma table 4. Reference numeral 9 refers to the gamma table 5, and reference numerals 10 and 11 refer to the gamma table 6.

  In FIG. 8, the same index pixel group with the index number 0 arranged at the position of the displacement vector (± 2, ± 3) is 2 in the upper left 4 × 6 small pixel matrix as shown in the figure. 24 are arranged in the 16 × 18 pixel matrix of FIG. Therefore, in this embodiment, a plurality (9 types) of diffusion gamma tables are assigned to the representative gamma table 0 of the same index pixel group having the index number 0. In addition, a plurality of diffusion gamma tables are distributed in a plane in the same index pixel group, and are preferably distributed at random.

  FIG. 9 is a diagram showing an index table for the diffusion gamma table according to the present embodiment. In the present embodiment, 80 identical index pixel groups with index 0 arranged in a wide pixel matrix (32 × 30 pixel matrix) as will be described later with reference to FIG. 12 are assigned to nine types of diffusion matrices. Only a part of the pixel matrix is shown in the index table 120A of FIG. The correspondence table 121B shown at the bottom in FIG. 9 shows the correspondence between the index numbers in FIGS. 8 are assigned index numbers 0, 12, 24, 36, 48, 60, 72, 84, and 96 of the diffusion gamma table. Moreover, the allocation method is a random arrangement in a plane. Similarly, nine or eight types of diffusion gamma tables are assigned to the same index pixel group having other index numbers in FIG. The index number of the assigned diffusion gamma table is shown in the correspondence table 121B.

  FIG. 10 is a diagram showing an example of output gradation values that can be taken by the diffusion gamma table shown in FIG. With respect to the output gradation value 161 of the square X shown in FIG. 6, the 4-bit discrete point X1 (output gradation value 153) and the discrete point X2 (output gradation value 170) are the diffusion gamma tables 0, 12, 24, 36, 48, 60, 72, 84, 96 are randomly assigned. Of the nine types of diffusion gamma tables associated with the representative gamma table 0 of 24 pixels shown here, the 13 diffusion gamma tables have 10 discrete points X1 (output gradation value 153). Discrete points X2 (output gradation value 170) are randomly assigned to the diffusion gamma table. The allocation ratio is roughly 13: 10 = 5: 4. In this way, the arrangement of the plurality of diffusion gamma tables assigned to the representative gamma table is randomized, and discrete points near the output tone value of the representative gamma table are also randomly assigned to the diffusion gamma table. A gradation value that is very close to the output gradation value 161 of the gamma table can be spatially expressed. Also for the other diffusion gamma tables shown in FIG. 9, 4-bit discrete points in the vicinity of the output gradation values of the corresponding representative gamma table are randomly assigned.

  FIG. 11 is a diagram illustrating an example of an 8-bit output representative gamma table index table and its representative gamma table. The index table 120 is the same as the index table 20 in FIG. 8, and the pixel size is expanded to 32 × 30. As described with reference to FIG. 8, the same index pixel group of index 0 (representative gamma table 0) in the index table 120 is arranged at the position of the same displacement vector (± 3, ± 4). In the pixel matrix 32 × 30, there are a total of 80 identical index pixels with index 0. The representative gamma table 0 has an 8-bit output pulse width as the input gradation increases, as shown by the black square in FIG. If the representative gamma table 0 is simply changed to a 4-bit output pulse width, only discrete values can be obtained as shown by white squares in FIG.

  FIG. 11 includes indexes of representative gamma tables 2, 3, 4, 6, 7, 8, 9, 10, and 11 in addition to index 0 of representative gamma table 0. These pixel groups having the same index are also arranged at positions on the same displacement vector.

  FIG. 12 is a diagram showing an index table and a gamma table for the diffusion gamma table according to the present embodiment. The index table 120A is the same as the index table 120A of FIG. 9, and the pixel size is enlarged to 32 × 30. In the figure, the pixel group of index 0 corresponding to the representative gamma table 0 is displayed in halftone. Nine types of diffusion gamma tables are assigned to 80 identical index pixel groups (index 0) in FIG. That is, the representative gamma table 0 is dispersed or diffused into nine diffusion gamma tables 0 and 12, eight diffusion gamma tables 124, and nine diffusion gamma tables 36, 48, 60, 72, 84, and 96. In the index table 120A, index numbers of a plurality of diffusion gamma tables are assigned to other representative gamma tables.

