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.
The in-memory built-in is not quantized and does not have this
problem, as soon as you serialize it, you got quantization.
So, that is because the XYZ representation of file format. You
may want to use LUT based profiles instead.
Regards
Marti
Quoting Elle Stone <ellest...@ninedegreesbelow.com>:
> There are discrepancies when converting to an lcms built-in profile,
> compared to converting to the same profile previously saved to disk.
>
> I noticed the discrepancies while testing GIMP 2.9 16-bit integer and
> 32-bit floating point ICC profile conversions. The same discrepancies
> happen when using tificc and also when using transicc. So the
> discrepancies seems to be consistent across various ways of accessing
> lcms built-in profiles.
>
> Here is transicc output showing the discrepancies:
>
> Converting to a copy of the lcms built-in sRGB profile that was
> previously saved to disk ("-o sRGB-lcms-built-in.icc"):
>
> $ transicc -w -t1 -c0 -b -i sRGB-elle-V4-g100.icc -o sRGB-lcms-built-in.icc
> LittleCMS ColorSpace conversion calculator - 4.2 [LittleCMS 2.06]
>
> Enter values, 'q' to quit
> R? 255 255 255
> G? B?
> R=255.0019 G=254.9968 B=255.0028
>
> Enter values, 'q' to quit
> R? 255 0 0
> G? B?
> R=255.0450 G=0.0839 B=-0.0372
>
> Enter values, 'q' to quit
> R? 0 255 0
> G? B?
> R=-1.3516 G=254.9937 B=-0.3004
>
> Enter values, 'q' to quit
> R? 0 0 255
> G? B?
> R=0.0858 G=0.0080 B=255.0143
>
> Converting to the transicc built-in sRGB profile ("-o *sRGB"):
>
> $ transicc -w -t1 -c0 -b -i sRGB-elle-V4-g100.icc -o *sRGB
> LittleCMS ColorSpace conversion calculator - 4.2 [LittleCMS 2.06]
>
> Enter values, 'q' to quit
> R? 255 255 255
> G? B?
> R=255.0007 G=254.9997 B=255.0009
>
> Enter values, 'q' to quit
> R? 255 0 0
> G? B?
> R=255.0428 G=0.1139 B=-0.0725
>
> Enter values, 'q' to quit
> R? 0 255 0
> G? B?
> R=-1.3259 G=254.9944 B=-0.2851
>
> Enter values, 'q' to quit
> R? 0 0 255
> G? B?
> R=0.0893 G=0.0423 B=255.0130
>
>
> Here's the code that I used to save a copy of the lcms built-in sRGB
> profile to disk:
>
> After declaring/defining the variables:
>
> profile = cmsCreate_sRGBProfile();
> cmsWriteTag(profile, cmsSigCopyrightTag, copyright);
> cmsWriteTag(profile, cmsSigDeviceMfgDescTag, manufacturer);
> description = cmsMLUalloc(NULL, 1);
> cmsMLUsetASCII(description, "en", "US", "sRGB-lcms-built-in.icc.");
> cmsWriteTag(profile, cmsSigProfileDescriptionTag, description);
> filename = "sRGB-lcms-built-in.icc";
> cmsSaveProfileToFile(profile, filename);
> cmsMLUfree(description);
>
>
> Elle Stone
>
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