A color separation method for offset printing with direct conversion between source color space and target color space

Abstract

The invention discloses an offset color separation method for direct conversion between a source color space and a target color space, comprising: S010, color matching and color selection; S020, designing 8 color sequence combinations according to collected hue data; S030, designing a combined color table of the color sequence combinations; S040, determining publishing conditions, printing equipment conditions, optimal printing density of each color ink and an optimal printing compensation curve through several experiments; S050, determining the printing equipment to be used based on a comparison of 5-color, 6-color, 7-color and 8-color printing equipment; S060, establishing a conversion model between RGB spectrum data and collected spectrum data; achieving the reduction of color loss in the color conversion process and the expansion of the color rendering range of four-color offset printing.

Description

Technical Field

[0001] The invention relates to the field of offset color separation technology equipment, and in particular to an offset color separation method for directly converting a source color space and a target color space. Background Art

[0002] The mainstream image conversion method on the market is to use Photoshop to perform color separation and color conversion between related color spaces. The basic principle of Photoshop conversion is to use ICC profile files to convert from RGB space to Lab space and then to CMYK space. Four mapping methods can be used during conversion: relative value, absolute value, saturation, and visual. The traditional ICC color profile file conversion method uses an intermediate connecting color space (PCS) as a bridge to convert between two spaces. This method has the following limitations:

[0003] After converting from CMYK four-dimensional color space to Lab three-dimensional color space and then back to CMYK four-dimensional color space, the color precision is lost greatly and the accuracy is also affected to a certain extent. The current ICC color profile conversion method has only about 90% color restoration accuracy.

[0004] Due to the limitations of the CMYK four-color gamut, some products on the market still use spot color inks to compensate for specific color gamuts. These spot colors cannot be overprinted, and there are step jumps between four colors and spot colors. They are only suitable for printing specific uniform solid color blocks, and are not suitable for printing pictures with gradations and transitions such as artworks, books, and periodicals. Moreover, their spot color channels cannot be adjusted, so they are printed in the same way as they are printed. The entire hue depends entirely on the type of spot color. Different spot colors are needed to supplement the production of different products, making the production site and quality even more difficult to control.

[0005] Therefore, there is an urgent need for an offset color separation method for directly converting a source color space and a target color space that can solve one or more of the above problems. Summary of the invention

[0006] In order to solve one or more problems existing in the prior art, the present invention provides an offset color separation method for direct conversion between a source color space and a target color space. The technical solution adopted by the present invention to solve the above problems is: an offset color separation method for direct conversion between a source color space and a target color space, comprising:

[0007] S010, mix blue, red, yellow, orange, green, purple and black inks, use a color development instrument to develop colors, and collect, calculate, and adjust and improve color groups;

[0008] S020, designing 8 color sequence combinations according to the collected hue data, wherein the colors in the color sequence combinations are freely combined and / or used alone;

[0009] S030, designing a combined color table of the color sequence combinations, wherein all combination situations of each color sequence combination can be within the combined color table;

[0010] S040, through several experiments, determine the publishing conditions, printing equipment conditions, the best printing density of each color ink and the optimal printing compensation curve;

[0011] S050, determine the printing equipment to be used based on the comparison of 5-color, 6-color, 7-color and 8-color printing equipment;

[0012] S060, after the printed combined color table is completely dry, collecting and exporting the spectral data of the combined color table, comparing the RGB spectral data with the collected spectral data, and establishing a conversion model between the RGB spectral data and the collected spectral data through the RGB spectral data and the collected spectral data.

[0013] Furthermore, S011, based on the CIE LCH color model, collected: black: L: 11.96, C: 0.447, H: 130.462°;

[0014] Blue: L: 51.00, C: 64.252, H: 238.324°;

[0015] Red: L: 50.24, C: 81.968, H: 346.432°;

[0016] Yellow: L: 87.28, C: 99.467, H: 93.112°;

[0017] Orange: L:72.32, C:94.714, H:65.085°;

[0018] Green: L: 65.25, C: 66.462, H: 180.422°;

[0019] Purple: L: 31.83, C: 76.487, H: 311.189°;

[0020] Among them, L refers to color brightness, C refers to color saturation, and H refers to angle, using the CIE LCH standard.

