Correct edges_too_close

If the next edge has 0 fDX, we should add a SLACK = 1 so two edges
with fX less than 1 pixel apart should be considered close, and
noRealBlitter should be true to force the use of AdditiveBlitter and
the cumulation of alpha.  The changed GM will show bleed through if
SLACK is 0.

The artifact without the fix can be seen at:
https://fiddle.skia.org/c/f6912f1af6c14e054f5b5935a93380ea

Bug: skia:
Change-Id: I15f9c3aef25a0357cd11d447e7bf0b4fbac0ce67
Reviewed-on: https://skia-review.googlesource.com/67804
Commit-Queue: Yuqian Li <liyuqian@google.com>
Reviewed-by: Cary Clark <caryclark@google.com>

1 files changed, 16 insertions, 0 deletions

diff --git a/gm/aaa.cpp b/gm/aaa.cpp
index 614ff84258..4f81fc9a2c 100644
--- a/gm/aaa.cpp
+++ b/gm/aaa.cpp
@@ -123,6 +123,22 @@ protected:
         p.setStrokeWidth(5);
         canvas->drawPath(path, p);
         canvas->restore();
+
+
+        // The following two paths test if we correctly cumulates the alpha on the middle pixel
+        // column where the left rect and the right rect abut.
+        p.setStyle(SkPaint::kFill_Style);
+        canvas->translate(0, 300);
+        path.reset();
+        path.addRect({20, 20, 100.4999f, 100});
+        path.addRect({100.5001f, 20, 200, 100});
+        canvas->drawPath(path, p);
+
+        canvas->translate(300, 0);
+        path.reset();
+        path.addRect({20, 20, 100.1f, 100});
+        path.addRect({100.9f, 20, 200, 100});
+        canvas->drawPath(path, p);
     }
 
 private: