fix repeat/mirror sampling bleed

I think this has been broken since we tried to simplify this in
  https://skia-review.googlesource.com/16547

The HSW backend does still look a little wrong, but improved,
and the others seem fixed.  Can you see how this affects your
test cases, layout tests, etc?

BUG=skia:6783

Change-Id: I17957ac8100331bea5b64d674bf43105048b72f6
Reviewed-on: https://skia-review.googlesource.com/20548
Commit-Queue: Mike Klein <mtklein@google.com>
Reviewed-by: Florin Malita <fmalita@chromium.org>
Reviewed-by: Herb Derby <herb@google.com>

1 files changed, 47 insertions, 0 deletions

diff --git a/gm/bug6783.cpp b/gm/bug6783.cpp
new file mode 100644
index 0000000000..5b54e1ce6a
--- /dev/null
+++ b/gm/bug6783.cpp
@@ -0,0 +1,47 @@
+/*
+* Copyright 2017 Google Inc.
+*
+* Use of this source code is governed by a BSD-style license that can be
+* found in the LICENSE file.
+*/
+
+#include "gm.h"
+#include "SkSurface.h"
+
+// This GM reproduces skia:6783, which demonstrated a bug in repeat and mirror
+// image sampling tiling modes as implemented in software.  We want to tile to
+// [0,limit), and the old incorrect logic was:
+//
+//    limit = ulp_before(limit)
+//    val = val - floor(val/limit)*limit    (This is repeat; mirror is similar.)
+//
+// while the correct logic is more like:
+//
+//    val = val - floor(val/limit)*limit
+//    val = min(val, ulp_before(limit))
+//
+// You would see ugly jaggies on the blue/yellow edge near the bottom left if
+// the bug were still present.  All stripes should now look roughly the same.
+
+DEF_SIMPLE_GM(bug6783, canvas, 500, 500) {
+    sk_sp<SkSurface> surface = SkSurface::MakeRasterN32Premul(100, 100);
+
+    SkPaint p;
+    p.setColor(SK_ColorYELLOW);
+    surface->getCanvas()->drawPaint(p);
+    p.setColor(SK_ColorBLUE);
+    surface->getCanvas()->drawRect(SkRect::MakeWH(50, 100), p);
+
+    sk_sp<SkImage> img = surface->makeImageSnapshot();
+
+    SkMatrix m = SkMatrix::Concat(SkMatrix::MakeTrans(25, 214),
+                                  SkMatrix::MakeScale(2, 2));
+    m.preSkew(0.5f, 0.5f);
+
+    // The bug was present at all filter levels, but you might not notice it at kNone.
+    p.setFilterQuality(kLow_SkFilterQuality);
+
+    // It's only important to repeat or mirror in x to show off the bug.
+    p.setShader(img->makeShader(SkShader::kRepeat_TileMode, SkShader::kClamp_TileMode, &m));
+    canvas->drawPaint(p);
+}