тээк значит механизм проблемы найдено на photo.net. переводить лень, но общий смысл в том что фотошоп не умеет работать с негативныим значениями RGB, которые возникают в самых насыщенных цветах при конвертировании туда-обратно, а RawTherapee умеет. хотя вероятность найти на фотографиях в реальной жизни такие чистые и насыщенные цвета приближается к 0, все же както обидно както подсознательно...
[quot]Bill C , Oct 07, 2013; 07:02 p.m. I took your example of "blue colors beginning from 0 0 255 to somewhere 0 0 170 will be mapped after LAB to uniform flat 31 0 168," and ran it through Bruce Lindbloom's calculator.
An original Prophoto RGB 0, 0, 255 maps to LAB 0.08, 90, -172. Since Photoshop can't go this far, I'd expect it to use LAB = 0, 90, -128. Converting THAT back to Prophoto RGB actually calculates to 30, -18, 168. But, since RGB values can't be negative, it presumably will clip to 30, 0, 168, which is pretty close to what you say you actually got.
So it seems pretty clear that your "errors" in Photoshop are because it has limited LAB a* and b* values to the range of -128 to 127. Presumably RawTherapee doesn't set those limits.
I think that Mark M has stated the situation very well. Regarding the imaginary primaries,it is impossible to have an actual such light source. In a way, they are like something I saw in cartoons as a kid - someone finds a "black light," and when they point it at something, it casts a black spot. Imaginary primaries are something of this sort; they have no real physical meaning, but are very useful mathematically.[/quot]
LAB 
