fix SkJumper radial gradient precision

rcp(rsqrt(x)) doesn't have enough precision when x is a coordinate.
(It's fine when x is a color, like in the softlight blend mode.)

Adds a GM to test this.  It used to look quite ugly.

Change-Id: Icec295c2e2f50ae7a5e3e33c62270f632a58f65c
Reviewed-on: https://skia-review.googlesource.com/16914
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>

1 files changed, 22 insertions, 0 deletions

diff --git a/gm/radial_gradient_precision.cpp b/gm/radial_gradient_precision.cpp
new file mode 100644
index 0000000000..b4e9a11bb1
--- /dev/null
+++ b/gm/radial_gradient_precision.cpp
@@ -0,0 +1,22 @@
+/*
+ * 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 "SkGradientShader.h"
+
+// All we're looking for here is that we see a smooth gradient.
+DEF_SIMPLE_GM(radial_gradient_precision, canvas, 200, 200) {
+    SkPoint  center   = {100000, 100000};
+    SkScalar radius   = 40;
+    SkColor  colors[] = {SK_ColorBLACK, SK_ColorWHITE};
+
+    SkPaint p;
+    p.setShader(SkGradientShader::MakeRadial(center, radius,
+                                             colors, nullptr, SK_ARRAY_COUNT(colors),
+                                             SkShader::kRepeat_TileMode));
+    canvas->drawPaint(p);
+}