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

[Øyvind Kolås <pippin gimp org>]

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 
> 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