JP2011244285A - Method for coding image data - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for coding image data which can code a K value within CMYK image data in a restorable manner, while keeping favorable decoded images and compression efficiency.SOLUTION: In the method, for each pixel, CMYK data is converted into an RGB value to express a color equivalent to its CMYK value (S1), the maximum K value within the CMYK value expressing the color equivalent to the RGB value, when each color component of the CMYK is positive or zero, is indicated as K', and coding is performed by using a gray substitution rate x corresponding to a rate of the K to the K' value in original data and color information consisted of predetermined components not including a K color component (YCCin Fig. 6, other than that, RGB, CMY, etc.) to reproduce a color equivalent to the pixel.

Description

  The present invention relates to an image data encoding method for encoding image data composed of color components such as CMYK.

  Except for gold and silver, all colors can be expressed in three primary colors such as R (red) G (green) B (blue) and C (cyan) M (magenta) Y (yellow). In a color copying machine or the like, a color image is formed on a recording sheet using four color components of C, M, Y, and K (black). By providing the K component, black reproducibility and reduction in the amount of color material used (for example, the black character portion is reproduced only with the K color material) and the like are achieved.

  By the way, since a color image has a large amount of data, it is usually encoded (compressed) by an irreversible method with a good compression rate when stored in a storage unit. For example, Patent Document 1 below discloses an image processing apparatus that separately sets and encodes a compression parameter related to K and a compression parameter related to CMY in order to encode CMYK image data more appropriately in an irreversible manner. It is disclosed. In this image processing apparatus, for example, the compression parameter is set so that the restoration accuracy of the K component is increased in the character area. As the compression parameter, for example, there is a scale factor by which a quantization table in the JPEG (Joint Photographic Experts Group) system is multiplied.

  Patent Document 2 below discloses an image processing apparatus that performs compression coding by DCT (Discrete Cosine Transform) on each of CMYK image signals generated by inking processing by removing high frequency components. It is disclosed. In this image processing apparatus, when image data obtained by reading a document with a scanner is encoded, compression efficiency is improved by suppressing image noise by a low-pass filter effect.

Japanese Patent Laid-Open No. 2003-8907 JP 2008-31264 A

  The K component represents black that can be reproduced with three colors of CMY, and has a larger influence on the image than each component of CMY. Therefore, when encoding CMYK image data, the K component is independent of CMY. If the value of K fluctuates for some reason, such as the influence of image processing, the visual influence will appear greatly in the decoded image. In order to avoid this, it is necessary to manage the value of K more strictly than information such as other color components and encode it. However, if this is done, the amount of code increases and the compression efficiency decreases.

  Note that the present invention is not limited to CMYK but also in image data having color components of basic colors such as CMY and RGB and one or more special colors that can reproduce approximate colors by combining these basic colors at a predetermined ratio. Regarding spot colors, problems similar to those of the K component occur.

  The present invention is intended to solve the above-described problem, and is an image data code that can encode CMYK image data in such a manner that the value of K can be restored while ensuring a good decoded image and compression efficiency. The purpose is to provide a conversion method. In addition, image data having a color component of a basic color and a special color that can reproduce an approximate color by combining the basic color at a predetermined ratio is used to set a value of the special color while ensuring a good decoded image and compression efficiency. An object of the present invention is to provide an image data encoding method which can be reconstructably encoded.

  The gist of the present invention for achieving the object lies in the inventions of the following items.

[1] An image data encoding method for encoding image data having four colors of C (cyan), M (magenta), Y (yellow), and K (black) as color components,
Each pixel is converted to an R (red), G (green), and B (blue) value that represents a color equivalent to the CMYK value, and each color component is positive or zero. In order to reproduce the color equivalent to the pixel and the replacement rate corresponding to the ratio between K ′ and the K value of the original data. An image data encoding method characterized by encoding using color information composed of a predetermined component that does not use a K color component.

  In the above invention, the CMYK image data is converted into RGB values representing equivalent colors, and the maximum value of K that can be taken when converting the RGB values into CMYK values of equivalent colors is obtained as K ′. A replacement rate (gray replacement rate) indicating a ratio between 'and the K value of the original data is obtained. Each pixel is encoded using color information that represents a color equivalent to the original CMYK image data and includes a predetermined component that does not include the K component, and a replacement rate. In the invention according to [1], K ′ is obtained by once converting it into RGB values. Color information for reproducing a color equivalent to the original pixel and composed of predetermined components that do not use the K color component includes RGB values, CMY values, luminance and two color components, etc. What is necessary is just to be comprised with the arbitrary components which do not contain K component. The color information is preferably composed of three components to reduce the amount of information.

