Combine analytic edges with tolerance

If not, we sometimes would end up with only one edge for a convex path. That
either triggers SkASSERT(count >= 2) failure in debug build or SIGSEGV in
release build. After the change, we should return 0 edges for such a path
because everything is totally combined.

Note that this change also makes the SkAnalyticEdge's CombineVertical function
behave more similarly to SkEdge's CombineVertical function: SkEdge only
compares fFirstY and fLastY which are integer values, which is equivalent to
setting our tolerance to SK_Fixed1 (our current tolerance is 0x100, 1/256 of
SK_Fixed1).  And this is intentional.

BUG=chrome:662914
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2477373002

Review-Url: https://codereview.chromium.org/2477373002

1 files changed, 8 insertions, 4 deletions

diff --git a/src/core/SkEdgeBuilder.cpp b/src/core/SkEdgeBuilder.cpp
index 1a160882d4..22ee9ed346 100644
--- a/src/core/SkEdgeBuilder.cpp
+++ b/src/core/SkEdgeBuilder.cpp
@@ -63,6 +63,10 @@ SkEdgeBuilder::Combine SkEdgeBuilder::CombineVertical(const SkEdge* edge, SkEdge
     return kNo_Combine;
 }
 
+static inline bool approximatelyEqual(SkFixed a, SkFixed b) {
+    return SkAbs32(a - b) < 0x100;
+}
+
 SkEdgeBuilder::Combine SkEdgeBuilder::CombineVertical(
         const SkAnalyticEdge* edge, SkAnalyticEdge* last) {
     SkASSERT(fAnalyticAA);
@@ -75,14 +79,14 @@ SkEdgeBuilder::Combine SkEdgeBuilder::CombineVertical(
             last->fY = last->fUpperY;
             return kPartial_Combine;
         }
-        if (edge->fUpperY == last->fLowerY) {
+        if (approximatelyEqual(edge->fUpperY, last->fLowerY)) {
             last->fLowerY = edge->fLowerY;
             return kPartial_Combine;
         }
         return kNo_Combine;
     }
-    if (edge->fUpperY == last->fUpperY) {
-        if (edge->fLowerY == last->fLowerY) {
+    if (approximatelyEqual(edge->fUpperY, last->fUpperY)) {
+        if (approximatelyEqual(edge->fLowerY, last->fLowerY)) {
             return kTotal_Combine;
         }
         if (edge->fLowerY < last->fLowerY) {
@@ -96,7 +100,7 @@ SkEdgeBuilder::Combine SkEdgeBuilder::CombineVertical(
         last->fWinding = edge->fWinding;
         return kPartial_Combine;
     }
-    if (edge->fLowerY == last->fLowerY) {
+    if (approximatelyEqual(edge->fLowerY, last->fLowerY)) {
         if (edge->fUpperY > last->fUpperY) {
             last->fLowerY = edge->fUpperY;
             return kPartial_Combine;