Gamut index and other metrics for comparing colour gamuts

Transcription

1 ICC Grahic Arts Colour Exerts Day, Frankfurt 12 th June 201 Gamut index and other metrics for comaring colour gamuts Kiran Deshande 1,2 & Phil Green 1London College of Communication London 2Chesaeake, UK Gjøvik University College, Norway

2 Outline Background Proosed metrics and imlementation Evaluation of metrics: RPC-8 candidate exchange sace Alications Summary

3 Background We often need to comare two or more colour gamuts The difference in gamut volumes alone is a oor indicator It can't tell if the gamuts intersect sufficiently to meet the reroduction aims Two gamuts having the same volume may not coincide Metric needs to include both relative volume and intersection

4 Gamut comarison index

5 Gamut comarison index Gamut comarison index between two gamuts shows how closely they match GCI = V V i x V V i y Vx : gamut volume of the medium x Vy : gamut volume of the medium y Vi : volume of intersection of the two gamuts (Vx Vy)

6 New metrics [V i / V x ]: how much of gamut x is covered by gamut y [V i / V y ]: how much of gamut y is covered by gamut x [(V x V i ) / V x ]: how much of gamut x is outside the gamut y [(V y V i ) / V y ]: how much of gamut y is outside the gamut x [V x / V y ]: ratio of gamut x to gamut y [V y / V x ]: ratio of gamut y to gamut x

7 Finding the volume 1) Use any method of determining coordinates (vertices) on gamut boundary 2) Determine surface triangles (e.g. by n- dimensional Delaunay algorithm) ) Generate list of triangle faces 4) Determine set of tetrahedra that fill gamut volume 5) Comute volumes of tetrahedra

8 Finding the volume For this fragment of gamut surface Vertices = Faces = Tetrahedra = b a L b a L b a L b a L L a b

9 Evaluation Selected 7 samle data sets in ISO DIS 159-1: reference rinting conditions (RPCs) which are aroximately equal in searation of rimaries RPC-1 Coldset Newsaer RPC-2 Heatset Newsaer RPC- Premier Uncoated Paer RPC-4 Suer-Calendared Paer RPC-5 Publication Coated Paer RPC-6 Premium Coated Paer RPC-7 Extra Large Gamut Gamut metrics calculated against RPC-8 Candidate (Exchange Sace)

10 Evaluation Gamut comarison index RPC-8 Candidate vs. other RPCs Gamut comarison index RPC-1 RPC-2 RPC- RPC-4 RPC-5 RPC-6 RPC-7 Reference rinting conditions

11 Alications - Digital Printing Three digital rinting resses comared against the digital rinting exchange sace rofile (RPC-8 Candidate) Digital Press A: CMYK Digital Press B: CMYK Digital Press B: CMYKOGV Gamut boundaries derived using convex hull, gamut volume comuted in CIELAB sace Gamut metrics calculated in Matlab

12 Alications - Digital Printing

13 Alications - Digital Printing Gamut x Gamut y Gamut x Volume (V x ) Gamut y Volume (V y ) Volume of Intersection (V i ) Gamut Comarison Index (V i / V x )(V i / V y ) Proortion of Gamut x outside the Gamut y (V x V i )/ V x Proortion of Gamut y outside the Gamut x (V y V i )/ V y Digital Press A - CMYK Exchange Sace (PRMG) % 49% Digital Press B - CMYK Exchange Sace (PRMG) % 55% Digital Press B - CMYKOGV Exchange Sace (PRMG) % 21%

14 Alications Proofing Gamut Volumes FOGRA9L = Proofer A = Proofer B = GCI Values FOGRA9L & Proofer A = FOGRA9L & Proofer B = 0.520

15 Alications RPC Validation Gamut Volumes FOGRA9L = Offset Press1 = Offset Press2 = GCI Values FOGRA9L & Press1 = FOGRA9L & Press2 = 0.86

16 Alications Photograhic Images Comare an image gamut to a device gamut degree of mismatch can guide reroduction rocess e.g. scene-referred colorimetry of image against srgb gamut GCI = 0. Eric Walowit 1 st Jan 201

17 Summary Gamut comarison index a single metric for comaring two gamuts redicting the ability of one gamut to match colours in another gamut Gamut metrics work well with standard methods of encoding gamut boundaries Metrics values can be extracted and comuted automatically (e.g. from ICC rofile) without requiring visual comarison Metrics would be useful to guide the reroduction rocess: gamut-maing and re-rendering

18 Thank You!