OK, colour spaces are very confsing, but a very basic point is that the
human spectral sensitivity has a pronounced peak in the green region,
This is why any function deriving the perceived brightness or luminance
from RGB, such as Rec709Luminance (0.212656 * R + 0.715158 * G +
0.072186 * B) weights the green far more than red and blue. This just
reflects the spectral sensitivity of the human eye.
The perceived brightness of purely red image at, say, 100 bit will be
far less than that of a purely green image at 100 bit. Correspondingly,
the latter should result in a leighter equivalent grey than the former.
Gimp proceeds this way when converting a colour image to grey, as I have
100 R --> 21 G
100 G --> 72 G
100 B --> 7 G
This is very close to Rec709Luminance.
I just suggest that it should offer such a weighting in the histogramm
for a colour image, i.e. the equivalent grey value at Gimp itself would
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