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Add comment about additive blending alpha 'hack'.

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This commit is contained in:
Jennifer Taylor 2021-05-23 20:31:39 +00:00
parent 9dada6fb79
commit 0a6635d993
2 changed files with 16 additions and 0 deletions
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8
bemani/format/afp/blend.py
View File

@ -64,6 +64,14 @@ except ImportError:
clamp(dest[0] + (src[0] * srcpercent)),
clamp(dest[1] + (src[1] * srcpercent)),
clamp(dest[2] + (src[2] * srcpercent)),
# Additive blending doesn't actually make sense on semi-transparent destinations,
# as that implies that the semi-transparent pixel will be later displayed on top
# of something else. That doesn't work since additive blending needs to non-linearly
# mix with the destination. So, in reality, we should be doing what subtractive
# blending does and keeping the destination alpha (which should always be 255),
# but if somebody renders an animation with additive blending meant to go over a
# background onto a transparent or semi-transparent background this will make the
# resulting graphic look more correct.
clamp(dest[3] + (255 * srcpercent)),
)

8
bemani/format/afp/blendaltimpl.cxx
View File

@ -121,6 +121,14 @@ extern "C"
clamp(dest.r + (src.r * srcpercent)),
clamp(dest.g + (src.g * srcpercent)),
clamp(dest.b + (src.b * srcpercent)),
// Additive blending doesn't actually make sense on semi-transparent destinations,
// as that implies that the semi-transparent pixel will be later displayed on top
// of something else. That doesn't work since additive blending needs to non-linearly
// mix with the destination. So, in reality, we should be doing what subtractive
// blending does and keeping the destination alpha (which should always be 255),
// but if somebody renders an animation with additive blending meant to go over a
// background onto a transparent or semi-transparent background this will make the
// resulting graphic look more correct.
clamp(dest.a + (255 * srcpercent)),
};
}