[0021] Further, S021, the obtained eight color sequence combinations include: the first one: black, blue, red and yellow;

[0022] The second type: orange, black, blue, red and yellow;

[0023] The third type: purple, black, blue, red and yellow;

[0024] The fourth type: green, black, blue, red and yellow;

[0025] Fifth: black, blue, red, yellow, purple and green;

[0026] The sixth type: black, blue, red, yellow, purple and orange;

[0027] Seventh: black, blue, red, yellow, green and orange;

[0028] Eighth: black, blue, red, yellow, purple, green and orange.

[0029] Further, S031, 8 combination color tables are provided, and each combination color table corresponds to each color sequence combination.

[0030] Further, S032, the combination basis of the colors in the color sequence combination is: 1≤number of participating colors≤total number of colors in the color sequence combination, 0%≤color participation ratio≤100%, and the color participation ratio is set as needed.

[0031] Further, S033, the combined color table can be obtained according to the design: 1068 color blocks of the first color sequence combination, 1473 color blocks of the second color sequence combination, 2010 color blocks of the third color sequence combination, 1473 color blocks of the fourth color sequence combination, 2170 color blocks of the fifth color sequence combination, 2170 color blocks of the sixth color sequence combination, 2010 color blocks of the seventh color sequence combination, and 2170 color blocks of the eighth color sequence combination;

[0032] S034, specifying that the printing color sequence of the combined color table is the same as the color sequence combination.

[0033] Further, S051, determine to use a 5-color printing device, and print using the 5-color printing device, and the printing color sequence is based on the color sequence combination.

[0034] Further, S052, in the case of using a five-color printing device, when the number of printing colors is ≤5, select single wet-on-wet printing; when the number of printing colors is >5, select two wet-on-dry printings, that is, print five colors on the machine first, and then print the remaining colors on the machine a second time after the ink is dry.

[0035] The beneficial value achieved by the present invention is: by using the above method, the present invention reduces the loss in the color conversion process: the color matching method of directly converting the source color space to the target color space does not need to pass through the intermediate connection color space, avoids the loss of information of the image in the color conversion process, and has a more ideal restoration ability; expands the color rendering range of four-color offset printing: the present invention uses spot colors to participate in overprinting, and the spot colors are saturated and pure, which can expand the offset printing color gamut, so that the color reproduction color gamut of the printed image is large and the restoration effect is good, optimizes the "loss" phenomenon of the gradation of the traditional super-color gamut RGB image printing, makes the color of the printed product saturated and rich in layers, and improves the quality and work efficiency of the printing process. The method for printing and processing super-color gamut RGB images is added: the color channels separated by the color separation process used in the present invention can be edited in Photoshop, which refreshes the traditional high-fidelity printing that can only compensate for the part beyond the color gamut, but cannot control the degree and amount of its compensation. The pre-press file can be processed by compensation processing, so that the super-color gamut RGB images can also be consistent with the three of printed products, screen soft proofing, and digital proofing. The production cost is saved: the five-color printing method of the present invention can meet the requirements of 90% of the ultra-color gamut images on the market. Compared with the traditional high-fidelity printing, the requirements for equipment are lower. At the same time, the five-color printing makes the printing registration simpler and the screen collision phenomenon is less likely to occur, which can greatly reduce the frequency of production accidents. The above greatly improves the practical value of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 It is a schematic block diagram of the process of the present invention;

[0037] Figure 2 The present invention is a schematic block diagram of the implementation process. DETAILED DESCRIPTION

[0038] In order to make the above-mentioned objects, features and advantages of the present invention more understandable, the specific embodiments of the present invention are described in detail below in conjunction with the accompanying drawings. In the following description, many specific details are set forth to facilitate a full understanding of the present invention. However, the present invention can be implemented in many other ways different from this description, and those skilled in the art can make similar improvements without violating the connotation of the present invention, so the present invention is not limited by the specific embodiments disclosed below.

