fb126fa96e
The most interesting parts of this are how plus interacts with partial
coverage. Plus needs its clamp to happen after the lerp.
Luckily, some of its math folds away:
d' = clamp[ d*(1-c) + (s+d)*c ] ==
clamp[ d - dc + sc + dc ] ==
clamp[ d + sc ]
What's nice there is that coverage can be folded into the src term.
This suggests that we can re-write the plus stage to clamp internally
(and thus, be viable for 8-bit) if we always pre-scale with coverage.
We don't have a way to pre-scale with 565 coverage until now, but
it's only a step or two away from there. We can use the alternate
formulation we derived for alpha for lerp_565, calculating the alpha
coverage from red, green, and blue coverages _and_ the values of src
and dst alpha.
While we already pre-scale srcover today for 8-bit or constant coverage,
we cannot do the same for 565. When evaluating the expression
d' = s + (1-a)d
we need the a term to be pre-scaled with red's coverage when calculating
dr', with blue's when calculating db', etc. Essentially we need to
carry around a bunch of extra values, and we've got no way to do that.
So instead, we'll just carefully pre-scale plus with any coverage, and
keep post-lerping srcover when we have 565 coverage.
Change-Id: I7a7a52eec7d482e1b98bb8a01ea0a3d5e67bef65
Reviewed-on: https://skia-review.googlesource.com/38300
Commit-Queue: Mike Klein <mtklein@chromium.org>
Reviewed-by: Florin Malita <fmalita@chromium.org>
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| dm | ||
| docs | ||
| example | ||
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| fuzz | ||
| gm | ||
| gn | ||
| include | ||
| infra | ||
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| README.chromium | ||
| whitespace.txt | ||
Skia is a complete 2D graphic library for drawing Text, Geometries, and Images. See full details, and build instructions, at https://skia.org.