[2] Conversion to an RGB value representing a color equivalent to the CMYK value of the original pixel is performed using a predetermined adjustment coefficient α (0 <α <1).
R value = 1-min (1, C value x (1-αK value) + αK value)
G value = 1-min (1, M value × (1-αK value) + αK value)
B value = 1-min (1, Y value × (1-αK value) + αK value)
The image data encoding method according to [1], wherein the image data encoding method is performed by:

  In the above invention, by providing the adjustment coefficient α, CMY value information is ensured even when the value of the K component is 1. The adjustment coefficient α is preferably a value close to 1 in order to suppress distortion.

[3] An image data encoding method for encoding image data having four colors of C (cyan), M (magenta), Y (yellow), and K (black) as color components,
For each pixel, a value obtained by adding the K value to the minimum value among the C value, the M value, and the Y value is obtained as K ′, and the replacement rate corresponding to the ratio of K ′ and K value is equivalent to the pixel. An image data encoding method comprising: encoding using color information for reproducing a color and color information composed of a predetermined component that does not use a K color component.

  In the above-described invention, K ′ is obtained with the CMYK values as they are without being converted into RGB values. Others are the same as [1].

[4] An image data encoding method for encoding image data having four colors of C (cyan), M (magenta), Y (yellow), and K (black) as color components,
For each pixel, color information representing a color equivalent to the pixel and including a predetermined component that does not use the K color component is obtained, and the color information and the K value of the original image data are obtained. An image data encoding method comprising: encoding using auxiliary information for specifying.

  In the above invention, the color information composed of a plurality of predetermined color components that represent a color equivalent to the original pixel and does not include the K component is obtained, and the auxiliary information for specifying the K value of the original image data And information. The auxiliary information may be the replacement rate described in [1] or the like, or the K value itself of the original image data.

[5] An image data encoding method for encoding image data having color components of a plurality of basic colors and a special color that can represent an approximate color by combining these basic colors at a predetermined ratio,
For each pixel, obtain the value of each color component when the color equivalent to that pixel is represented by only the basic color, and when the color equivalent to the color represented by these color component values is represented by the basic color and the special color The value of the special color component when the value of the special color component is the maximum among the values of each color component and the values of all the basic color components are positive or zero is obtained as S ′, and S ′ and the special color component of the original data Encoding using a replacement rate corresponding to a ratio to the value of color and color information that is a color information that represents a color equivalent to the pixel and that includes a predetermined component that does not use a special color component. A characteristic image data encoding method.

  The above-described invention corresponds to a component obtained by replacing the K component in [1] with a spot color component obtained by combining basic colors at a predetermined ratio. The basic color is not limited to CMY, but may be RGB or the like.

[6] An image data encoding method for encoding image data having color components of a plurality of basic colors and a special color that can represent approximate colors by combining these basic colors at a predetermined ratio,
For each pixel, color information representing a color equivalent to the pixel and including a predetermined component that does not use the special color component is obtained, and the color information and the value of the special color component in the original image data are obtained. An image data encoding method comprising: encoding using auxiliary information for specifying.

  In the above invention, color information composed of a plurality of predetermined color components that represent a color equivalent to the original pixel and do not include the special color component is obtained, and the value of the special color component in the original image data is specified. And ancillary information. The auxiliary information may be the replacement rate described in [5] or the value of the spot color component of the original image data.

[7] The color information composed of the predetermined component is composed of one data mainly representing a luminance component and two data mainly representing a color component. [1] to [1] 4]. The image data encoding method according to any one of 4).

  In the said invention, the color information comprised by the luminance component and two color components is produced | generated. By adopting such a format, the information on the two color components is less affected by visual distortion due to the compression distortion than the luminance component, so that the compression rate can be increased.

[8] The image data encoding method according to [7], wherein at least the replacement rate among the luminance component, the color component, and the replacement rate is encoded while suppressing the resolution.

  In the above invention, even if the compression distortion increases, the replacement ratio has little influence on the restored image such as color reproducibility, so the resolution is lowered and the compression ratio is increased.

[9] Divide the image to be encoded into the local blocks;
The pixels included in the local block whose data variation degree in the local block is less than the reference value are encoded while suppressing the resolution for at least the color component in addition to the replacement rate,
Pixels included in a local block whose local data variation degree is greater than or equal to the reference value are coded with a bit width (number of gradations) suppressed at least for the tint component from the local block whose degree of dispersion is less than the reference value [8] The image data encoding method according to [8].

