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.

Here's another blend mode example, this time for the Divide blend mode:
http://ninedegreesbelow.com/gimpgit/unbounded-srgb-divide-blend-mode.html

Conclusion:

The Divide blend mode assumes the image RGB values are in fact bounded 
by 0.0 and 1.0, which means technical uses of Divide fail completely 
after converting a saturated image from its original wider gamut color 
space to unbounded mode sRGB.

For example, dividing an image by the reddest possible red should send 
the green and blue channels of all the pixels to 1.0, leaving behind 
only varying shades of cyan. So after converting a saturated image from 
its original camera-sized RGB color space to the unbounded mode sRGB 
color space, dividing by 100% saturated red ("camera red" or sRGB red) 
failed.

Artistic use of Divide blend mode produces different colors after 
converting an image to unbounded mode sRGB. In a line drawing produced 
in the ProPhotoRGB color space, the space between the lines was almost 
white. In the unbounded mode sRGB color space, the same procedure 
produced a line drawing with a yellow color cast over much of the image.


Elle