Re: [Lcms-user] Profile linearization and softproofing

[Aurélien P. <re...@au...>]

Hi,

Le 04/11/2018 à 23:38, Graeme Gill a écrit :
> 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.
I know, for now I'm trying to debug and understand what's going on,
elegant solutions will come later.
>> 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.

Oh God, yes, you are right. darktable labels it L but it is L* (looking
at the code). So I was mislead the whole time and everything is
perfectly normal.

Thanks a lot for your time,

Aurélien.

>
> ( L* = linear_light^1/3 * 116 - 16)
>
> Cheers,
> 	Graeme Gill.
>