Re: [Gimp-developer] Some blend modes break in unbounded mode sRGB

[Elle Stone <ellestone ninedegreesbelow com>]

On 04/13/2014 09:34 AM, Michael Henning wrote:
> You are correct in the sense that *right now*, most of the babl color
> spaces are based on the sRGB chromaticities. There is nothing
> preventing us from adding different color spaces in the future,
> including ProPhoto RGB.

Can you add my custom RGB color space that works best with my 
interpolated raw files? And the next person's custom RGB working space? 
I've done a lot of experimenting with RGB color spaces over the years. 
Editing my interpolated raw files in my camera-sized RGB working space 
space gives the best results. The colors stay the truest. All channel 
data is available and can be manipulated without having the channel data 
contaminated by converting the image to another RGB color space with 
different chromaticities.

> > And if the operation is better done in in some other color space *model*,
> > not RGB but rather perhaps CIELAB or HSL or whatever, then the conversion is
> > now and will always be from *sRGB* to CIELAB, HSL or whatever, yes?
>
> No, not always. Gimp already has the option of storing your image in a
> linear or perceptual gray color space, as well as different indexed
> formats. As above, new formats can be added easily.

If there is a way to input the chromaticities of my chosen RGB color 
space, and do ALL editing using those chromaticities, that would be 
wonderful. The thing is, without a way for the user to input his or her 
chosen chromaticities, you are severely limiting the user's creative use 
of different color spaces. Not all color spaces are equally suited for 
all editing purposes.


On 04/13/2014 09:34 AM, Michael Henning wrote:
> Can I ask: what
> difference does it make? If I convert an image from ProPhoto to
> CIELAB, or if I convert from sRGB to CIELAB, I will get the same
> result. The gegl operation doesn't care how the data is stored; it
> only cares what format it receives the data in.

On 04/12/2014 06:45 PM, Øyvind Kolås wrote:
> Some GEGL operations make most sense in linear color spaces others
> make most sense in perceptual color spaces. For consistency in
> behavior operations should always be made in the same space regardless
> of whichever source space the images originated.

On 04/13/2014 07:54 AM, Teo Mazars wrote:
> even though
> conversions are done internally through linear sRGB, the fact that
> this is an invertible linear transformation of the XYZ color space
> makes it mathematically equivalent in terms of gamut.


Assume unbounded mode ICC profile conversions from one linear gamma 
color space to another (and putting aside all concern with things like 
exporting to jpeg, which apparently does require a perceptually uniform 
TRC):

The following operations are (probably, I'm still testing some of them) 
*completely independent* of the RGB color space chromaticities:

1. Multiplying and dividing by gray, black, or white.
2. Adding and subtracting.
3. Any blend modes that makes use of adding and subtracting, including 
grain merge, grain extract, difference.
4. Normal blend mode.
5. Levels upper sliders (which produce the same result as dividing or 
multiplying by gray).
6. Gaussian blur.
7. Unsharp Mask.
8. Scaling.


The following operations are *highly dependent* on the RGB color space 
chromaticities:

1. Multiplying and dividing by any color other than gray, black, or white.
2. Any blend mode that involves multiplying or dividing, including burn, 
dodge, soft light, hard light.
3. The lighten only, darken only, hue, saturation, color, and value 
blend modes.
4. Channel mixer.
5. Any editing move that involves pulling over an individual channel and 
using it as a blending layer in a layer stack.


As long as you limit your edits to the chromaticity *in*dependent set of 
editing operations, you get absolutely consistent results *regardless* 
of what unbounded mode color space you convert to. The chromaticities 
don't matter.

As soon as you employ any of the chromaticity *dependent* operations, 
then you have problems. Lettuce turns yellow when using Divide to make a 
line drawing. Channel mixer can't be used desaturate a saturated yellow 
flower. The blue channel in that same yellow flower can't be used as a 
blending layer in a conversion to black and white. And so on.


Elle