  In the above invention, when the local block is an image region including an edge portion, the resolution is emphasized so that the sharpness of the edge is not lost, and the bit width of the color component is reduced to reduce the amount of information. On the other hand, in the case of a relatively flat image such as a non-edge portion, the compression rate is increased by suppressing the resolution, and a bit width is secured for the color component, and importance is placed on the reproducibility of subtle color changes.

[10] The image data encoding method according to [9], wherein the encoding is performed so that a code amount of a local block unit is a fixed length.

  In the above invention, by selecting the resolution-oriented and the bit width (gradation-oriented), the encoded data is set to a fixed length in both the edge portion and the non-edge portion.

  According to the image data encoding method of the present invention, instead of encoding the values of the K component and the spot color component itself, the replacement that can specify the value of the K component and the spot color component to be extracted from the color information and restored. Since the rate and auxiliary information are obtained and encoded, the visual influence can be suppressed even if the replacement rate and auxiliary information slightly change, and CMYK image data and image data composed of basic colors and special colors can be reduced. Thus, K and spot color values can be reconstructed while ensuring a good decoded image and good compression efficiency.

It is explanatory drawing which shows the conversion from CMYK image data to various image data of an equivalent color. It is explanatory drawing which shows the concept of a gray replacement rate. It is explanatory drawing which shows the concept of a spot color replacement rate. It is a flowchart which shows the outline of the encoding process by the image data encoding method of this invention. It is explanatory drawing which shows the data flow in the image data encoding method of this invention. It is explanatory drawing which shows an example of the color conversion used with the image data encoding method of this invention. It is explanatory drawing which shows the computing equation which concerns on color conversion at the time of introduce | transducing the color adjustment coefficient (alpha). It is explanatory drawing which shows an example of the compression condition table | surface showing the compression conditions of the edge part and the non-edge part. It is a flowchart which shows the detail of the compression process (encoding process) which concerns on this invention. 10 is a flowchart showing a process of decompressing (decoding) encoded data obtained by the process of FIG. 9. It is explanatory drawing which compares the encoding method of this invention and another encoding system about various items, and shows the effect of this invention. It is explanatory drawing which shows the example of application of the encoding method which concerns on this invention to an image forming apparatus.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

  First, the basic principle of the present invention will be described. Of the image data composed of CMYK color components, the K color component is usually a color obtained by combining the C, M, and Y color components at a predetermined ratio such as 1: 1: 1. In the following description, the K color is described as a color approximated by a color obtained by combining CMY at a ratio of 1: 1: 1.

  Therefore, CMYK image data (for example, image data 10 in FIG. 1) can be converted into CMY image data 11 representing an equivalent color by adding a K value to each value of CMY.

  Further, the CMY image data 11 can be converted into CMYK image data representing an equivalent color. That is, when image data composed of CMY color components is converted into CMYK image data, the K value may be subtracted from each CMY value by the amount corresponding to the generated K value.

  At this time, the K value can be set to any value within a range in which the CMY values after subtraction do not become negative. For example, if each color component value can take a range of 0 to 100 and each CMY value in the CMY image data 11 is C = 70, M = 80, and Y = 90, K can be replaced with the K component. The maximum value K ′ is 70, which is the minimum value in CMY.

  When the CMY image data 11 is converted into CMYK image data 12 with a K value of 10, the CMY value is a value obtained by subtracting 10 from each value of the CMY image data 11 (C = 60, M = 70. Y = 80). If the K value is converted to CMYK image data 13 with 20, the value obtained by subtracting 20 from each value of the CMY image data 11 (C = 50, M = 60.Y = 70) may be used. Is converted to CMYK image data 14 by setting 40 to a value obtained by subtracting 40 from each value of CMY (C = 30, M = 40.Y = 50). The CMYK image data 12, 13, and 14 all represent substantially the same color as the CMY image data 11.

  When converting CMY image data into CMYK image data in this way, an image representing substantially the same color (color represented by the original CMY image data) regardless of the K value within the possible range. Data can be obtained.

  Therefore, in the present invention, when the image data 10 composed of CMYK is encoded, the image data 10 composed of CMYK is temporarily composed of an image composed of a plurality of components not including K components such as CMY and RGB. Data (this is color information; in the example of FIG. 1, CMY image data 11), and how much K component should be extracted from the converted image data and the color represented by the image data, In other words, encoding is performed using auxiliary information indicating how much K component is extracted to reproduce the original K value.

  As described above, when CMY image data is converted to CMYK image data, almost the same color is expressed regardless of the K value, so that the coding accuracy (restoration accuracy) of auxiliary information is reduced to some extent. However, there is little influence on the color reproduced by decoding, and sufficient color reproducibility can be ensured. In addition, there are an infinite number of combinations of CMYK that can be converted from color information such as CMY that represents the same color as the CMYK image data. However, by using auxiliary information, the original CMYK image can be obtained using this as a clue. K values comparable to the data can be reproduced.