[0039] like Figure 1-Figure 2 As shown, the present invention discloses an offset color separation method for direct conversion between a source color space and a target color space, which comprises:

[0040] S010, mix blue, red, yellow, orange, green, purple and black inks (select matte paper), use a color display instrument (any specification and model) to display the colors, and collect, calculate, adjust and improve the color groups;

[0041] S011, based on the CIE LCH color model, the following are collected: black: L: 11.96, C: 0.447, H: 130.462°;

[0042] Blue: L: 51.00, C: 64.252, H: 238.324°;

[0043] Red: L: 50.24, C: 81.968, H: 346.432°;

[0044] Yellow: L: 87.28, C: 99.467, H: 93.112°;

[0045] Orange: L:72.32, C:94.714, H:65.085°;

[0046] Green: L: 65.25, C: 66.462, H: 180.422°;

[0047] Purple: L: 31.83, C: 76.487, H: 311.189°;

[0048] Where L refers to color brightness, C refers to color saturation, and H refers to angle, using the CIE LCH standard;

[0049] S020, designing a fixed group and a free group according to the collected hue data, wherein the fixed group includes: blue, red, yellow, and black, and the free group includes: orange, green, and purple, and the fixed group and the free group are freely combined to obtain 8 color sequence combinations, and the colors in the color sequence combinations are freely combined and / or used alone;

[0050] S021, the obtained eight color sequence combinations include: the first one: black, blue, red and yellow;

[0051] The second type: orange, black, blue, red and yellow;

[0052] The third type: purple, black, blue, red and yellow;

[0053] The fourth type: green, black, blue, red and yellow;

[0054] Fifth: black, blue, red, yellow, purple and green;

[0055] The sixth type: black, blue, red, yellow, purple and orange;

[0056] Seventh: black, blue, red, yellow, green and orange;

[0057] Eighth: black, blue, red, yellow, purple, green and orange;

[0058] S030, designing a combined color table of the color sequence combinations, wherein all combination situations of each color sequence combination can be within the combined color table;

[0059] S031, the combination color tables are provided with 8 kinds, and each combination color table corresponds to each color sequence combination;

[0060] S032, the combination basis of the colors in the color sequence combination is: 1≤number of participating colors≤total number of colors in the color sequence combination, 0%≤color participation ratio≤100%, and the color participation ratio is set as needed; in the case of uncertain color participation ratio, in theory, there are countless combinations of each color in each color sequence combination, and the color participation ratio here is flexibly set by the user according to actual conditions (cost, difference);

[0061] A large number of color simulations can be performed through the combined color table in S030. If the target is a commonly used color, the simulation cost can be reduced by adjusting the color participation ratio. In addition, the fixed group is combined with the free group to effectively avoid the situation where too few colors lead to the failure to simulate colors (common colors) or too many colors lead to high costs. Special colors (very rarely used colors) can also be taken into account by selecting more colors and participation ratios.

[0062] For example: S033, the combined color table can be designed to obtain: 1068 color blocks of the first color sequence combination, 1473 color blocks of the second color sequence combination, 2010 color blocks of the third color sequence combination, 1473 color blocks of the fourth color sequence combination, 2170 color blocks of the fifth color sequence combination, 2170 color blocks of the sixth color sequence combination, 2010 color blocks of the seventh color sequence combination, 2170 color blocks of the eighth color sequence combination, and the number of the above color blocks is calculated by the device according to the algorithm. The number of the above color blocks is obtained by the user adjusting the color participation ratio according to the needs, and the specific color participation ratio is not described in detail here. S034, it is stipulated that the printing color sequence of the combined color table is the same as the color sequence combination. It should be pointed out that the color block size is designed to be 8mm*8mm. This format can not only make all the color blocks arranged regularly, but also ensure the convenience of subsequent data collection;

[0063] S040, through several experiments, determine the publishing conditions, printing equipment conditions, the best printing density of each color ink and the optimal printing compensation curve;

