Hi Elle,
Sorry, with the profiles you sent and transicc from lcms2-2.6, this does not happen. I got the expected values. Please check the version of lcms2 you are using. BTW, -c0 is not needed in lcms2 and may cause problems. This may be the source of the issue in versions previous to 2.6 M:\lcms2-2.6\bin>transicc -c0 -t1 -w -o sRGB-V2-g10.icc -i sRGB-V4-g20.icc LittleCMS ColorSpace conversion calculator - 4.2 [LittleCMS 2.06] Enter values, 'q' to quit R? 255 G? 0 B? 0 R=255.0000 G=0.0000 B=-0.0000 Enter values, 'q' to quit R? 255 G? 255 B? 0 R=255.0000 G=255.0000 B=0.0000 Enter values, 'q' to quit Regards Marti Quoting Elle Stone <ellest...@ninedegreesbelow.com>: > Transicc and also tificc both produce oddly wrong results when > converting near-zero and zero RGB channel values from one RGB matrix > profile to a second RGB matrix profile with a different TRC but the same > colorants (as per converting a linear gamma sRGB image to regular sRGB). > > Below are some transicc examples. The profile names indicate the profile > spec version and the TRC. All the profiles were made using LCMS2 and > have identical colorants and chad/white point. > > First case: Both profiles have a true gamma TRC (V4 vs V2 makes no > difference): > > transicc -c0 -t1 -w -o sRGB-V2-g10.icc -i sRGB-V4-g20.icc > > 255 255 0 > R=255.0000 G=255.0000 B=0.0134 (the B channel should be zero) > > 255 0 0 > R=255.0000 G=0.0282 B=0.0000 (the G channel should be zero) > > 255 0 255 > R=255.0000 G=0.0331 B=255.0000 (the G channel should be zero) > > 255 0.001 0.001 > R=255.0000 G=0.5037 B=0.5051 (G and B should be equal to each other) > > > Going the other way produces negative RGB channel values: > > transicc -c0 -t1 -w -i sRGB-V4-g20.icc -o sRGB-V2-g10.icc > > 255 0 0 > R=255.0000 G=0.0000 B=-0.0000 (negative B channel) > > 255 0.001 0.001 > R=255.0000 G=0.0000 B=-0.0000 (negative B channel) > > 70000 0.001 0.001 > R=19215684.3750 G=-0.3511 B=0.0270 (the G and B channel values should be > equal and positive) > > > Second case: The destination profile has the V4 sRGB parametric TRC: > > transicc -c0 -t1 -w -i sRGB-V4-g10.icc -o sRGB-V4-srgbtrc.icc > > 0.0001 255 0.0001 > R=0.0015 G=255.0000 B=0.0013 (R and G channels should be equal) > > 255 0 0 > R=255.0000 G=0.0000 B=-0.0000 (negative B) > > 0 255 0 > R=-0.0002 G=255.0000 B=-0.0000 (negative B) > > 0 0 255 > R=-0.0001 G=0.0000 B=255.0000 (negative R) > > > When using GIMP 2.9 from git (which uses LCMS2) to do the same profile > conversions, it turns out that the "zero" and "-zero" channel values are > actually slightly positive and slightly negative at floating point > precision. At 16- and 32-bit integer precision the "zero" values are > slightly positive. > > If the destination profile has the "V4-when-saved-as-V2" 4096-point sRGB > TRC, then there are no negative channel values, and the near-zero > channel values are always equal to each other if the channel values are > equal to each other in the source color space. However, the resulting > RGB channel values in the destination color space are quantized, which > means information is lost. > > Why the odd conversion results for zero and near zero channel values? Is > there a workaround? Or perhaps I'm using the wrong LCMS ICC profile > conversion parameters? > > Best regards, > Elle > > > ------------------------------------------------------------------------------ > Want excitement? > Manually upgrade your production database. > When you want reliability, choose Perforce > Perforce version control. Predictably reliable. > http://pubads.g.doubleclick.net/gampad/clk?id=157508191&iu=/4140/ostg.clktrk > _______________________________________________ > Lcms-user mailing list > Lcms-user@lists.sourceforge.net > https://lists.sourceforge.net/lists/listinfo/lcms-user ------------------------------------------------------------------------------ Want excitement? Manually upgrade your production database. When you want reliability, choose Perforce Perforce version control. Predictably reliable. http://pubads.g.doubleclick.net/gampad/clk?id=157508191&iu=/4140/ostg.clktrk _______________________________________________ Lcms-user mailing list Lcms-user@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/lcms-user