  Note that the color information for representing the same color as the original CMYK image data is not limited to CMY, but may be RGB or the like. Further, it may be represented by luminance information and color difference information.

  The auxiliary information is, for example, the maximum K value that can be extracted from the K value in the original CMYK image data 10 and the CMY or RGB color information representing the same color as the original CMYK image data 10. (This will be referred to as the “gray replacement rate”).

  In the example of FIG. 1, assuming that the maximum value of K that can be extracted from the CMY image data 11 is K ′ and the K value included in the original CMYK image data 10 is k, the gray replacement rate is k / K ′.

  FIG. 2 is another explanatory diagram showing the concept of the gray replacement rate. In this example, the original CMYK image data 20 is C = 60, M = 70, Y = 30, and K = 20. The maximum value that can be replaced with the K component from the CMY color components included in the original CMYK image data 20 within a range in which they are not negative is 30 which is the minimum value of CMY.

  Therefore, the CMY data 21 composed of C = 30, M = 30, and Y = 30 becomes the gray replacement portion, and this is replaced with the K component (data 22) of K = 30. The portion obtained by removing the CMY data 21 for gray replacement from the original CMYK image data 20 is image data 23 for non-gray replacement. In this example, C = 30, M = 40, Y = 0, K = 20. It becomes.

  An image after gray replacement in which the K component (data 22) obtained by replacing the CMY data 21 and the non-gray replacement image data 23 represent the same color as the original CMYK image data 20 Data 24. In the image data 24 after gray replacement, the C component value is C ′, the M component value is M ′, the Y component value is Y ′, and the K component value is K ′. The gray replacement rate is represented by k / K ′ and is 20/50, where k is the K value included in the original CMYK image data.

  Not only CMYK but also image data with multiple basic colors and special colors that can represent the same color or approximate color by combining these basic colors at a predetermined ratio, it corresponds to the gray replacement rate. It is possible to define the spot color replacement rate, which is the calculated value.

  For example, when encoding CMYS image data having color components of a basic color composed of CMY and a special color S obtained by combining CMY at a predetermined ratio, a special color replacement rate S as shown in FIG. ′ Is obtained and encoded.

  In the example of FIG. 3, the spot color S is a color approximated by combining C, M, and Y at a ratio of 2: 3: 6. In this example, it is assumed that there is a correspondence relationship of (C: M: Y): S = (2: 3: 6): 6. The original CMYS image data 30 is C = 40, M = 15, Y = 55, and S = 10. Among these, the CMY data 31 including C = 10, M = 15, and Y = 30 is the maximum amount (spot color replacement) that can be replaced with the spot color S, and is replaced with the S component (data 32) of S = 30. .

  A portion obtained by removing the special color replacement CMY data 31 from the original CMYS image data 30 becomes non-spot color replacement image data 33. In this example, C = 30, M = 0, Y = 25, S = 10. It becomes. The image after the spot color replacement in which the S component (data 32) obtained by replacing the CMY data 31 and the non-spot color replacement image data 33 represents the same color as the original CMYS image data 30 Data 34. In the image data 34 after the spot color replacement, the C component value is C ′, the M component value is M ′, the Y component value is Y ′, and the S component value is S ′. The spot color replacement rate is represented by s / S ′ and becomes 10/40, where s is the S value included in the original CMYS image data.

  FIG. 4 shows a schematic flow of encoding processing by the image data encoding method of the present invention, and FIG. 5 shows the data flow. The original CMYK image data to be encoded is input (step S101), and this is color-converted to obtain color information and a gray replacement rate (FIG. 4: step S102, FIG. 5: P1). Here, the color information is obtained as data in a format represented by a luminance component and two color components.

  FIG. 6 shows an example of color conversion. First, the CMYK image data is converted into RGB image data (S1). Here, it is assumed that the value of each color component of CMYK can range from 0 to 1. Moreover, the symbol which shows each component in a figure shall show the value of the component.

Next, the maximum K value K ′ that can be extracted from the converted RGB is obtained (S2), and the gray replacement rate x is obtained from the K value and K ′ of the original CMYK image data (S3). The RGB image data is further subjected to gamma conversion with reference to a conversion LUT (Look Up Table) (S4), and then converted to YC ₁ C ₂ image data using a color conversion matrix (S5). ). In YC ₁ C ₂ , Y is luminance component data, and C ₁ and C ₂ are color difference data indicating color components, respectively. As a result of the color conversion, color information (YC ₁ C ₂ ) and a gray replacement rate x are generated.

