Hi, You are right; absolute colorimetric is not stored in the profile. Instead, BToA1 tag is used with a white point scaling. The algorithm is rather simple, it is performed on XYZ color space and it is only a plain scaling.
AbsCol = RelCol * (MediaWhite / D50) For each of the tristimulous: Xabs = Xrel * (Xwtpt/0.9642) Yabs = Yrel * (Ywtpt/1.0) Zabs = Zrel * (Zwtpt/0.8249) Latest revision of ICC spec, however, specifies a DToB3 tag, which stores floating-point absolute colorimetric intent. But this is an optional tag and AFAIK very few CMM (if any) actually supports this tag (lcms doesn't) Hope this helps Marti Maria The littleCMS project www.littlecms.com -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of John Weissberg Sent: Friday, September 07, 2007 2:14 PM To: lcms-user@lists.sourceforge.net Subject: [Lcms-user] [Fwd: Using BtoA1 table with rendering intent absolute] Consider the following case: Converting from Lab -> CMYK using the BtoA tables of an ICC profile with "Absolute Colorimetric" as a rendering intent. As I understand it, the ICC profile does not contain an Absolute colorimetric BtoA table. Rather, it will use the relative colorimetric BtoA table, along with whitepoint data, for any look-ups it makes. Can anyone explain what algorithm is used for these conversions between relative and absolute colorimetric? Is it simply adding/subtracting the DeltaE between the whitepoint and 100,0,0 ? Is this delta value added for all points in the color space? Only for points where L is above a certain limit? Any information is appreciated. -- John Weissberg PerfectProof - Accuracy Made Easy www.perfectproof.com www.printers-website.com www.proofmaster.net pakready.perfectproof.com USA (888) 623 3662 Tel. (888) 623 3662 FAX Europe +32 15 272502 Tel. +32 15 273740 FAX -- John Weissberg PerfectProof - Accuracy Made Easy www.perfectproof.com www.printers-website.com www.proofmaster.net pakready.perfectproof.com USA (888) 623 3662 Tel. (888) 623 3662 FAX Europe +32 15 272502 Tel. +32 15 273740 FAX ------------------------------------------------------------------------- This SF.net email is sponsored by: Splunk Inc. Still grepping through log files to find problems? Stop. Now Search log events and configuration files using AJAX and a browser. Download your FREE copy of Splunk now >> http://get.splunk.com/ _______________________________________________ Lcms-user mailing list Lcms-user@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/lcms-user ------------------------------------------------------------------------- This SF.net email is sponsored by: Microsoft Defy all challenges. Microsoft(R) Visual Studio 2005. http://clk.atdmt.com/MRT/go/vse0120000070mrt/direct/01/ _______________________________________________ Lcms-user mailing list Lcms-user@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/lcms-user