[0064] S050, determine the printing equipment to be used based on the comparison of 5-color, 6-color, 7-color and 8-color printing equipment;

[0065] S051, determine to use a 5-color printing device, and use the 5-color printing device for printing, the printing color sequence is based on the color sequence combination, the reason for using the 5-color printing device is: under the premise of ensuring quality, high restoration and low cost effects are achieved, and the 5-color printing device is used for 15 micron high-precision printing during implementation;

[0066] S052, when a 5-color printing device is used, when the number of printing colors is ≤5, a single wet-on-wet printing is selected, and when the number of printing colors is >5, a double wet-on-dry printing is selected, that is, the first printing is for 5 colors, and the second printing is performed on the machine after the ink is dried to print the remaining colors. If a 6-color or 7-color printing device is used here, when printing the first to fourth color sequence combinations, the device will idle, wasting resources;

[0067] S060, after the printed combined color table is completely dry, the spectral data of the combined color table is collected and exported, and the RGB spectral data and hue data are compared with the collected spectral data and hue data. Through the RGB spectral data and hue data and the collected spectral data and hue data (that is, the color and hue data in the two spaces are comprehensively configured), a conversion model between the RGB spectral data and the collected spectral data (the conversion relationship between the two spaces) is established.

[0068] Furthermore, a conversion method and a production method for converting a super color gamut RGB image into black, blue, red, yellow, orange, green and purple are provided.

[0069] In practice, when the colors in the fixed group are simulated in conjunction with any one of the colors in the free group (i.e., 5 colors), the colors required by most customers can be simulated, and the simulation of 6 and 7 colors is performed for special needs. Such needs are very rare and have a narrow application range, and the 5-color device can also meet this need during secondary printing. On this basis, the use of the 5-color device can also be well coordinated with the fixed group plus the free group to simulate most colors, saving costs.

[0070] refer to Figure 2 As shown, the implementation includes: R&D test: using standard printing methods, operating the Komori printing press to perform sample printing tests, collecting equipment status data, determining the optimal printing density and printing compensation curve for each color, and after debugging, carrying out color table printing operations according to the color sequence of each color combination. The specifications are as follows:

[0071] Printing density:

[0072] color K C M Y V G O Wet density 1.95 1.70 1.60 1.15 1.65 1.45 1.31

[0073] Network expansion:

[0074] percentage 10 20 30 40 50 60 70 80 90 black 8.4 13.1 16.4 18.8 18.4 17.2 14.3 10.8 6.2 green 7.6 12.0 16.5 17.3 18.6 17.6 14.5 11.5 6.6 Taste 8.5 13.4 15.2 17.4 17 16 13.1 10.6 6.4 yellow 8.4 11.7 15.1 16.6 16.9 16.7 13.8 9.5 5.4

[0075] Publishing, developing and printing conditions:

[0076]

[0077] CTP publishing plate data:

[0078]

[0079]

[0080] Printing Linearization Curves:

[0081] Net value% 5 10 20 30 40 50 60 70 80 90 95 CTP / Black 2.7 5.5 11 16.7 23.5 31.0 39.7 49.2 59.1 71.6 81.5 CTP / Blue 3.1 6.0 11.5 17.5 24.0 32.0 40.6 50.7 61.5 75.5 86.5 CTP / Red 3.2 6.4 12.5 18.4 26.0 33.3 43 53.0 63.8 78.8 88.7 CTP / Yellow 3.1 6.0 10.6 15.6 21.2 27.4 35.4 46.3 59.0 75.3 85.5 CTP / Purple 5 10 20 30 40 50 60 70 80 90 95 CTP / Green 5 10 20 30 40 50 60 70 80 90 95 CTP / Orange 5 10 20 30 40 50 60 70 80 90 95

[0082] Data collection: After the color chart is completely dried for 48 hours, use a densitometer as a data collection tool to scan and collect the color chart spectral data and hue data (Lab value, LCH value).