  In the diffusion gamma table 124 shown in FIG. 12, 4-bit output pulse widths of nine types of diffusion gamma tables are plotted. That is, for the representative gamma table G indicated by a broken line, 4-bit discrete values X1 and X2 adjacent to the output value are assigned as output values of the diffusion gamma table. That is, one of the discrete values X1 and X2 is assigned to the output values of nine types of diffusion gamma tables. Therefore, a total of nine types of diffusion gamma tables are assigned to the discrete values X1 and X2. This discrete value is a single or a plurality of 4-bit discrete values that are close to the output value of the representative gamma table G, and a plurality of diffusion values such that these discrete values approximate the output value of the representative gamma table on average. Assigned to gamma table.

  FIGS. 13, 14, and 15 are diagrams showing the arrangement and specific table values of the nine types of diffusion gamma tables in FIG. 13 shows the arrangement and table values in the index table of the diffusion gamma tables 0, 12, and 24, and FIG. 14 shows the arrangement and table values in the index table of the tables 36, 48, and 60 in FIG. Indicates the arrangement of the tables 72, 84, and 96 in the index table and the table values, respectively. The index numbers in the index table are the same as those in FIG. However, according to the index tables of FIGS. 13 to 15, the position and number of each diffusion gamma table that is randomly diffused can be confirmed. The table value of the diffusion gamma table has a 4-bit discrete value. In this example, one of the upper and lower discrete values close to the output value of the representative gamma table indicated by the broken line is assigned to each diffusion gamma table. . Therefore, the diffusion gamma table of FIGS. 13 to 15 can confirm whether each diffusion gamma table has an upper or lower discrete value in each gradation value from the input gradation value 0 to 25. That is, it is the same as the diffusion gamma table shown in FIG. 7, and the nine types of diffusion gamma tables have discrete values approximating the output values of the broken line representative gamma table as output values, and the allocation ratio of the discrete values. By controlling this, the average value of the output values of the diffusion gamma table becomes very close to the output value of the representative gamma table.

  From the above description, the lookup table in the present embodiment no longer has a representative gamma table, and has a plurality of diffusion gamma tables instead of the representative gamma table. In that sense, a virtual representative gamma table is assigned to the same index pixel group, and the virtual representative gamma table has a plurality of diffusion gamma tables. Then, the average of the output values of the plurality of diffusion gamma tables approximates the output value of the representative gamma table.

  FIG. 16 is a diagram illustrating the gamma tables of FIGS. 11 and 12 and the difference in output pulse width with respect to changes in input gradation values. It is a figure of the same kind as FIG. FIG. 16A shows the difference between the representative gamma tables 0, 2, 3, 4, 6, 7, 8, 9, 10, and 11 shown in FIG. Show. That is, as in FIG. 4A, the total output pulse value changes with respect to all 255 changes in the input gradation value.

  On the other hand, FIG. 16B shows the difference between the diffusion gamma table shown in FIG. 12 and the total output pulse value with respect to the change of the input gradation value. That is, as shown in the correspondence table 121B of FIG. 9, the representative gamma table 0 is diffused to the diffusion gamma tables 0 to 96, and the representative gamma tables 2 and 3 are diffused to the diffusion gamma tables 2 to 98 and 3 to 87. The representative gamma table 4 is diffused to the diffusion gamma tables 4 to 100, the representative gamma tables 6 and 7 are diffused to the diffusion gamma tables 6 to 102 and 7 to 103, and the representative gamma tables 8 and 9 are diffused gamma. The representative gamma tables 10 and 11 are diffused to the diffusion gamma tables 10 to 106 and 11 to 107, respectively.

  As shown in FIG. 16B, the average output value can be made equal to that of the representative gamma table even if the output value is a 4-bit discrete value by diffusing or dispersing in a plurality of diffusion gamma tables. . As a result, the total output pulse width difference changes in all 255 stages with respect to the change in the input gradation value, and the tendency of the difference is the same as in FIG. However, the scale of the vertical axis is 360 in FIG. 16B, which is nine times that of 40 in FIG.

