Re: [Gimp-developer] Questions about the histogram

On 08/11/2019 04:34 PM, Elle Stone wrote:
> On 07/30/2019 09:07 AM, Elle Stone wrote:
> 
>> On approximately 08/11/2019, Steve Pricks wrote:
> (text below copied from Nabble)
>> However, I have not seen buttons like these in any other image editing 
>> software, neither commercial nor free/libre (not even in RT with its 
>> technology filled GUI).

An actually helpful evaluation of and suggestions for improving the 
histogram buttons does require a good understanding of how GIMP works 
"under the hood".

* Many operations (eg Normal/Addition?Subtract/Multiply/Divide blend 
modes (and a few other blend modes), Gaussian and other blurs, 
down-scaling, Exposure, radiometrically meaningful Levels adjustments 
including white balancing, Channel mixing, conversions to Luminance 
black and white - the list goes on and on) *require* being done on 
linear RGB to avoid "gamma" artifacts and produce radiometrically 
correct results.

* Many other operations are *designed* to work on perceptually uniform 
RGB, for which purpose GIMP uses the sRGB TRC instead of the more 
appropriate (imho) Lab "L" TRC. These operations include Luma 
conversions to black and white, plus various layer blend modes such as 
Soft Light/Hard Light/Screen/Burn/etc, plus no doubt a plethora of 
legacy operations and plug-ins.

Either way, and unlike various other editing programs, GIMP does the 
appropriate transform between linear and perceptual encoding "under the 
hood" to produce correct results, with "correct" defined for each 
operation. This is one reason (not the only reason) for the two buttons 
(linear and perceptual) in GIMP-2.10.

I'm assuming based on preliminary testing - and hopefully Pippin will 
correct me if I'm wrong - that the third button in GIMP-2.99 is to 
accomodate use cases such as opening a ProPhotoRGB image encoded using 
gamma=1.8 TRC, and wanting to produce results that match PhotoShop when 
the "linear" option is disabled in the PS Color Management settings. As 
an aside, please note: "linear" option in PS CM settings doesn't affect 
all 16-bit editing operations, and 32f editing in PS is handled somewhat 
differently than 16-bit integer editing. See this article and try the 
tests using PhotoShop at various bit depths: 
https://ninedegreesbelow.com/photography/test-for-linear-processing.html

That's all I can write right now. Hope it helps. I'll try to continue 
responding to your post tomorrow or hopefully later today.

Best,
Elle
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