Re: [Gimp-developer] Different results between RGB to Grayscale conversion and Desaturate (luminosity) filter

On Mon, Jun 15, 2015 at 2:29 PM,  <thomas manni free fr> wrote:
in master branch, using Colors > Desaturate (luminosity) and Image > Mode > Grayscale on a RGB image 
(whatever the image precision) gives different results.

This does not appear in branch gimp-2-8, since the 2 actions are computed the same way (using 

In master, for Desaturate (luminosity) filter, GIMP_RGB_LUMINANCE is used on a R'G'B'A buffer.
For Grayscale conversion, drawables' buffers formats are converted to babl_format Y (or Y' according to the 
initial format).

As these 2 actions seem to have the same goal (obtain the relative luminance), shouldn't we obtain the same 
result ?

I am not sure they should behave the same way / give the same result -
nor that any of the results are ideal yet.

What is GIMP_RGB_LUMINANCE? is it the equivalent of the V component of

It is possible that GIMP could ask the user how they want to derive a
grayscale/desaturated version. Perhaps only ask when doing desaturate
and do the best we can do for the implicit conversion when going from
color to grayscale mode - something like the Luminance component of
CIE Lab normalized to 0.0-1.0 range.


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