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
