How to use the Perceptual Reference Medium Gamut

Transcription

1 How to use the Percetual Reference Medium Gamut Phil Green Gjøvik University College, Norway

2 Outline What is a colour gamut? Gamut maing in ICC workflows What is the PRMG? Gamut maing in ICC v4 New gamut boundary encoding

3 What is a colour gamut? Range of colours that exist within a colour encoding Usually defined as the boundary of the encoding in CIELAB colour sace Colour gamut can be obtained from characterization data or the ICC rofile for the colour encoding Colour gamut of FOGRA51

4 ICC colour management workflow In an ICC color managed workflow, rofiles convert between source and destination color encodings Source rofile Destination rofile Source color data Destination color data The characterization model, viewing condition adjustments and gamut maing are incororated into the transform. The Percetual intent also includes reference adjustments

5 ICC v2 Percetual gamut maing In ICC v2: The source rofile does not know the destination gamut Source rofile Destination rofile? Source color data Destination color data The destination rofile does not know the source gamut Result: rofile creator has to guess what gamut to match

6 What is the Percetual Reference Medium Gamut? The ICC v4 secification introduced a reference intermediate gamut PRMG This was ublished as ISO :2005 and corresonds aroximately to the gamut of real surface colours

7 Using the PRMG in ICC workflows Colorimetric rendering intents: Entire CIELAB encoding is maed to outut gamut Percetual rendering intent: Profiles ma to and from PRMG Source rofile Destination rofile Source rofile Destination rofile

8 Gamut maing in ICC v4 Colorimetric intent Source transform converts source encoding to CIELAB PCS Destination transform clis PCS colorimetry to destination gamut

9 Gamut maing in ICC v4 Percetual intent Source transform re renders source encoding to PRMG using reference criterion Destination transform mas from PRMG to destination gamut using subjective accuracy criterion

10 Gamut maing in ICC v4 How to use the PRMG Use the srgb v4 reference rofile to render to the PRMG Destination transform mas from PRMG to destination gamut using subjective accuracy criterion

11 Using the PRMG Use the srgb v4 reference rofile to ma the source image to the PRMG

12 Using the PRMG Use an outut rofile that renders from the PRMG to the outut encoding

13 Using the PRMG How to test that the outut rofile renders from the PRMG to the outut encoding? Use rofile with good PRMG round tri interoerability Round tri reort from Profile Dum (available on ICC web site)

14 New gamut boundary encoding in iccmax ICC GamutBoundaryDescritionTye Byte Position Field Length (bytes) Content Encoded as 'gbd ' ( h) tye signature Reserved, shall be Number of PCS Channels (P) uint16number Number of Device Channels (Q) uint16number Number of vertices (V) uint32number Number of faces (F) uint32number F*12 F*12 Array of vertex IDs for each face uint32number 20+F* F*12+V*P*4 V*P Array of PCS coordinates for each vertex float32number 20+F*12+V*P*4 end V*Q Array of device coordinates for each vertex float32number

15 Encoding the gamut boundary A gamut boundary can be encoded as a list of vertices on gamut surface + a list of indices into the vertices list which form triangular faces on surface Vertices = Faces = n n n b a L b a L b a L b a L b a L L a b n n n

16 Extracting GBD from a rofile 1) Assign rofile to gamut boundary target (Green, 2002) and convert to CIELAB as destination with Colorimetric intent 2) Comute array of faces by steing through atches in gamut boundary target (ensuring indices in each face are in clockwise order) 3) Write vertex and face arrays into GBD structure

17 Gamut comarison 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

18 Gamut comarison index

19 Gamut comarison index

20 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)

21 Gamut comarison index (V i / V x ) how much of gamut x is outside the intersection (V i / V y ) how much of gamut y is outside the intersection

22 Summary The Percetual Reference Medium Gamut is a rendering target for ICC v4 workflows It enables consistent and otimal maing between source and destination encodings Percetual intent Profiles are available to render to and from the PRMG PRMG comatibility can be easily evaluated ICC has introduced a new method of encoding the gamut boundary in iccmax ISO, ICC and CIE are in the rocess of defining standard methods of describing a gamut boundary and comaring two gamuts

23 Thank you!