On 08/29/2015 03:54 PM, Wolthera wrote:
> On Sat, Aug 29, 2015 at 7:42 PM, <marti.ma...@littlecms.com
> <mailto:marti.ma...@littlecms.com>> wrote:
>
> Regarding TRC, this is just a stage of the internal conversion
> tables present on the profiles. Legacy matrix-shaper profiles like
> sRGB or AdobeRGB relies more on those curves, but modern V4 profiles
> may chain CLUT, parametric curves and matrices in a complex way and
> then there is no real physical sense on those elements other that
> provide the math. Things will go works on iccMAX, where there is a
> programming language on calc elements. If you plan to only work with
> that very old profiles, messing with TRC is ok. For new profiles the
> possibilities are so huge that is not practical to worry the user
> with implementation details. Gamma curves in those cases are not a
> single element, but are distributed within many others.
Standard RGB working spaces are defined by Red, Green, and Blue
primaries, a color space white point and a TRC that is the same in all
three channels, plus chromatic adaptation from the color space white
point to the profile illuminant.
Do phrases like "legacy matrix-shaper profiles" and "only work with that
very old profiles" - mean that using standard RGB working spaces (and
linear gamma versions of same) is outdated or substandard for RGB
digital painting or image editing, such as one might do using Krita,
GIMP, or PhotoShop?
>
> Well, I am not very interested in the exact implementation either. As a
> Krita programmer, I need three things: A way to linearise values from a
> color in a given color space, a way to de-linearise linear values into
> colors in a given colorspace(both for luminance calculations in
> filters), and a way to give feedback to the user on what they should
> expect with this profile. This would be either a curve widget but I am
> suspecting that a black-to-white gradient drawn in the given colorspace
> and then transformed to sRGB for display ought to give plenty of
> feedback as well.
AFAIK, the "legacy matrix-shaper profiles" (such as sRGB, AdobeRGB,
Rec.2020, Rec.709, ACEScg, ProPhotoRGB, and etc) have TRCs that are
variously defined by either point curve approximations of well-defined
"sets" of equations (eg linear in the shadows, a gamma curve elsewhere),
or by parametric equations that express? approximate? these "sets" of
equations (Table 52 in the LCMS 2.8 API), or by simple gamma curves.
Given a standard RGB working space profile with a parametric curve TRC,
for example the sRGB TRC, defined using:
cmsFloat64Number srgb_parameters[5] = { 2.4, 1.0 / 1.055, 0.055 /
1.055, 1.0 / 12.92, 0.04045 }
is there a way to retrieve these parameters from the profile and then
use them to generate a point curve approximation that can be plotted?
Best regards,
Elle Stone
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