  FIG. 7 shows an arithmetic expression related to color conversion when the color adjustment coefficient α is introduced. When converting CMYK image data into RGB image data, if K is 1 in the arithmetic expression shown in S1 of FIG. 6, information on C, M, and Y is lost. Therefore, an adjustment coefficient α is introduced so that C, M, and Y information remain even when K is 1. The adjustment coefficient α is a value relatively close to 1 with 0 <α <1. For example, it is set to a value such as 0.95. An arithmetic expression group 41 in FIG. 7 is an arithmetic expression when converting CMYK image data to RGB image data, and an arithmetic expression group 42 is used when converting from RGB image data and gray replacement rate to CMYK image data. An arithmetic expression is shown.

  Returning to FIG. 4, the description will be continued. The input image is divided into local blocks of N pixels × N pixels (for example, 2 pixels × 2 pixels) in units (step S103), and the degree of variation of data (particularly color components) in each local block is obtained (step S104). Then, it is determined whether or not the degree of variation is less than the reference value (step S105). Then, the local block whose variation degree is less than the reference value (step S105; Yes) is encoded (compressed) with emphasis on gradation information (step S106).

  For example, encoding is performed assuming that the compression rate of gradation information is “low”, the compression rate of resolution information is “high”, and the compression rate of the gray replacement rate is “high”. In other words, a local block whose variation degree is less than the reference value is considered to be a non-edge part (an image area in which there is no abrupt change like the edge part and can be a gentle and subtle change). Encoding is performed with emphasis on the luminance and the number of gradations of the color component so that the change is not impaired.

  On the other hand, local blocks with a variation degree equal to or greater than the reference value (step S105; No) are compressed with emphasis on resolution information (step S107). For example, encoding is performed assuming that the compression rate of gradation information is “high”, the compression rate of resolution information is “low”, and the compression rate of the gray replacement rate is “high”. That is, a local block having a variation degree equal to or greater than a reference value is considered as an image area where an edge portion exists. Therefore, encoding is performed with emphasis on resolution so that the clarity of the edges is not lost.

  Steps S104 to S107 are repeated until the processing is completed for all local blocks (step S108; No). When encoding is completed for all local blocks (step S108; Y), the processing is ended.

  In steps S105 and S107, encoding is performed so that the code amount of the local block unit becomes a fixed length. Specifically, when importance is attached to the resolution, for example, the resolution is the same as the original image data, and the bit width of the luminance and color component data is reduced as compared with the case where the resolution is not important. On the other hand, when importance is attached to gradation information (the number of gradations of luminance and color components), for example, the resolution is lowered as compared with the case where importance is attached to resolution, and the bit width of luminance and color component data attaches importance to resolution. Secure more than the case.

FIG. 8 shows a compression condition table 50 for an edge portion (variation is greater than or equal to a reference value) and a non-edge portion (variation is less than the reference value). The compression condition table 50 also describes the amount of information after compression. The resolution is 1 when information is given for each pixel. At the edge portion, the resolution of Y (luminance), C ₁ , and C ₂ is 1/1, and the resolution of the gray replacement rate is 1/2. Bit width, Y is 5bit, _{C 1} is 4bit, _{C 2} is 5bit, rate replaced gray and 4bit, has become a half of the bit width in each case a non-edge portion. In the non-edge portion, the resolution for Y is 1/1, and the resolutions of C ₁ , C _{2 and the} gray replacement rate are 1/2.

  When the resolution is halved, the information amount is halved in each of the X and Y directions, so the information amount is ¼. For example, if the information is provided for each pixel and the resolution is 1, the information is provided in a local block unit of 2 pixels × 2 pixels, so that the resolution is halved and the information amount is ¼.

  By applying the compression condition shown in FIG. 8, the image compression rate can always be ½ or less while ensuring the edge shape of the edge portion and the color reproducibility of the non-edge portion. For example, if the original CMYK image data is 32 bits / pixel, the edge portion is compressed to 15 bits / pixel, and the non-edge portion is also compressed to 15 bits / pixel. Actually, an edge part identification code (1 bit) indicating whether the coding is performed under the compression condition of the edge part or the non-edge part is added.

  Of the luminance component, the color component, and the replacement rate, at least for the replacement rate, regardless of the edge portion or the non-edge portion, it can be represented by a representative value in units of local blocks or by encoding an average value. Encode with reduced resolution. As a result, it is possible to reduce the code amount while suppressing the visual influence during restoration.

  In the edge portion, for example, the bit width of the luminance component may not be suppressed as long as the bit width of the color component is suppressed. In the non-edge portion, at least the color component may be encoded with the resolution suppressed. The resolution suppression method is not limited to encoding the average value for the representative value of each local block. For example, the resolution is suppressed by encoding the representative value or the average value for a plurality of pixels of a part of the block. May be.

