Add fiddle demonstrating alpha when used on saveLayer
Change-Id: I4ba2d911a035871512f26201d561d64e9580556b
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/277465
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
diff --git a/docs/examples/Canvas_saveLayer_4.cpp b/docs/examples/Canvas_saveLayer_4.cpp
new file mode 100644
index 0000000..41aa4b9
--- /dev/null
+++ b/docs/examples/Canvas_saveLayer_4.cpp
@@ -0,0 +1,58 @@
+// Copyright 2020 Google LLC.
+// Use of this source code is governed by a BSD-style license that can be found in the LICENSE file.
+#include "tools/fiddle/examples.h"
+// HASH=7d3751e82d1b6ec328ffa3d6f48ca831
+REG_FIDDLE(Canvas_saveLayer_4, 256, 256, false, 3) {
+void draw(SkCanvas* canvas) {
+ SkPaint pRed;
+ pRed.setColor(SK_ColorRED);
+
+ SkPaint pSolidBlue;
+ pSolidBlue.setColor(SK_ColorBLUE);
+
+ SkPaint pThirtyBlue;
+ pThirtyBlue.setColor(SK_ColorBLUE);
+ pThirtyBlue.setAlphaf(0.3);
+
+ SkPaint alpha;
+ alpha.setAlphaf(0.3);
+
+ // First row: Draw two opaque red rectangles into the 0th layer. Then draw two blue
+ // rectangles overlapping the red, one is solid, the other is 30% transparent.
+ canvas->drawRect(SkRect::MakeLTRB(10, 10, 60, 60), pRed);
+ canvas->drawRect(SkRect::MakeLTRB(150, 10, 200, 60), pRed);
+
+ canvas->drawRect(SkRect::MakeLTRB(30, 10, 80, 60), pSolidBlue);
+ canvas->drawRect(SkRect::MakeLTRB(170, 10, 220, 60), pThirtyBlue);
+
+ // Second row: Draw two opaque red rectangles into the 0th layer. Then save a new layer;
+ // when the 1st layer gets merged onto the 0th layer (i.e. when restore() is called), it will
+ // use the provided paint to do so. In this case, the paint is set to have 30% opacity, but
+ // it could also have things set like blend modes or image filters.
+ canvas->drawRect(SkRect::MakeLTRB(10, 70, 60, 120), pRed);
+ canvas->drawRect(SkRect::MakeLTRB(150, 70, 200, 120), pRed);
+
+ canvas->saveLayer(nullptr, &alpha);
+
+ // In the 1st layer, draw the same blue overlapping rectangles as in the first row. Notice in
+ // the final output, we have two different shades of purple. The layer's alpha made the
+ // opaque blue rectangle transparent, and it made the transparent blue rectangle even more so
+ canvas->drawRect(SkRect::MakeLTRB(30, 70, 80, 120), pSolidBlue);
+ canvas->drawRect(SkRect::MakeLTRB(170, 70, 220, 120), pThirtyBlue);
+
+ canvas->restore();
+
+ // Third row: save the layer first, before drawing the two red rectangle, followed by the
+ // overlapping blue rectangles. Notice that the blue overwrites the red in the same way as
+ // the first row because the alpha of the layer is not applied until the layer is restored.
+ canvas->saveLayer(nullptr, &alpha);
+
+ canvas->drawRect(SkRect::MakeLTRB(10, 130, 60, 180), pRed);
+ canvas->drawRect(SkRect::MakeLTRB(150, 130, 200, 180), pRed);
+
+ canvas->drawRect(SkRect::MakeLTRB(30, 130, 80, 180), pSolidBlue);
+ canvas->drawRect(SkRect::MakeLTRB(170, 130, 220, 180), pThirtyBlue);
+
+ canvas->restore();
+}
+} // END FIDDLE