Decrease the presumed thickness of polygons.

This addresses some of the common expectations that single digit differences in
Z value will change rendering order.

BUG=499397
R=flackr@chromium.org
TEST=Rendering of Z stacked boxes, test case in bug.
CQ_INCLUDE_TRYBOTS=tryserver.blink:linux_blink_rel

Review URL: https://codereview.chromium.org/1412143004

Cr-Commit-Position: refs/heads/master@{#358416}

2 files changed, 23 insertions, 2 deletions

diff --git a/cc/quads/draw_polygon.cc b/cc/quads/draw_polygon.cc
index 9ff1af6..02c1283 100644
--- a/cc/quads/draw_polygon.cc
+++ b/cc/quads/draw_polygon.cc
@@ -16,7 +16,7 @@ static const float coplanar_dot_epsilon = 0.001f;
 // This threshold controls how "thick" a plane is. If a point's distance is
 // <= |compare_threshold|, then it is considered on the plane. Only when this
 // boundary is crossed do we consider doing splitting.
-static const float compare_threshold = 1.0f;
+static const float compare_threshold = 0.1f;
 // |split_threshold| is lower in this case because we want the points created
 // during splitting to be well within the range of |compare_threshold| for
 // comparison purposes. The splitting operation will produce intersection points
@@ -27,7 +27,7 @@ static const float compare_threshold = 1.0f;
 // this range.
 // This is really supposd to be compare_threshold / 2.0f, but that would
 // create another static initializer.
-static const float split_threshold = 0.5f;
+static const float split_threshold = 0.05f;
 
 static const float normalized_threshold = 0.001f;
 }  // namespace
diff --git a/cc/quads/draw_polygon_unittest.cc b/cc/quads/draw_polygon_unittest.cc
index 517e131..caf0f27 100644
--- a/cc/quads/draw_polygon_unittest.cc
+++ b/cc/quads/draw_polygon_unittest.cc
@@ -32,6 +32,27 @@ static void ValidatePoints(const DrawPolygon& polygon,
   }
 }
 
+// Two quads are nearly touching but definitely ordered. Second one should
+// compare in front.
+TEST(DrawPolygonSplitTest, NearlyTouchingOrder) {
+  std::vector<gfx::Point3F> vertices_a;
+  vertices_a.push_back(gfx::Point3F(0.0f, 10.0f, 0.0f));
+  vertices_a.push_back(gfx::Point3F(0.0f, 0.0f, 0.0f));
+  vertices_a.push_back(gfx::Point3F(10.0f, 0.0f, 0.0f));
+  vertices_a.push_back(gfx::Point3F(10.0f, 10.0f, 0.0f));
+  std::vector<gfx::Point3F> vertices_b;
+  vertices_b.push_back(gfx::Point3F(0.0f, 10.0f, -1.0f));
+  vertices_b.push_back(gfx::Point3F(0.0f, 0.0f, -1.0f));
+  vertices_b.push_back(gfx::Point3F(10.0f, 0.0f, -1.0f));
+  vertices_b.push_back(gfx::Point3F(10.0f, 10.0f, -1.0f));
+  gfx::Vector3dF normal(0.0f, 0.0f, 1.0f);
+
+  CREATE_NEW_DRAW_POLYGON(polygon_a, vertices_a, normal, 0);
+  CREATE_NEW_DRAW_POLYGON(polygon_b, vertices_b, normal, 1);
+
+  EXPECT_EQ(BSP_BACK, DrawPolygon::SideCompare(polygon_b, polygon_a));
+}
+
 // Two quads are definitely not touching and so no split should occur.
 TEST(DrawPolygonSplitTest, NotTouchingNoSplit) {
   std::vector<gfx::Point3F> vertices_a;