  FIG. 16C shows a table converted into the average value of the nine types of diffusion gamma tables in FIG. 16B, and the difference in output pulse width with respect to the change in input gradation value in that case. It can be understood that the average value is almost the same as the representative gamma table of FIG. Further, the difference in the output pulse width is different from that in FIG. 16B only in the scale of the vertical axis, and is the same scale and the same difference as the vertical axis of FIG. What can be confirmed in FIG. 16C is that each representative gamma table is diffused or dispersed into a plurality of diffusion gamma tables. First, even if the number of output gradations is reduced and the number of output gradations is small, Very close output values can be reproduced (refer to the gamma table). Second, even if the number of output gradations is reduced and the number of output gradations is small, a high gradation characteristic of 255 can be obtained (refer to the output pulse width difference). ). Therefore, even if the halftone processing unit converts to image reproduction output data with a reduced bit, it can have high-accuracy density characteristics and high gradation characteristics equivalent to high-bit output data.

  In the above embodiment, when converting 8-bit input gradation data into 4-bit output pulse width data, the representative gamma table is diffused or dispersed into a plurality of diffusion gamma tables. That is, the output of the representative gamma table is 8 bits and 256 gradations, whereas the output of the diffusion gamma table is 4 bits and 16 gradations, which is smaller than that. Even in such a case, high gradation characteristics and high-precision density characteristics are realized by diffusing or dispersing in a plurality of diffusion gamma tables. The present embodiment is not limited to this. Even when the 8-bit input gradation data is converted into 8-bit output pulse width data using the diffusion gamma table, each representative gamma table is the same as that. The image quality can be improved by diffusing or dispersing the plurality of diffusion gamma tables having 8-bit output data. In this case, it is possible to express a high gradation output density that cannot be represented by the 8-bit representative gamma table. In other words, the 8-bit representative gamma table can express only 256 gradations, but by diffusing or dispersing in a plurality of diffusion gamma tables, gradation characteristics of 256 gradations or more can be given to the output data. However, it is necessary to replace one representative gamma table with a plurality of diffusion gamma tables, which increases the memory capacity of the gamma table. In that case, for example, in addition to a common representative gamma table, by providing a plurality of diffusion gamma tables having only a difference value with a small number of bits, an increase in the overall memory capacity can be suppressed. In addition, the halftone processing unit may randomly output a difference value to the output data of the representative gamma table and output it as output data of the diffusion gamma table.

  In the image processing configuration diagram of FIG. 1, the halftone processing unit 110 refers to the look-up table 122 and outputs image reproduction output data OUT and L / R. However, such conversion processing can also be performed by software processing using a microprocessor. In that case, the processor refers to the high-speed memory storing the lookup table 122 based on the input gradation data IN and the pixel position information (X, Y), and converts the input gradation data IN into the image reproduction output data OUT, L / R is generated.

[Calibration process]
As described above, by using the diffusion gamma table for the lookup table, it is possible to suppress degradation in image quality even if the output data has a small number of gradations. Further, higher image quality can be provided. As shown in FIG. 1, the diffusion gamma table optimized in the design is stored in the ROM 126. However, if the accumulated usage time of the print engine exceeds a predetermined time or the photosensitive drum is replaced, the print engine has characteristics different from those at the time of design, and calibration processing is necessary. However, it is not appropriate to directly correct the diffusion gamma table because the image quality after correction deteriorates.

  FIG. 18 is a flowchart of calibration processing in the present embodiment. FIG. 19 is a diagram illustrating a relationship between tables in the calibration process. In the present embodiment, in response to the calibration request (S30), the processor 128 shown in FIG. 1 corrects the normal diffusion gamma table in the ROM via the representative gamma table to generate a corrected diffusion gamma table. And stored in the lookup table 122. First, the processor 128 refers to the normal diffusion gamma table in the ROM and forms a predetermined calibration pattern on the photosensitive drum in the print engine (not shown) (S32). Then, the processor 128 causes the optical density sensor in the print engine to detect the density of the calibration pattern, and receives the detected density data from the print engine (S34). The processor 128 generates the correction gamma table 202 based on the detected density data (S38).

