GrTessellator: fix for collinear out-of-range intersections.

Change 3b5a3fa8b1c11d4bd4499b040311f4c3553ebf8c introduced support
for out-of-range intersections, which is necessary when the top and
bottom vertices of an edge differ by only one ULP in the primary sort
order and can't be split in-order.

However, some out-of-range intersections produce edges which cancel
each other out on splitting, in particular when the intersection is
collinear with the newly-computed edge. This undoes the effect of the
split.  The tessellator then rewinds, re-detects the intersection, resplits,
an infinite loop.

The fix is to check for out-of-range intersections which are also
collinear, and ignore them. This is ok, because these are not
the cases we care about it change 3b5a3f above, which are
never collinear.

Bug: 753867
Change-Id: I590231e0e6f19c98f1ccf46cb7acc8a63ba35a9d
Reviewed-on: https://skia-review.googlesource.com/34925
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Stephen White <senorblanco@chromium.org>

1 files changed, 17 insertions, 0 deletions

diff --git a/src/gpu/GrTessellator.cpp b/src/gpu/GrTessellator.cpp
index 5f25e5a5fb..2cb9adcbf8 100644
--- a/src/gpu/GrTessellator.cpp
+++ b/src/gpu/GrTessellator.cpp
@@ -1078,6 +1078,17 @@ uint8_t max_edge_alpha(Edge* a, Edge* b) {
     }
 }
 
+bool out_of_range_and_collinear(const SkPoint& p, Edge* edge, Comparator& c) {
+    if (c.sweep_lt(p, edge->fTop->fPoint) &&
+        !Line(p, edge->fBottom->fPoint).dist(edge->fTop->fPoint)) {
+        return true;
+    } else if (c.sweep_lt(edge->fBottom->fPoint, p) &&
+        !Line(edge->fTop->fPoint, p).dist(edge->fBottom->fPoint)) {
+        return true;
+    }
+    return false;
+}
+
 bool check_for_intersection(Edge* edge, Edge* other, EdgeList* activeEdges, Vertex** current,
                             VertexList* mesh, Comparator& c, SkArenaAlloc& alloc) {
     if (!edge || !other) {
@@ -1086,6 +1097,12 @@ bool check_for_intersection(Edge* edge, Edge* other, EdgeList* activeEdges, Vert
     SkPoint p;
     uint8_t alpha;
     if (edge->intersect(*other, &p, &alpha) && p.isFinite()) {
+        // Ignore any out-of-range intersections which are also collinear,
+        // since the resulting edges would cancel each other out by merging.
+        if (out_of_range_and_collinear(p, edge, c) ||
+            out_of_range_and_collinear(p, other, c)) {
+            return false;
+        }
         Vertex* v;
         LOG("found intersection, pt is %g, %g\n", p.fX, p.fY);
         Vertex* top = *current;