Color Halftoning Jan Allebach School of Electrical and Computer Engineering Purdue University West Lafayette, IN 47907 -2035 allebach@purdue. edu Advanced Topics in Digital Halftoning – 17 -19 October 2016
Big Picture of Pipeline for NPAC Halftoning Color Space Transformation Color Gamut Mapping and Tetrahedral Interpolation Source colors (s. RGB color space) XYZ color space Source s. RGB color space in YCx. Cz coordinates (8 s. RGB NPs: W, C, M, Y, CM, CY, MY, CMY) Color mapping Linearization Tone Correction Curve XYZ color space Table 1: NP area cover (NPAC) and the corresponding YCx. Cz coordinates when R, G, B changing form 0 to 255 Table 2: YCx. Cz coordinates of 8 NPs Destination color (CMY color space) Input continuous image YCx. Cz color space (8 Indigo NPs: W, C, M, Y, CM, CY, MY, CMY) + Tetrahedral interpolation Advanced Topics in Digital Halftoning – 17 -19 October 2016 DBS Output halftone image
Outline • Color Management – Step 1: Source and Destination Color Space Transfer – Step 2: Color Gamut Mapping – Step 3: Get the percentage of NPs based on Tetrahedral Interpolation Method – Separation Image Advanced Topics in Digital Halftoning – 17 -19 October 2016
Step 1: Color Space Transformation Source colors (s. RGB color space) XYZ color space Source s. RGB color space in YCx. Cz coordinates (8 s. RGB NPs: W, C, M, Y, CM, CY, MY, CMY) Getting from reflectance file Destination color (CMY color space) XYZ color space YCx. Cz color space (8 Indigo NPs: W, C, M, Y, CM, CY, MY, CMY) Advanced Topics in Digital Halftoning – 17 -19 October 2016 Color mapping
Substep 1: Source Color Transfer • Transfer from s. RGB to XYZ color space, which is independent device color space, then to YCx. Cz color space. or (R, G, B) in s. RGB color space (X, Y, Z) in XYZ color space s. RGB to XYZ Advanced Topics in Digital Halftoning – 17 -19 October 2016 (Y, Cx, Cz) in YCx. Cz color space XYZ to YCx. Cz
Source Gamut in YCx. Cz Color Space • Source gamut – The source image gamut is distributed in the cube composed by 8 NPs W C M Y CM CY MY CMY YCx. Cz coordinate Y Cx Cz 115. 9894 -5. 6843 E-14 0 91. 3255 -110. 6201 -38. 9720 33. 0333 169. 4708 -121. 1371 107. 6200 -58. 8507 160. 1091 8. 3694 58. 8507 -160. 1091 82. 9561 -169. 4708 121. 1371 24. 6639 110. 6201 38. 9720 0 Advanced Topics in Digital Halftoning – 17 -19 October 2016
Substep 2: Destination Color Transfer • 1). Get XYZ coordinates based on reflectance spectra – The HP Indigo reflectance can getting from ‘glossydata. txt’. – The illuminant and color matching functions (CMFs) are shown in bottom figures. D 50 Illuminance Color Matching Function (CMFs) Advanced Topics in Digital Halftoning – 17 -19 October 2016
Substep 2: Destination Color Transfer • 2). Transform from XYZ to YCx. Cz (C, M, Y) in printer color space (X, Y, Z) in XYZ color space Advanced Topics in Digital Halftoning – 17 -19 October 2016 (Y, Cx, Cz) in YCx. Cz color space
Destination Gamut in YCx. Cz Color Space • The region spanned by the printed colors in YCx. Cz is larger than the polyhedron defined by the 8 NPs • The mismatch is caused by printer • Ignore the error caused by the mismatch for now color W C M Y CM CY MY CMY YCx. Cz coordinate Y Cx Cz 98. 4799 3. 4639 E-14 1. 3323 E-14 26. 5236 -36. 8302 -77. 9275 20. 3525 90. 9011 3. 2936 84. 9223 -12. 2956 130. 0522 2. 8509 7. 8085 -22. 5646 19. 7844 -49. 8316 21. 0008 19. 4581 82. 719 29. 5053 2. 176 1. 2295 -0. 1831 Advanced Topics in Digital Halftoning – 17 -19 October 2016
Outline • Color Gamut Mapping – Step 1: Source and Destination Color Space Transfer – Step 2: Color Gamut Mapping – Step 3: Get the percentage of NPs based on Tetrahedral Interpolation Method – Separation Image Advanced Topics in Digital Halftoning – 17 -19 October 2016
Step 2: Color Gamut Mapping • The coordinates of W and CMY NPs are different between source and destination color space Advanced Topics in Digital Halftoning – 17 -19 October 2016
