Hello,
IEEE754 single-precision float has only 7 digit precision (log10(2^24) ≈ 7.225), since your conversion goes from gamma 1 to gamma 2 space, the operation you do is rgb^1 -> [Matrix1] -> [Matrix2]^-1 -> rgb^(1/2) Matrix1 and 2 are stored as doubles, but values are evaluated stage by stage and stored as floats, and therefore quantized. Then (1E-7)^(1/2) = 0.0003162277, if you move to 0..255 domain, (1E-7)^(1/2) * 255 = 0.0806 which is the maximum error you could get. The value you report is below this. What you see is a precision loss due to quantization to floats and gamma 1.0 space. Regards Marti Quoting Elle Stone <ellest...@ninedegreesbelow.com>: > On 09/15/2014 04:43 AM, marti.ma...@littlecms.com wrote: >> >> >> Hi Elle, >> >> I've tried RedHat 6 with gcc 4.4.7 and a old Mac PowerPC running darwin9 >> 4.0.1, in both cases I got no issues, and the return values were the >> expected ones. I wonder how you do to get this particular result: >> >>> 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) > > Hmm, my apologies, I might have mixed up the "to-from" commands in > the initial email. > > I checked conversions going from linear gamma to gamma=2.0 and also > going from gamma=2.0 to linear gamma. > > From linear gamma to gamma=2.0: > > transicc -c0 -t1 -w -i sRGB-V2-g10.icc -o sRGB-V4-g20.icc > LittleCMS ColorSpace conversion calculator - 4.2 [LittleCMS 2.06] > > Enter values, 'q' to quit > R? 255 255 0 > G? B? > R=255.0000 G=255.0000 B=0.0134 > > > From gamma=2.0 to linear gamma: > > 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? 255 255 0 > G? B? > R=255.0000 G=255.0000 B=0.0000 > > >> Maybe you used a >> different profiles? Could you check the profiles you sent? > I will sent you a new set in a private email. Results are the same > with both sets. The only difference is the naming convention. > > 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