  FIG. 20 is a diagram illustrating an example of detected density data and a correction gamma table. The detected density data 200 has a sensor output value with respect to an input gradation value. The sensor output value is normalized to the density on the paper. In this example, the density characteristics are not ideal with respect to the input gradation, but the density rapidly increases at a low input gradation and reaches the maximum density before reaching the maximum input gradation. That is, the output density is higher than the ideal characteristic. Therefore, it is necessary to correct the design gamma table so as to reduce the output pulse width. The corrected gamma table C-GT in the figure is obtained by inverting the X axis and the Y axis of the detected density data 200. That is, in the corrected gamma table C-GT, the input gradation of the corrected gamma table in which the X axis indicates the input gradation of the corrected gamma table and the Y axis has the output pulse width to be given to the input gradation. Indicates the value (corrected gradation value). The correction method using the correction gamma table will be clarified in later explanation.

  FIG. 20 shows an enlarged view C-GT (A) of the correction gamma table. The enlarged view C-GT (A) of this correction gamma table shows the Y axis (correction gradation value) at 0 to 30 on the X axis. In this example, the X-axis is 24, and the Y-axis corrected gradation value (the input gradation value of the design gamma table to be referred to) is 1. Therefore, with respect to 1 to 23 on the X axis, the corrected gradation value on the Y axis is set to a real number (a number including a small number) obtained by interpolation calculation. For example, 0.04, 0.09, etc., that is, a value between discrete integer values. As a result, the correction gamma table has highly accurate correction characteristics.

  Returning to FIGS. 18 and 19, the processor 128 generates a normal representative gamma table R-RGT from the normal diffusion gamma table R-DGT stored in the ROM (S38). The typical representative gamma table R-RGT is obtained by calculating the average of the output gradation values of a plurality of corresponding diffusion gamma tables.

  FIG. 21 is a diagram showing a normal diffusion gamma table R-DGT and its representative gamma table R-RGT. This example corresponds to the diffusion gamma table and the representative gamma table shown in FIGS. The normal diffusion gamma table of FIG. 21 has six groups of normal diffusion gamma tables generated by diffusing six representative gamma tables. Each diffusion gamma table R-DGT has output pulse width data of 4 bits and 16 gradations. The six representative gamma tables R-RGT obtained by calculating the average value of the normal diffusion gamma table R-DGT have 8 bits of output pulse width data and 256 gradations.

  18 and 19, the processor 128 corrects the normal representative gamma table R-RGT based on the corrected gamma table C-GT, and generates a corrected representative gamma table C-RGT (S40). This correction processing refers to the correction gamma table C-GT for each of the input gradation values 0 to 255, reads the correction gradation value (FIG. 20) as the output, and uses the correction gradation value as the input gradation. The output pulse value of the normal representative gamma table R-RGT is acquired. If the corrected gradation value has a decimal point between integer discrete values, the output pulse value is obtained by linear interpolation. Therefore, the obtained output pulse value may be a real number with a decimal point. This output pulse value becomes the output data of the corrected representative gamma table C-RGT. Hereinafter, a description will be given with reference to the drawings.

  FIG. 22 is a diagram illustrating a method for correcting the representative gamma table. The first quadrant of FIG. 22 shows a normal representative gamma table R-RGT and a corrected representative gamma table C-RGT that has been corrected. In the fourth quadrant, the correction gamma table C-GT is shown, the X axis corresponds to the input, the Y axis corresponds to the output (correction gradation value), and is inverted upside down. In the third quadrant, a typical representative gamma table R-RGT is given to the first quadrant and the origin object. The second quadrant simply converts the X-axis value into the Y-axis as it is.

  In FIG. 22, corrected output pulse values pa and pb for the input gradations ia and ib are obtained. When the correction gamma table C-GT is referred to corresponding to the input gradation ia, the correction gradation value ixa is obtained. Therefore, when the normal representative gamma table R-RGT is referred to with the corrected gradation value ixa as the input gradation, the output pulse value pa is obtained. This output pulse value pa becomes a corrected output pulse value as shown in the first quadrant. Next, referring to the correction gamma table C-GT corresponding to the input gradation ib, it is assumed that a real number ixb between integer discrete values ixn and ixn + 1 is obtained. As described above, the correction gamma table C-GT has a real number with a decimal number between discrete values of integers. In this case, output pulse values pn and pn + 1 corresponding to the discrete values ixn and ixn + 1 are obtained from the normal representative gamma table R-RGT, and are interpolated by linear interpolation using the discrete values ixn and ixn + 1 and the real number ixb therebetween. The output pulse value pb is obtained. The output pulse value pb is a real number having a decimal number. Then, the real output pulse value pb is given to the corrected representative gamma table C0RGT as the corrected output pulse value. In this way, by providing the correction gamma table with real numbers, the correction representative gamma table can have real correction output pulse values. By using the diffusion gamma table, it is possible to reproduce such real correction output pulse values. As described above, in the correction process S40, for each normal representative gamma table, the processor 128 obtains a corrected output pulse value by repeatedly referring to each table and linear interpolation for the input gradations 0 to 255. Repeat the process.

