Fix double counting of coverage in analytic round capped stroked circles.

Change-Id: I28d39f8620d786180b0bf0204b18ca4d1fec597b
Reviewed-on: https://skia-review.googlesource.com/122086
Reviewed-by: Jim Van Verth <jvanverth@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>

1 files changed, 4 insertions, 1 deletions

diff --git a/src/gpu/ops/GrOvalOpFactory.cpp b/src/gpu/ops/GrOvalOpFactory.cpp
index 10159bba77..9dc1c91e5b 100644
--- a/src/gpu/ops/GrOvalOpFactory.cpp
+++ b/src/gpu/ops/GrOvalOpFactory.cpp
@@ -195,12 +195,15 @@ private:
                 }
                 fragBuilder->codeAppend("edgeAlpha *= clip;");
                 if (cgp.fInRoundCapCenters) {
+                    // We compute coverage of the round caps as circles at the butt caps produced
+                    // by the clip planes. The inverse of the clip planes is applied so that there
+                    // is no double counting.
                     fragBuilder->codeAppendf(
                             "half dcap1 = circleEdge.z * (%s - length(circleEdge.xy - "
                             "                                         roundCapCenters.xy));"
                             "half dcap2 = circleEdge.z * (%s - length(circleEdge.xy - "
                             "                                         roundCapCenters.zw));"
-                            "half capAlpha = max(dcap1, 0) + max(dcap2, 0);"
+                            "half capAlpha = (1 - clip) * (max(dcap1, 0) + max(dcap2, 0));"
                             "edgeAlpha = min(edgeAlpha + capAlpha, 1.0);",
                             capRadius.fsIn(), capRadius.fsIn());
                 }