Speed up handling of "bowtie" joins in the hw tessellator
Bowtie joins are only used on internal chops, and in that case, they
are only needed on cusps. Rather than measuring the angle to see if we
need the bowtie, we can just detect if the chop was at a cusp instead.
Bug: chromium:1172543
Change-Id: I5efe4df1fb10dd850b65e0f85415d7791e196cd3
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/373797
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Chris Dalton <csmartdalton@google.com>
diff --git a/src/gpu/tessellate/GrStrokeHardwareTessellator.cpp b/src/gpu/tessellate/GrStrokeHardwareTessellator.cpp
index 03b05ba..79d4592 100644
--- a/src/gpu/tessellate/GrStrokeHardwareTessellator.cpp
+++ b/src/gpu/tessellate/GrStrokeHardwareTessellator.cpp
@@ -503,13 +503,35 @@
SkPoint endPt) {
if (prevJoinType == JoinType::kBowtie) {
SkASSERT(fHasLastControlPoint);
- // Bowties need to go in their own patch if they will have >1 segment. TODO: Investigate
- // if an optimization like "x < fCosRadiansPerSegment" would be worth it.
+ // Bowtie joins are only used on internal chops, and internal chops almost always have
+ // continuous tangent angles (i.e., the ending tangent of the first chop and the
+ // beginning tangent of the second both point in the same direction). The tangents will
+ // only ever not point in the same direction if we chopped at a cusp point, so that's
+ // the only time we actually need a bowtie.
SkPoint nextControlPoint = (p[1] == p[0]) ? p[2] : p[1];
- float rotation = SkMeasureAngleBetweenVectors(p[0] - fLastControlPoint,
- nextControlPoint - p[0]);
- if (rotation * fTolerances.fNumRadialSegmentsPerRadian > 1) {
- this->internalJoinTo(prevJoinType, p[0], nextControlPoint);
+ SkVector a = p[0] - fLastControlPoint;
+ SkVector b = nextControlPoint - p[0];
+ float ab_cosTheta = a.dot(b);
+ float ab_pow2 = a.dot(a) * b.dot(b);
+ // To check if tangents 'a' and 'b' do not point in the same direction, any of the
+ // following formulas work:
+ //
+ // 0 != theta
+ // 1 != cosTheta
+ // 1 != cosTheta * abs(cosTheta) [Still false when cosTheta == -1]
+ //
+ // Introducing a slop term for fuzzy equality gives:
+ //
+ // 1 !~= cosTheta * abs(cosTheta) [tolerance = epsilon]
+ // (ab)^2 !~= (ab)^2 * cosTheta * abs(cosTheta) [tolerance = (ab)^2 * epsilon]
+ // (ab)^2 !~= (ab * cosTheta) * (ab * abs(cosTheta)) [tolerance = (ab)^2 * epsilon]
+ // (ab)^2 !~= (ab * cosTheta) * abs(ab * cosTheta) [tolerance = (ab)^2 * epsilon]
+ //
+ // Since we also scale the tolerance, the formula is unaffected by the magnitude of the
+ // tangent vectors. (And we can fold "ab" in to the abs() because it's always positive.)
+ if (!SkScalarNearlyEqual(ab_pow2, ab_cosTheta * fabsf(ab_cosTheta),
+ ab_pow2 * SK_ScalarNearlyZero)) {
+ this->internalJoinTo(JoinType::kBowtie, p[0], nextControlPoint);
fLastControlPoint = p[0]; // Disables the join section of this patch.
prevJoinFitsInPatch = true;
}