  FIG. 23 is a diagram showing a corrected representative gamma table C-RGT and a corrected diffusion gamma table C-DGT. As shown in the corrected representative gamma table C-RGT, six representative gamma tables are corrected by the correction method shown in FIG. Compared to the normal representative gamma table R-RGT before correction shown in FIG. 21, the corrected representative gamma table C-RGT has a slightly larger inclination and has a characteristic of moving slightly to the right. That is, in the sensor output characteristic 200 of FIG. 20, since the density changes slightly higher than the ideal characteristic, the output pulse value is suppressed as a result of correction.

  18 and 19, the processor 128 obtains a corrected diffusion gamma table C-DGT from the corrected representative gamma table C-RGT (S42). This process is the same as the process shown in the flowchart of FIG. That is, the corrected output pulse width X (pa, pb in FIG. 22) corresponding to the input gradation value IN of the corrected representative gamma table is acquired (S12), and the discrete points xl and xs adjacent thereto are set to N diffusion numbers. Distribute to obtain L and (N−L) distribution numbers (S14), and randomly give discrete points xl and xs as output pulse values of N correction diffusion gamma tables (S16). This process is repeated for all input gradation values IN = 0 to 255. Then, the processor 128 stores the obtained corrected diffusion gamma table C-DGT in the lookup table memory of the SRAM (S44). An example of this corrected diffusion gamma table C-DGT is shown in FIG.

  In the present embodiment, the index table 120 is not changed in the calibration process. Therefore, there is no change in the arrangement of the N diffusion gamma tables. However, as described above, the output pulse values of the diffusion gamma table are corrected corresponding to the output pulse values pa and pb of the corrected representative gamma table. Even if the output pulse value pb is a real number including a small number, it is possible to reproduce such a real number by diffusing it into N diffusion gamma tables C-DGT. This calibration process is performed by the processor 128 executing a calibration program. That is, the processor 128 corresponds to calibration means.

  In the above embodiment, the typical representative gamma table may be stored in the ROM in the image processing apparatus 100. In that case, the processor 128 corrects the normal representative gamma table in the ROM, and generates a corrected diffusion gamma table from the corrected representative gamma table.

  As described above, according to the calibration processing of the present embodiment, when the normal diffusion gamma table at the time of design is stored in the memory, the normal representative gamma table is obtained from the normal diffusion gamma table and corrected by the correction gamma table. It is returned to the corrected diffusion gamma table again. Therefore, it is possible to correct the gamma table correctly. Since a real number is given to the reference output value of the correction gamma table and the output pulse value of the corrected representative gamma table is obtained by using the real number, correction with higher accuracy can be performed.