[0083] It should be noted that the Lab standard value calculation method is: L value = (L value at white paper - L value at 100%) * (1-tone value) + L value at 100%, a value = a value at white paper * (1-tone value); b value = b value at white paper * (1-tone value). This calculation method is used in the data collection process of the color blocks of the color table in S060. This calculation method is used to determine all color block data and then convert the data on the color table into computer-readable data, and to establish the conversion relationship between the color table and the target color (customer-required color) after matching other color separation algorithms.

[0084] Establishing a color conversion relationship: According to the color separation algorithm, a color conversion relationship between the RGB color space and the 4-color, 5-color, 6-color, and 7-color of the present invention is established, that is, a color space conversion relationship of RGB to black-blue-red-yellow, RGB to orange-black-blue-red-yellow, RGB to green-black-blue-red-yellow, RGB to purple-black-blue-red-yellow, RGB to black-blue-red-yellow-purple-green, RGB to black-blue-red-yellow-purple-orange, RGB to black-blue-red-yellow-green-orange, and RGB to black-blue-red-yellow-purple-green-orange is established.

[0085] Take Shader syntax as an example: in RGB mode, color A is mixed with color B to obtain color C, and the transparency is Alpha (α), then A is the cache color, B is the current color, and C is the output color.

[0086] Adopt normal transparency blending algorithm: output color = current color * current transparency + (1-current transparency) * cache color. Shader: fixe4C = B*Bα+A*(1-Bα), the current color, cache color, and output color algorithm data are RGB components, namely R, G, B. Similarly, when the RGB color is the same as the color light components of the invention in RGB mode, the color conversion purpose can be achieved.

[0087] For example, using the crawler (Python) language, design a batch processing program for color conversion of RGB images based on the selectable color combination space. The logic is as follows:

[0088] Reading in a file

[0089] filename = 'file address'

[0090] Read in pictures and collect pixel RGB data

[0091] Image=Image.open(filename).convert('RGB')

[0092] RGB color value separation

[0093] r,g,b = image.split()

[0094] result_r,result_g,result_b=[],[],[]

[0095] Pixels are processed one by one

[0096] for pixel_r,pixel_g,pixel_b

[0097] in zip(r.getdata(),g.getdata(),b.getdata());

[0098] Convert LCH

[0099] L,C,H=colorsys.rgb_to_hsv(pixel_r255.,pixel_b / 255.,pixel_b / 255.)

[0100] Merge pictures

[0101] Image=image.merge('LCH',(L,C,H))

[0102] Output image

[0103] Image.save('target address')

[0104] Finish

[0105] In summary, the present invention reduces the loss in the color conversion process by using the above method: the color matching method of directly converting the source color space to the target color space does not need to go through the intermediate connection color space, avoids the loss of information of the image during the color conversion process, and has a more ideal restoration ability; expands the color rendering range of four-color offset printing: the present invention uses spot colors to participate in overprinting, and the spot colors are saturated and pure, which can expand the offset printing color gamut, so that the color reproduction color gamut of the printed image is large and the restoration effect is good, optimizes the "loss" phenomenon of the gradation of the traditional super-color gamut RGB image printing, makes the color of the printed product saturated and rich in layers, and improves the quality and work efficiency of the printing process. The method for printing and processing super-color gamut RGB images is added: the color channels separated by the color separation process used in the present invention can be edited in Photoshop, which refreshes the traditional high-fidelity printing that can only compensate for the part beyond the color gamut, but cannot control the degree and amount of its compensation. The pre-press file can be processed by compensation processing, so that the super-color gamut RGB images can also be consistent with the three of printed products, screen soft proofing, and digital proofing. The production cost is saved: the five-color printing method of the present invention can meet the requirements of 90% of the ultra-color gamut images on the market. Compared with the traditional high-fidelity printing, the requirements for equipment are lower. At the same time, the five-color printing makes the printing registration simpler and the screen collision phenomenon is less likely to occur, which can greatly reduce the frequency of production accidents. The above greatly improves the practical value of the present invention.

[0106] The above-described embodiments only express one or more implementation modes of the present invention, and the description thereof is relatively specific and detailed, but it cannot be understood as limiting the patent of the present invention. It should be pointed out that, for a person of ordinary skill in the art, several variations and improvements can be made without departing from the concept of the present invention, and these all belong to the protection scope of the present invention. Therefore, the protection scope of the present invention shall be subject to the attached claims.

