Re: [Lcms-user] Profile linearization and softproofing

[Graeme G. <gr...@ar...>]

Aurélien Pierre wrote:

Hi,

> I understand the theory of profiling and error reverting with LUT or TRC, it's on the
> actual implementation that I'm lost, and what actually happens in the pixelpipe.

seems then that this is really a question for those who understand
darktable's internals.

Note though, that changing the display profile to allow for a change
in the source color encoding, is 100% wrong.
Instead you need to make the source color encoding match the
source profile. Note that in this context "source" means the
input to the conversion to display space made using lcms,
which is the output of darktables photo processing.

> output Lab values are off, too bright : data 50 % Lab (measured on a RAW photo of a ColorChecker grey
> patch with white patch properly exposed) is shown as 75 % screen Lab. Given that 0.5^(1/2.4) = 0.75,
> that sounds like a gamma issue.

Hmm. That's an odd way of measurement, given that gamma usually relates linear light to
gamma encoded, while L* is a non-linear light measure.

Are you sure that your "50 % Lab" is actual L*, and not a linear light % ?

i.e. a 50% linear light value would be expected to measure approximately
L* 76 if white is L*100.

( L* = linear_light^1/3 * 116 - 16)

Cheers,
	Graeme Gill.