GrTessellator: fix overlap outer boundary winding.

When two outer boundary edges are collinear, they are part of an
overlap region. As of 77169c8fd6d, we (correctly) leave these edges
connected, but we must also adjust the winding to magnitude 1 to
keep the winding rules correct.

Note: this adds a new test case to the concavepaths GM, affecting all platforms.

Bug: 863389

Change-Id: I7e3a06df537cd189101e7ad39a4815a78be8fbdd
Reviewed-on: https://skia-review.googlesource.com/141952
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Stephen White <senorblanco@chromium.org>

1 files changed, 17 insertions, 0 deletions

diff --git a/gm/concavepaths.cpp b/gm/concavepaths.cpp
index 50c1abfebb..40152364af 100644
--- a/gm/concavepaths.cpp
+++ b/gm/concavepaths.cpp
@@ -367,6 +367,22 @@ void test_bowtie_coincident_triangle(SkCanvas* canvas, const SkPaint& paint) {
     canvas->restore();
 }
 
+// Collinear outer boundary edges. In the edge-AA codepath, this creates an overlap region
+// which contains a boundary edge. It can't be removed, but it must have the correct winding.
+void test_collinear_outer_boundary_edge(SkCanvas* canvas, const SkPaint& paint) {
+    SkPath path;
+    canvas->save();
+    canvas->translate(400, 400);
+    path.moveTo(20, 20);
+    path.lineTo(20, 50);
+    path.lineTo(50, 50);
+    path.moveTo(80, 50);
+    path.lineTo(50, 50);
+    path.lineTo(80, 20);
+    canvas->drawPath(path, paint);
+    canvas->restore();
+}
+
 // Coincident edges (big ones first, coincident vert on top).
 void test_coincident_edges_1(SkCanvas* canvas, const SkPaint& paint) {
     SkPath path;
@@ -456,6 +472,7 @@ DEF_SIMPLE_GM(concavepaths, canvas, 500, 600) {
     test_degenerate(canvas, paint);
     test_coincident_edge(canvas, paint);
     test_bowtie_coincident_triangle(canvas, paint);
+    test_collinear_outer_boundary_edge(canvas, paint);
     test_coincident_edges_1(canvas, paint);
     test_coincident_edges_2(canvas, paint);
     test_coincident_edges_3(canvas, paint);