  FIG. 9 shows a more detailed flow of the compression process (encoding process). The flow is basically the same as that shown in FIG. Here, the local block is 2 pixels × 2 pixels. First, a preparatory process for encoding is performed (step S201). Creation of a gamma conversion LUT, creation of color conversion coefficients, setting of condition parameters, securing of input / output buffers (for two lines), opening of an encoding target image (CMYK image data) file, and the like are performed.

  Next, the processing for every two lines is repeated until the entire image is completed (Lp1s to Lp1e). When the entire image is completed, post-processing such as file close and buffer release is performed (step S211), and the process ends.

  In the processing for every two lines, the image data for two lines is read (step S202), and the processing of the local block unit of 2 pixels × 2 pixels is repeated for the entire two lines (Lp2s to Lp2e).

In the processing in units of local blocks, first, color conversion is performed for each pixel (Lp3s to Lp3e). Specifically, RGB image data and a gray replacement rate are obtained from CMYK image data (step S203), the RGB image data is gamma-converted (step S204), and then a YC ₁ C ₂ image is obtained using a color conversion matrix. Data is converted (step S205). The processing contents of the color conversion are the same as those shown in FIGS.

Next, the degree of variation (edge degree) of the local block to be processed is calculated (step S206). Here, determined the square of the difference between the maximum and minimum values of C ₁ of each pixel in the local block, the sum of the square of the difference between the maximum value and the minimum value of C ₂ as the degree of variation e, Whether or not the edge portion is an edge is determined based on whether or not the variation degree e is equal to or greater than a reference value (threshold value) (step S207). The reference value may be set as appropriate.

  When the variation degree e is larger than the reference value (step S207: True), compression is performed under the edge compression condition of FIG. 8 (step S208). If the variation degree e is equal to or less than the reference value (step S207: False), compression is performed under the non-edge portion compression condition in FIG. 8 (step S209). The Y, C1, C2, and gray replacement rate data obtained by compression under the edge or non-edge compression conditions, and the edge identification that indicates whether the compression is performed under the compression conditions of the edge or non-edge The code is output as code data of the local block (step S210).

  FIG. 10 shows the flow of decompression (decoding) processing. In this process, the code data obtained by compression in the process of FIG. 9 is expanded. First, preparation processing for decoding is performed (step S301). Creation of a gamma conversion LUT, creation of color conversion coefficients, setting of condition parameters, securing of input / output buffers (for two lines), opening of a decoding target code data file, etc. are performed.

  Next, the processing for every two lines is repeated until the entire encoded data to be decoded is completed (Lp1s to Lp1e), and after the entire processing is completed, post-processing such as file close and buffer release is performed (step S310). finish.

  In the processing for every two lines, code data for two lines is read (step S302), and the decoding process in units of local blocks of 2 pixels × 2 pixels is repeated for the entire two lines (Lp2s to Lp2e).

  In the decoding process in units of local blocks, first, it is determined by the edge part identification code whether the edge part or the non-edge part is encoded (step S303), and if it is an edge part (step S303; Ture). ), Each pixel data is expanded based on the compression condition of the edge portion of FIG. 8 (step S304). On the other hand, if the edge part identification code indicates a non-edge part (step S303; False), the pixel data is expanded based on the compression condition of the non-edge part of FIG. 8 (step S305).

After expanding each pixel data in step S304 or S305, color conversion processing is performed for each pixel (Lp3s to Lp3e). More specifically, YC ₁ C ₂ image data is converted into R′G′B ′ image data using a color conversion matrix (step S306), which is further converted into RGB image data by gamma conversion (step S306). In step S307, CMYK image data is generated from the RGB image data and the gray replacement rate (step S308). The processing contents of the color conversion correspond to what is performed in FIG. 6 from the right side to the left side. Then, the generated CMYK image data is output (step S309).

FIG. 11 is a list showing the effects of the present invention by comparing the image data coding method of the present invention with other coding methods for various items. The A method and the B method are conventional compression methods. The A method corresponds to, for example, the BTC method, and the C, M, Y, and K components are independently encoded. The B system is such that YC ₁ C _{2 and} other luminance and two color components are converted and encoded, and JPEG or the like is applicable. In the B method, there is no clue as to how many K values should be used when returning to CMYK image data, and the K component cannot be restored to a K value equivalent to the original CMYK image data.

