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

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.


[Date Prev][Date Next]   [Thread Prev][Thread Next]   [Thread Index] [Date Index] [Author Index]