It is a block diagram of the image processing apparatus in this Embodiment. It is a figure which shows the relationship between a general index table and a gamma table. FIG. 3 is a diagram illustrating dot growth of a pixel when an input gradation value is increased in the example of FIG. 2. It is a figure which shows the variation | change_quantity of the output gradation value with respect to the change of the input gradation value of a gamma table. It is an enlarged view of a gamma table when the output gradation is 4 bits. It is a figure explaining the principle of the diffusion gamma table in this Embodiment. It is a figure showing an example at the time of assigning a plurality of diffusion gamma tables G1, G2, G3 to representative gamma table G. It is a figure which shows the same index pixel group which has the representative gamma table in this Embodiment. It is a figure which shows the index table with respect to the diffusion gamma table by this Embodiment. It is a figure which shows an example of the output gradation value which the diffusion gamma table shown in FIG. 9 can take. It is a figure which shows an example of the index table of an 8-bit output representative gamma table, and its representative gamma table. It is a figure which shows the index table with respect to the diffusion gamma table by this Embodiment, and a gamma table. It is a figure which shows arrangement | positioning and table value of nine types of diffusion gamma tables of FIG. It is a figure which shows arrangement | positioning and table value of nine types of diffusion gamma tables of FIG. It is a figure which shows arrangement | positioning and table value of nine types of diffusion gamma tables of FIG. It is a figure which shows the difference of the output pulse width with respect to the gamma table of FIG. 11 and FIG. 12, and the change of an input gradation value. It is a flowchart figure which produces | generates a diffusion gamma table from the representative gamma table in this Embodiment. It is a flowchart figure of the calibration process in this Embodiment. It is a figure which shows the relationship between the tables in a calibration process. It is a figure which shows an example of detection density data and a correction | amendment gamma table. It is a figure which shows normal spreading | diffusion gamma table R-DGT and its representative gamma table R-RGT. It is a figure explaining the correction method of a representative gamma table. It is a figure which shows the correction | amendment representative gamma table C-RGT and the correction | amendment diffusion gamma table C-DGT.

Explanation of symbols

110: Halftone processing unit, 120: Index table 122: Look-up table, 128: Processor, 130: Pulse width modulation unit IN: Gradation input data, (X, Y): Pixel position OUT: Image reproduction output data, L / R: Right justification / left justification information

Claims (7)