Step 2: Color Gamut Mapping • Idea: Only change the lightness and saturation during the mapping, but don’t change the hue of colors • Procedure – Mapping white and black NPs. – Remain the hue unchanged, based on the compression color mapping method to get the corresponding coordinates of all source colors Advanced Topics in Digital Halftoning – 17 -19 October 2016
Procedure of Color Gamut Mapping • Step 1: Mapping white and black NPs. • Step 2: Remain the hue unchanged, based on the compression color mapping method to get the corresponding coordinates of all source colors Step 2 Step 1 Mapping white and black NPs Advanced Topics in Digital Halftoning – 17 -19 October 2016 Color gamut mapping result
Color Gamut Mapping Result • Two example of color mapping – Mapping NP: magenta (as indicated by the green arrow) – Mapping NP: yellow (as indicated by the green arrow) o (33. 03, 169. 47, -121. 14) o (46. 46, 27. 24, -20. 16) Advanced Topics in Digital Halftoning – 17 -19 October 2016 o (107. 62, -58. 85, 160. 11) o (71. 91, -18. 65, 69. 62)
Color Gamut Mapping Result • Do the color mapping for each color in s. RGB color space, finally, we can get the mapping color in destination YCx. Cz color space. As shown in the figure: Source YCx. Cz color space Advanced Topics in Digital Halftoning – 17 -19 October 2016 Mapping result
Outline • Color Gamut Mapping – Step 1: Source and Destination Color Space Transfer – Step 2: Color Gamut Mapping – Step 3: Get the percentage of NPs based on Tetrahedral Interpolation Method – Separation Image Advanced Topics in Digital Halftoning – 17 -19 October 2016
Step 3: Tetrahedral Interpolation • Based on the mapping YCx. Cz coordinate of each source color, we will get the percentage of destination NPs based on tetrahedral interpolation. • The destination color gamut can be divided into several tetrahedrons Advanced Topics in Digital Halftoning – 17 -19 October 2016
Step 3: Tetrahedral Interpolation • 1). Determine which Tetrahedron is P(Y, Cx, Cz) belonging to – Each tetrahedron have four vertexes, we can get three linearly independent vectors – Any point /color in a tetrahedron can be expressed by three vectors The coordinate (Y, Cx, Cz) can be expressed by: tetrahedron z x (Y, Cx, Cz) * o Advanced Topics in Digital Halftoning – 17 -19 October 2016 y
Step 3: Tetrahedral Interpolation 1). Determine which Tetrahedron P(Y, Cx, Cz) belongs to z x (Y, Cx, Cz) * o P(Y, Cx, Cz) should belong to tetrahedron (W, C, M, CMY), if it satisfies the following constrains: Advanced Topics in Digital Halftoning – 17 -19 October 2016 y
Look Up Table (LUT) of Color Gamut Mapping Percentage of NPs One-to-one correspondence R G B NP(W) NP(Y) NP(CY) NP(MY) NP(CMY) Y 0 0 0 0 0 1 2. 175965 0 0 1 0 0 0 0. 003922 0. 996078 2. 247246 RGB … … … combination 255 254 0. 996078 0. 003922 0 0 0 98. 173543 in total 255 255 1 0 0 0 0 98. 479925 Mapping coordinate in destination color gamut Cx 1. 229499 1. 581153 … 0. 356475 0 Cz -0. 183083 -0. 169449 … 0. 012916 0 Express the destination NPs by s. RGB W(216, 223) C(0, 79, 177) M(187, 1, 44) Y(236, 188, 0) CM(6, 3, 41) Advanced Topics in Digital Halftoning – 17 -19 October 2016 CY(0, 67, 11) MY(186, 2, 2) CMY(7, 4, 5)
Outline • Color Gamut Mapping – Step 1: Source and Destination Color Space Transfer – Step 2: Color Gamut Mapping – Step 3: Get the percentage of NPs based on Tetrahedral Interpolation Method – Separation Image Advanced Topics in Digital Halftoning – 17 -19 October 2016
Separation Image • Based on the LUT of color gamut mapping, we can generate the separation image, which each separation image corresponding to a NP Getting the percentage of NPs corresponding to each pixel Original Image Table 1 Advanced Topics in Digital Halftoning – 17 -19 October 2016 W C Y M CM CY MY C MY
Separation Image • 8 Separation images corresponding to 8 NP channels • Black means the percentage of NP almost be 0 W C M Y CM CY MY CMY Advanced Topics in Digital Halftoning – 17 -19 October 2016
Скачать презентацию Color Halftoning Jan Allebach School of Electrical and
b870162b6fb7fa55c3cdd09b855c666c.ppt