Claims

1. An offset color separation method for direct conversion between a source color space and a target color space, characterized in that: include: S010, mix blue, red, yellow, orange, green, purple and black inks, use a color development instrument to develop colors, and collect, calculate, and adjust and improve color groups; S020, designing 8 color sequence combinations according to the collected hue data, wherein the colors in the color sequence combinations are freely combined or used individually; S030, designing a combined color table of the color sequence combinations, wherein all combination situations of each color sequence combination are within the combined color table; S040, through several experiments, determine the publishing conditions, printing equipment conditions, the best printing density of each color ink and the optimal printing compensation curve; S050, determine the printing equipment to be used based on the comparison of 5-color, 6-color, 7-color and 8-color printing equipment; S060, after the printed combined color table is completely dry, collecting and exporting the spectral data of the combined color table, comparing the RGB spectral data with the collected spectral data, and establishing a conversion model between the RGB spectral data and the collected spectral data through the RGB spectral data and the collected spectral data.

2. The offset color separation method for direct conversion between source color space and target color space according to claim 1, characterized in that: S011, based on the CIE LCH color model, the following are collected: black: L: 11.96, C: 0.447, H: 130.462°; Blue: L: 51.00, C: 64.252, H: 238.324°; Red: L: 50.24, C: 81.968, H: 346.432°; Yellow: L: 87.28, C: 99.467, H: 93.112°; Orange: L:72.32, C:94.714, H:65.085°; Green: L: 65.25, C: 66.462, H: 180.422°; Purple: L: 31.83, C: 76.487, H: 311.189°; Among them, L refers to color brightness, C refers to color saturation, and H refers to angle, using the CIE LCH standard.

3. The offset color separation method for direct conversion between source color space and target color space according to claim 1, characterized in that: S021, the obtained eight color sequence combinations include: the first one: black, blue, red and yellow; The second type: orange, black, blue, red and yellow; The third type: purple, black, blue, red and yellow; The fourth type: green, black, blue, red and yellow; Fifth: black, blue, red, yellow, purple and green; The sixth type: black, blue, red, yellow, purple and orange; Seventh: black, blue, red, yellow, green and orange; Eighth: black, blue, red, yellow, purple, green and orange.

4. The offset color separation method for direct conversion between source color space and target color space according to claim 1 or 3, characterized in that: S031, there are 8 combination color tables, each of which corresponds to each color sequence combination.

5. The offset color separation method for direct conversion between source color space and target color space according to claim 4, characterized in that: S032, the combination basis of the colors in the color sequence combination is: 1≤number of participating colors≤total number of colors in the color sequence combination, 0%≤color participation ratio≤100%, and the color participation ratio is set according to needs.

6. The offset color separation method for direct conversion between source color space and target color space according to claim 5, characterized in that: S033, the combined color table is designed to obtain: 1068 color blocks of the first color sequence combination, 1473 color blocks of the second color sequence combination, 2010 color blocks of the third color sequence combination, 1473 color blocks of the fourth color sequence combination, 2170 color blocks of the fifth color sequence combination, 2170 color blocks of the sixth color sequence combination, 2010 color blocks of the seventh color sequence combination, and 2170 color blocks of the eighth color sequence combination; S034, specifying that the printing color sequence of the combined color table is the same as the color sequence combination.

7. The offset color separation method for direct conversion between source color space and target color space according to claim 1, characterized in that: S051, determine to use a 5-color printing device, and print using the 5-color printing device, and the printing color sequence is based on the color sequence combination.

8. The offset color separation method for direct conversion between source color space and target color space according to claim 7, characterized in that: S052, when using a 5-color printing device, when the number of printing colors is ≤5, select single wet-on-wet printing; when the number of printing colors is >5, select two wet-on-dry printings, that is, print 5 colors on the first machine, and then print the remaining colors on the second machine after the ink is dry.

Images