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4f gradient negative-dx interval fixup for kMirror_TileMode

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4f intervals are meant to be monotonically increasing in X.  This means
we invert the point order when dx < 0:

  [0...1) -> (1...0]

For kMirror_TileMode, we also append a duplicate/flipped interval
sequence and expand the range to 2:

  [0...1) -> [0...1)[1...2)

When dx < 0, the current logic yields

  (1...0] -> (1...0](2...1]

which violates the interval monotonicity.

To achieve the proper order, we need to swap the two halves:

  (1...0] -> (1...0](2...1] -> (2...1](1...0]

R=reed@google.com
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1735773003

Review URL: https://codereview.chromium.org/1735773003
This commit is contained in:
fmalita 2016-02-25 13:37:28 -08:00 committed by Commit bot
parent 00fddebe56
commit b976b2761f
1 changed files with 15 additions and 1 deletions
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16
src/effects/gradients/Sk4fGradientBase.cpp
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@ -207,8 +207,9 @@ GradientShaderBase4fContext::GradientShaderBase4fContext(const SkGradientShaderB
clamp_color, clamp_pos,
componentScale);
} else if (shader.fTileMode == SkShader::kMirror_TileMode) {
const int count = fIntervals.count();
// synthetic flipped intervals in [1 .. 2)
for (int i = fIntervals.count() - 1; i >= 0; --i) {
for (int i = count - 1; i >= 0; --i) {
const Interval& interval = fIntervals[i];
const SkScalar p0 = interval.fP0;
const SkScalar p1 = interval.fP1;
@ -216,6 +217,19 @@ GradientShaderBase4fContext::GradientShaderBase4fContext(const SkGradientShaderB
Sk4f c = Sk4f::Load(interval.fC0.fVec) + dc * Sk4f(p1 - p0);
fIntervals.emplace_back(c, dc * Sk4f(-1), 2 - p1, 2 - p0);
}
if (!dx_is_pos) {
// When dx is negative, our initial invervals are in (1..0] order.
// The loop above appends their flipped counterparts, pivoted in 2: (1..0](2..1]
// To achieve the expected monotonic interval order, we need to
// swap the two halves: (2..1](1..0]
// TODO: we can probably avoid this late swap with some additional logic during
// the initial interval buildup.
SkASSERT(fIntervals.count() == count * 2)
for (int i = 0; i < count; ++i) {
SkTSwap(fIntervals[i], fIntervals[count + i]);
}
}
}
SkASSERT(fIntervals.count() > 0);