fix 0.499999f rounding case for triangles

Bug: skia:7994
Change-Id: I83bb309a2c8fb0bddaf78ba32c0a07537e483900
Reviewed-on: https://skia-review.googlesource.com/129648
Commit-Queue: Mike Reed <reed@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>

1 files changed, 32 insertions, 1 deletions

diff --git a/src/core/SkScan_Path.cpp b/src/core/SkScan_Path.cpp
index 4110b832a9..43240eae37 100644
--- a/src/core/SkScan_Path.cpp
+++ b/src/core/SkScan_Path.cpp
@@ -738,6 +738,37 @@ static void sk_fill_triangle(const SkPoint pts[], const SkIRect* clipRect,
 //    walk_edges(&headEdge, SkPath::kEvenOdd_FillType, blitter, start_y, stop_y, nullptr);
 }
 
+/**
+ *  We need to match the rounding behavior of the line edge, which does this:
+ *  1. scale by 64 (to get into FDot6)
+ *  2. cast to an int
+ *  3. round that to an int (undoing the FDot6)
+ *  This should (in theory) be the same as sk_float_round2int, except for float values very very
+ *  close to 0.5 (like 0.49999997f). For those values, x + 0.5 gives 1.0 instead of 0.9999999,
+ *  and therefore they round2int differently as floats than as FDot6 values in the edge code.
+ *
+ *  A fix is to go into double temporarily, so that 0.49999997f + 0.5 stays < 1.0.
+ *
+ *  This sample triangle triggers the problem (if we just use SkRect::round() instead of
+ *  this double_round version.
+ *
+ *  {  0.499069244f, 9.63295173f },
+ *  {  0.499402374f, 7.88207579f },
+ *  { 10.2363272f,   0.49999997f },
+ *
+ *  Note: this version is basically just more correct than SkRect::round(). If we thought we could
+ *  afford the perf hit (assuming going to doubles cost more), then we might replace round()'s
+ *  impl with this.
+ */
+static SkIRect double_round(const SkRect& r) {
+    return {
+        sk_double_round2int(r.fLeft),
+        sk_double_round2int(r.fTop),
+        sk_double_round2int(r.fRight),
+        sk_double_round2int(r.fBottom),
+    };
+}
+
 void SkScan::FillTriangle(const SkPoint pts[], const SkRasterClip& clip,
                           SkBlitter* blitter) {
     if (clip.isEmpty()) {
@@ -757,7 +788,7 @@ void SkScan::FillTriangle(const SkPoint pts[], const SkRasterClip& clip,
         return;
     }
 
-    SkIRect ir = r.round();
+    SkIRect ir = double_round(r);
     if (ir.isEmpty() || !SkIRect::Intersects(ir, clip.getBounds())) {
         return;
     }