This adds a new output color effect: grayscale. It takes RGB color as
input and computes a gray pixel color using the luminance formula for
linear sRGB:
Y = 0.2126 * R + 0.7152 * G + 0.0722 * B
Just like the other color effects we have, this only works for sRGB and
are not enabled when color-management is on.
Note: although the technique is designed to be applied in linear, it's
costly to convert to linear and then back to electrical. As doing the
conversion in electrical still gives a reasonable result, we do it this
way. When we add support for color effects with color-management on,
we'll apply the effect in linear.
Signed-off-by: Leandro Ribeiro <leandro.ribeiro@collabora.com>
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