On Mon, Jun 15, 2015 at 2:29 PM,  <thomas.ma...@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 GIMP_RGB_LUMINANCE).
>
> 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
HSV (MAX(MAX(R,G),MAX(G,B)) ?

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.

/pippin
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