The C method is an encoding method according to the present invention, and is a method of encoding CMYK image data with RGB image data and a gray replacement rate. The D method is also an encoding method of the present invention, and is an encoding method based on luminance, color information based on _two color components (for example, YC ₁ C ₂ ), and a gray replacement rate. In both the C method and the D method, the K value of the original CMYK image data can be almost restored.

  In the A method, any of the components C, M, Y, and K has a large influence on the visual influence of the compression strain (see the column 61 in FIG. 11). In other words, the color identity between the decoded pixel and the original pixel cannot be ensured unless information is strictly managed and encoded for any component. Therefore, the compression space for each code element (room for increasing the compression rate) is small for any of C, M, Y, and K components (see column 62 in FIG. 11).

The B method has a large Y (luminance) component but a medium color component (C ₁ , C ₂ ) with respect to the visual impact of compression distortion (see column 63 in FIG. 11). The compression room for each code element is small for the Y (luminance) component and medium for the color components (C ₁ , C ₂ ) (see column 64 in FIG. 11).

  In the C method, the visual impact of compression distortion is large for the R, G, and B components, and the gray replacement rate is small (see column 65 in FIG. 11). Therefore, the compression room for each code element is small for the R, G, and B components and large for the gray replacement rate (see column 66 in FIG. 11). In other words, the gray replacement rate has a small influence on the color reproducibility even if the value fluctuates slightly due to compression, so that the gray replacement rate can be compressed at a high compression rate.

In the D method, the visual influence of compression distortion is large for the Y (luminance) component, medium for the color components (C ₁ , C ₂ ), and small for the gray replacement rate (see column 67 in FIG. 11). Therefore, the compression room for each code element is small for the Y (luminance) component, medium for the color components (C ₁ , C ₂ ), and large for the gray replacement rate (see column 68 in FIG. 11). In other words, in the D method, the Y component and the gray replacement rate have little influence on the color reproducibility even if their values fluctuate slightly due to compression, so that the compression can be performed at a high compression rate.

  The overall compression room is in the order of D method (about 1/2)> C method (about 4/5)> B method (about 2/3)> A method (about 1/1). The C method and D method can increase the compression rate compared to the other conventional methods A and B.

  FIG. 12 shows an application example of the encoding method according to the present invention to an image forming apparatus. The image forming apparatus includes a system control board 71, a video interface board 72, and a printer control board 73. The interruption of the figure shows a conventional data flow 75, and the lower part shows an example of a data flow 76 to which the present invention is applied.

  The system control board 71 performs RIP processing to generate CMYK image data. In the conventional data flow 75, the CMYK image data is transferred from the system control board 71 to the video interface board 72. However, in the data flow 76 to which the present invention is applied, the system control board 71 uses the encoding method of the present invention. Primary compression is performed by (for example, the process shown in FIG. 9). The primary encoded data obtained by the primary compression is transferred from the system control board 71 to the video interface board 72. Thus, the amount of data transferred from the system control board 71 to the video interface board 72 can be reduced.

  In the data flow 76 to which the present invention is applied, the primary encoded data is secondarily compressed by, for example, the BTC method in the video interface board 72 and stored in, for example, a hard disk device (HDD). At the time of printing, the encoded data is read from the HDD and transferred to the printer control board 73. The printer control board 73 first performs a decoding process corresponding to the secondary compression on the encoded data, and subsequently performs a decoding process corresponding to the primary compression (for example, the process shown in FIG. 10) to obtain CMYK image data. Are sent to the printing unit to print on the recording paper.

  The embodiment of the present invention has been described with reference to the drawings. However, the specific configuration is not limited to that shown in the embodiment, and there are changes and additions within the scope of the present invention. Are also included in the present invention.

  For example, in the embodiment, the gray replacement rate or the spot color replacement rate is used as the auxiliary information, but the auxiliary information is not limited to this. For example, the K value itself included in the original CMYK image data may be used as auxiliary information. In this case, the original CMYK image data is converted into color information such as CMY image data or RGB image data, and is encoded using this color information and the K value included in the original CMYK image data. The color information represents a color equivalent to the original CMYK image data. At the time of decoding, for example, equations 42c, 42m, and 42y for obtaining C, M, and Y in the arithmetic equation group 42 in FIG. 7 may be used, and the K value of the auxiliary information may be substituted as the K value. Even if the auxiliary information (K value) slightly varies depending on the compression distortion, the influence on the color reproducibility is small. Also, the original K component can be almost decoded. The same applies to the spot color S.

  Note that the image data encoding method and the decoding method corresponding to the image data encoding method according to the present invention are implemented, for example, by an image processing device having a CPU (Central Processing Unit) as a main part or by executing a program on a computer device. Is done. Further, the present invention is configured as an image processing apparatus that performs the image data encoding method and decoding method according to the present invention, or a program that causes a computer to execute the image data encoding method and decoding method according to the present invention. Also good.

