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

[thomas manni free fr]

Hi,
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 ?

Thomas Manni