GrTessellator: handle three consecutive collinear edges.

In some cases, splitting may produce three consecutive edges which
are collinear. The first one was being merged out, causing the third
one to be missed.

The fix is to switch the arguments to merge_edges_*, ensuring that the
second parameter (the destination edge) is never merged out.

Bug: 851409
Change-Id: I65be2e8222846c99f7bc8d17ea61ddead617cc31
Reviewed-on: https://skia-review.googlesource.com/138700
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Stephen White <senorblanco@chromium.org>

2 files changed, 19 insertions, 6 deletions

diff --git a/src/gpu/GrTessellator.cpp b/src/gpu/GrTessellator.cpp
index 6cd54380e6..78616d4e2f 100644
--- a/src/gpu/GrTessellator.cpp
+++ b/src/gpu/GrTessellator.cpp
@@ -1055,16 +1055,16 @@ void merge_collinear_edges(Edge* edge, EdgeList* activeEdges, Vertex** current,
     for (;;) {
         if (edge->fPrevEdgeAbove && (edge->fTop == edge->fPrevEdgeAbove->fTop ||
                                      !edge->fPrevEdgeAbove->isLeftOf(edge->fTop))) {
-            merge_edges_above(edge, edge->fPrevEdgeAbove, activeEdges, current, c);
+            merge_edges_above(edge->fPrevEdgeAbove, edge, activeEdges, current, c);
         } else if (edge->fNextEdgeAbove && (edge->fTop == edge->fNextEdgeAbove->fTop ||
                                             !edge->isLeftOf(edge->fNextEdgeAbove->fTop))) {
-            merge_edges_above(edge, edge->fNextEdgeAbove, activeEdges, current, c);
+            merge_edges_above(edge->fNextEdgeAbove, edge, activeEdges, current, c);
         } else if (edge->fPrevEdgeBelow && (edge->fBottom == edge->fPrevEdgeBelow->fBottom ||
                                      !edge->fPrevEdgeBelow->isLeftOf(edge->fBottom))) {
-            merge_edges_below(edge, edge->fPrevEdgeBelow, activeEdges, current, c);
+            merge_edges_below(edge->fPrevEdgeBelow, edge, activeEdges, current, c);
         } else if (edge->fNextEdgeBelow && (edge->fBottom == edge->fNextEdgeBelow->fBottom ||
                                             !edge->isLeftOf(edge->fNextEdgeBelow->fBottom))) {
-            merge_edges_below(edge, edge->fNextEdgeBelow, activeEdges, current, c);
+            merge_edges_below(edge->fNextEdgeBelow, edge, activeEdges, current, c);
         } else {
             break;
         }
@@ -1755,7 +1755,7 @@ void reconnect_all_overlap_edges(Vertex* src, Vertex* dst, Edge* current, Compar
 }
 
 void Event::apply(VertexList* mesh, Comparator& c, SkArenaAlloc& alloc) {
-    if (!fEdge) {
+    if (!fEdge || !fEdge->fTop || !fEdge->fBottom) {
         return;
     }
     Vertex* top = fEdge->fTop;
diff --git a/tests/TessellatingPathRendererTests.cpp b/tests/TessellatingPathRendererTests.cpp
index a9913769df..0c7a5ca4c5 100644
--- a/tests/TessellatingPathRendererTests.cpp
+++ b/tests/TessellatingPathRendererTests.cpp
@@ -560,7 +560,7 @@ static SkPath create_path_38() {
     return path;
 }
 
-// Reduction from crbug.com/851409.
+// Reduction from crbug.com/851409. Exercises collinear last vertex.
 static SkPath create_path_39() {
     SkPath path;
     path.moveTo(2072553216, 0);
@@ -571,6 +571,18 @@ static SkPath create_path_39() {
     return path;
 }
 
+// Another reduction from crbug.com/851409. Exercises two sequential collinear edges.
+static SkPath create_path_40() {
+    SkPath path;
+    path.moveTo(2072553216, 0);
+    path.lineTo(2072553216, 1);
+    path.lineTo(2072553472, -13);
+    path.lineTo(2072553216, 0);
+    path.lineTo(2072553472, -6);
+    path.lineTo(2072553472, -13);
+    return path;
+}
+
 static std::unique_ptr<GrFragmentProcessor> create_linear_gradient_processor(GrContext* ctx) {
 
     SkPoint pts[2] = { {0, 0}, {1, 1} };
@@ -670,4 +682,5 @@ DEF_GPUTEST_FOR_ALL_CONTEXTS(TessellatingPathRendererTests, reporter, ctxInfo) {
     test_path(ctx, rtc.get(), create_path_37());
     test_path(ctx, rtc.get(), create_path_38(), SkMatrix(), GrAAType::kCoverage);
     test_path(ctx, rtc.get(), create_path_39());
+    test_path(ctx, rtc.get(), create_path_40());
 }