In an image processing apparatus that generates image reproduction output data from gradation input data,
A halftone processing unit for converting the gradation input data into the image reproduction output data corresponding to a dot size of a pixel;
The gamma table having image reproduction output data corresponding to said grayscale input data, and a look-up table for several chromatic corresponding to the positions of the pixels of the grayscale input data,
The halftone processing unit converts the gradation input data into the image reproduction output data with reference to the gamma table corresponding to the pixel of the gradation input data,
In the lookup table, for each of the same index pixel group composed of a plurality of pixels arranged at the position of a predetermined displacement vector , a plurality of diffusion gamma tables distributed in a plane within the same index pixel group are provided. The plurality of diffusion gamma tables assigned as the gamma table have discrete image reproduction output data that differs at least in part corresponding to the gradation input data, and image reproduction of the plurality of diffusion gamma tables The average value of the output data substantially matches the output gradation value of the virtual representative gamma table of the same index pixel group,
Furthermore, during the calibration process, the output tone value of the representative gamma table of a virtual generated from the plurality of diffusion gamma tables, to generate corrected representative gamma table corrected based on the correction gamma table, the correction representative and generating a plurality of correction diffuse gamma table from gamma table have a calibration means to be stored in the look-up table,
The calibration means converts discrete image reproduction output data in the vicinity of the output gradation value of the corrected representative gamma table to the diffusion gamma in the same index pixel group as the output gradation value of the corrected representative gamma table. Distributed according to the number of tables and given as image reproduction output data of the corrected diffusion gamma table, and the average value of the image reproduction output data of the corrected diffusion gamma table substantially matches the output gradation value of the corrected representative gamma table An image processing apparatus. In an image processing apparatus that generates image reproduction output data from gradation input data,
A halftone processing unit for converting the gradation input data into the image reproduction output data corresponding to a dot size of a pixel;
The gamma table having image reproduction output data corresponding to said grayscale input data, and a look-up table for several chromatic corresponding to the positions of the pixels of the grayscale input data,
The halftone processing unit converts the gradation input data into the image reproduction output data with reference to the gamma table corresponding to the pixel of the gradation input data,
In the lookup table, a virtual representative gamma table is assigned to each of the same index pixel group consisting of a plurality of pixels arranged at the position of a predetermined displacement vector , and corresponding to each virtual representative gamma table. A plurality of diffusion gamma tables distributed in a plane within the same index pixel group is assigned as the gamma table, and the plurality of diffusion gamma tables correspond to the grayscale input data and correspond to the virtual representative. One of the discrete image reproduction output data in the vicinity of the output gradation value of the gamma table,
Furthermore, during the calibration process, the output tone value of the representative gamma table of a virtual generated from the plurality of diffusion gamma tables, to generate corrected representative gamma table corrected based on the correction gamma table, the correction representative and generating a plurality of correction diffuse gamma table from gamma table have a calibration means to be stored in the look-up table,
The calibration means converts discrete image reproduction output data in the vicinity of the output gradation value of the corrected representative gamma table to the diffusion gamma in the same index pixel group as the output gradation value of the corrected representative gamma table. Distributed according to the number of tables and given as image reproduction output data of the corrected diffusion gamma table, and the average value of the image reproduction output data of the corrected diffusion gamma table substantially matches the output gradation value of the corrected representative gamma table An image processing apparatus. 3. The output gradation value of the virtual representative gamma table according to claim 1 or 2, comprising a predetermined number of discrete values, and the calibration means outputs the discrete gradation value to at least one output gradation value of the corrected representative gamma table. An image processing apparatus characterized by giving a value between values.   3. The image processing apparatus according to claim 1, wherein the plurality of diffusion gamma tables are randomly distributed in the same index pixel group corresponding thereto. 3. The discrete image according to claim 1 or 2, wherein the plurality of diffusion gamma table image reproduction output data corresponds to the gradation input data and is in the vicinity of an output gradation value of the virtual representative gamma table. An image processing apparatus having any one of reproduction output data at random. In an image processing program for generating image reproduction output data corresponding to a pixel dot size from gradation input data,
Referring to a lookup table having a plurality of gamma tables having image reproduction output data corresponding to the gradation input data and corresponding to pixel positions of the gradation input data, the gradation input data is reproduced by the image. Let the computer execute the halftone processing procedure to convert to output data,
In the look-up table, a plurality of diffusion gamma tables arranged in a plane and distributed in the same index pixel group for each of the same index pixel group composed of a plurality of pixels arranged at the position of a predetermined displacement vector. The plurality of diffusion gamma tables assigned as the gamma table have discrete image reproduction output data that differs at least in part corresponding to the gradation input data, and image reproduction of the plurality of diffusion gamma tables The average value of the output data substantially matches the output gradation value of the virtual representative gamma table of the same index pixel group,
Further, a corrected representative gamma table is generated by correcting the output gradation value of the virtual representative gamma table generated from the plurality of diffusion gamma tables based on the corrected gamma table. Causing the computer to execute a calibration procedure to generate a corrected diffusion gamma table and store it in the lookup table ;
In the calibration procedure, discrete image reproduction output data in the vicinity of the output gradation value of the corrected representative gamma table is converted to the diffusion gamma in the same index pixel group as the output gradation value of the corrected representative gamma table. Distributed according to the number of tables and given as image reproduction output data of the corrected diffusion gamma table, and the average value of the image reproduction output data of the corrected diffusion gamma table substantially matches the output gradation value of the corrected representative gamma table An image processing program. In an image processing program for generating image reproduction output data corresponding to a pixel dot size from gradation input data,
Referring to a lookup table having a plurality of gamma tables having image reproduction output data corresponding to the gradation input data and corresponding to pixel positions of the gradation input data, the gradation input data is reproduced by the image. Let the computer execute the halftone processing procedure to convert to output data,
In the lookup table, a virtual representative gamma table is assigned to each of the same index pixel group consisting of a plurality of pixels arranged at the position of a predetermined displacement vector , and corresponding to each virtual representative gamma table. A plurality of diffusion gamma tables distributed in a plane within the same index pixel group is assigned as the gamma table, and the plurality of diffusion gamma tables correspond to the grayscale input data and correspond to the virtual representative. One of the discrete image reproduction output data in the vicinity of the output gradation value of the gamma table,
Further, a corrected representative gamma table is generated by correcting the output gradation value of the virtual representative gamma table generated from the plurality of diffusion gamma tables based on the corrected gamma table. Causing the computer to execute a calibration procedure to generate a corrected diffusion gamma table and store it in the lookup table ;
The calibration means converts discrete image reproduction output data in the vicinity of the output gradation value of the corrected representative gamma table to the diffusion gamma in the same index pixel group as the output gradation value of the corrected representative gamma table. Distributed according to the number of tables and given as image reproduction output data of the corrected diffusion gamma table, and the average value of the image reproduction output data of the corrected diffusion gamma table substantially matches the output gradation value of the corrected representative gamma table An image processing program.

Images