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

[Elle Stone <ellestone ninedegreesbelow com>]

On 04/09/2014 06:36 PM, Elle Stone wrote:
> For Lighten only, Darken only, Multiply, Divide, and some of the other
> blend modes, results are *highly dependent* on the color space in which
> the blending is done. Removing clipping code doesn't fix the problem.
>
> If the artist uses any of the color-space dependent blend modes:
>
> * Editing an image in a large gamut color space such as ProPhotoRGB
> produces one set of colors.
>
> * Converting the image to unbounded mode sRGB before editing it
> necessarily produces different colors - sometimes very different colors.
>
> For concrete examples showing how Lighten only and Darken only are
> color-space dependent, see
> http://ninedegreesbelow.com/gimpgit/unbounded-srgb-lighten-darken.html
>
> Can this problem be fixed? Is there a workaround such that an image can
> be converted from its source color space to unbounded mode sRGB, and
> then use the color-space dependent blend modes to produce the same
> colors that would have been produced if the image had been edited in the
> source color space instead of in unbounded mode sRGB?

I put up two more examples - not blend mode examples, rather channel 
mixing and blending examples:


1. Channel blending when converting to black and white
http://ninedegreesbelow.com/gimpgit/unbounded-srgb-channel-blend-convert-black-white.html

The example concludes:

My envisioned conversion to black and white was simple to achieve in my 
custom RGB working space: make a luminance-based conversion to black and 
white, pull over the blue channel and set it to multiply blend mode, 
season to taste.

Converting the image to unbounded mode sRGB made my envisioned 
conversion to black and white completely impossible.

Worse, if I hadn't examined the blue channel data before the conversion 
to the unbounded mode sRGB color space, I never would have seen the 
original blue channel data. As the original blue channel contained all 
the interesting information in this particular image, I would not have 
been inspired to convert the image to black and white. No doubt no loss 
to art! but that is not the point. The point is that a conversion to 
unbounded mode sRGB radically rearranges channel data, which in turn 
radically alters the information the artist has to work with.


2. Using Channel Mixer to decrease saturation
http://ninedegreesbelow.com/gimpgit/unbounded-srgb-channel-mixer-decrease-saturation.html

The example concludes:

Channel data is radically rearranged when a saturated image is converted 
from a wider gamut color space to the unbounded mode sRGB color space. A 
consequence is that many crucially important editing tools no longer 
function properly. Channel mixing is one such tool.

In the custom RGB color space using Channel Mixer to desaturate the 
yellow cone flower worked exactly as expected.

In the unbounded mode sRGB color space using Channel Mixer to desaturate 
the yellow cone flower failed completely. The flower wasn't actually 
desaturated, and excessive amounts of the blue channel polluted the 
image's original colors.


Thanks in advance for looking at the examples.

Elle