10 ... Original CMYK image data 11 ... CMY image data representing a color equivalent to the original image data 12 ... CMYK image data obtained by converting the CMY image data (K = 10)
13. CMYK image data obtained by converting CMY image data (K = 20)
14: CMYK image data obtained by converting CMY image data (K = 40)
20 ... Original CMYK image data 21 ... CMY data to be replaced with K 22 ... K component data obtained by replacement 23 ... Image data for non-gray replacement 24 ... Image data after gray replacement 30 ... Original CMYS image data 31 ... CMY data for special color replacement 32 ... Special color component data obtained by replacement 33 ... Image data for non-special color replacement 34 ... Image data after special color replacement 41 ... Arithmetic formula group 42 for converting CMYK to RGB 42 ... RGB Arithmetic expression group for conversion to CMYK 50 ... Compression condition table 71 ... System control board 72 ... Video interface board 73 ... Printer control board 75 ... Conventional data flow 76 ... Data flow to which the present invention is applied

Claims (10)

An image data encoding method for encoding image data having four color components of C (cyan), M (magenta), Y (yellow), and K (black),
Each pixel is converted to an R (red), G (green), and B (blue) value that represents a color equivalent to the CMYK value, and each color component is positive or zero. In order to reproduce the color equivalent to the pixel and the replacement rate corresponding to the ratio between K ′ and the K value of the original data. An image data encoding method characterized by encoding using color information composed of a predetermined component that does not use a K color component. Conversion to an RGB value representing a color equivalent to the CMYK value of the original pixel is performed using a predetermined adjustment coefficient α (0 <α <1).
R value = 1-min (1, C value x (1-αK value) + αK value)
G value = 1-min (1, M value × (1-αK value) + αK value)
B value = 1-min (1, Y value × (1-αK value) + αK value)
The image data encoding method according to claim 1, wherein the image data encoding method is performed by: An image data encoding method for encoding image data having four colors of C (cyan), M (magenta), Y (yellow), and K (black) as color components,
For each pixel, a value obtained by adding the K value to the minimum value among the C value, the M value, and the Y value is obtained as K ′, and the replacement rate corresponding to the ratio of K ′ and K value is equivalent to the pixel. An image data encoding method comprising: encoding using color information for reproducing a color and color information composed of a predetermined component that does not use a K color component. An image data encoding method for encoding image data having four color components of C (cyan), M (magenta), Y (yellow), and K (black),
For each pixel, color information representing a color equivalent to the pixel and including a predetermined component that does not use the K color component is obtained, and the color information and the K value of the original image data are obtained. An image data encoding method comprising: encoding using auxiliary information for specifying. An image data encoding method for encoding image data having color components of a plurality of basic colors and a special color that can represent an approximate color by combining these basic colors at a predetermined ratio,
For each pixel, obtain the value of each color component when the color equivalent to that pixel is represented by only the basic color, and when the color equivalent to the color represented by these color component values is represented by the basic color and the special color The value of the special color component when the value of the special color component is the maximum among the values of each color component and the values of all the basic color components are positive or zero is obtained as S ′, and S ′ and the special color component of the original data Encoding using a replacement rate corresponding to a ratio to the value of color and color information that is a color information that represents a color equivalent to the pixel and that includes a predetermined component that does not use a special color component. A characteristic image data encoding method. An image data encoding method for encoding image data having color components of a plurality of basic colors and a special color that can represent an approximate color by combining these basic colors at a predetermined ratio,
For each pixel, color information representing a color equivalent to the pixel and including a predetermined component that does not use the special color component is obtained, and the color information and the value of the special color component in the original image data are obtained. An image data encoding method comprising: encoding using auxiliary information for specifying. The color information composed of the predetermined component is composed of one data mainly representing a luminance component and two data mainly representing a color component. The image data encoding method as described in one. The image data encoding method according to claim 7, wherein at least the replacement rate among the luminance component, the color component, and the replacement rate is encoded while suppressing the resolution. Dividing the image to be encoded into the local blocks;
The pixels included in the local block whose data variation degree in the local block is less than the reference value are encoded while suppressing the resolution for at least the color component in addition to the replacement rate,
Pixels included in a local block whose local data variation degree is greater than or equal to the reference value are coded with a bit width (number of gradations) suppressed at least for the tint component from the local block whose degree of dispersion is less than the reference value The image data encoding method according to claim 8, wherein: The image data encoding method according to claim 9, wherein encoding is performed so that a code amount of a local block unit has a fixed length.

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