On 04/02/2014 04:43 PM, marti.ma...@littlecms.com wrote: > > > Hi, > > The ICC file format stores matrix of sRGB as 15.16 fixed point, that > means the precision you have is 1 / ((1 << 16) - 1) which is > about 1.5e-5 Since evaluating the matrix implies multiplication > and addition, the final precision is reduced to 1e-3, that means > only two decimal places are accurate. This is not a major problem > because is far below 1 dE. >
Hi Marti, My apologies, I didn't understand all of what you said. Are you saying that conversions to sRGB are only accurate to two decimal places, meaning somewhat less than five significant digits in terms of floating point precision? Previously I tested 32-bit floating point round-trip conversions from ProPhotoRGB to sRGB and back using your wonderful unbounded mode ICC profile conversions. The round trip conversion (using only profiles from disk and involving 4 sequential ICC profile conversions) is 100% lossless: http://ninedegreesbelow.com/photography/lcms2-unbounded-mode.html#pro2srgb I would have expected rounding errors during the unbounded mode round trip conversions if the conversions were accurate only to two decimal places. How is the round-trip conversion lossless despite only two decimal places of accuracy? My ability to visualize/grasp the math fails at this point. > > The in-memory built-in is not quantized and does not have this > problem, as soon as you serialize it, you got quantization. > Are you saying the conversion to the built-in profile that was created and stored in memory is more accurate? or less accurate? than the conversion to the same profile that's been saved to disk? It seems to me that it's less accurate. Here's another example of transicc output with less confusing RGB values. In this case transicc converts from a linear gamma version of the lcms built-in sRGB (cmsCreate_sRGBProfile) that had been created and saved to disk. When converting from linear to regular sRGB (both profiles using the same primaries and white point), the resulting output RGB values should match the source RGB values for all source values like (0,0,0), (255,255,255), (255,0,0), etc. Converting to the built-in lcms sRGB profile that had been created and saved to disk: $ transicc -w -c0 -b -t1 -i sRGB-lcms-primaries-wtpt-V4-g100.icc -o sRGB-lcms-primaries-wtpt-V4-srgbtrc.icc 255 255 255 R=255.0000 G=255.0000 B=255.0000 255 0 0 R=255.0000 G=0.0000 B=-0.0000 0 255 0 R=-0.0002 G=255.0000 B=-0.0000 0 0 255 R=-0.0001 G=0.0000 B=255.0000 The figures above are accurate to almost 4 decimal places. Converting to the lcms built-in sRGB profile: $ transicc -w -c0 -b -t1 -i sRGB-lcms-primaries-wtpt-V4-g100.icc -o *sRGB 255 255 255 R=254.9988 G=255.0029 B=254.9981 0 255 0 R=0.0254 G=255.0008 B=0.0153 0 0 255 R=0.0034 G=0.0343 B=254.9988 The figures above are accurate to 2 decimal places. So it looks to me like conversions to the profile that had been saved to disk are more accurate. GIMP from git does 32-bit unbounded mode floating point conversions from the source color space profile (which may be a linear gamma color space) to sRGB (and possibly to linear gamma sRGB), keeping the out-of-gamut RGB values rather than clipping them. So the conversions do need to be accurate. Can I modify the GIMP ICC profile conversion code to make conversions to the GIMP built-in sRGB *exactly* match conversions to the same profile that has been saved to disk? Perhaps there is an optimization switch that needs to be set or unset? Or is the only alternative rewriting the GIMP code to save the built-in profile to disk and then convert to the disk copy instead of to the copy in memory? > So, that is because the XYZ representation of file format. You > may want to use LUT based profiles instead. > This possible sRGB LUT profile, is there one that can be made with LCMS? Can it be used for unbounded mode ICC profile conversions? When converting to the LUT profile in memory, will the accuracy be better than or at least as good as the accuracy when converting to a matrix sRGB profile from disk? > Regards > Marti > Best regards, Elle ------------------------------------------------------------------------------ _______________________________________________ Lcms-user mailing list Lcms-user@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/lcms-user
