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>

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

include/core/SkCanvas.h

@@ -568,6 +568,7 @@ public:
         @return        depth of saved stack
 
         example: https://fiddle.skia.org/c/@Canvas_saveLayer
+        example: https://fiddle.skia.org/c/@Canvas_saveLayer_4
     */
     int saveLayer(const SkRect* bounds, const SkPaint* paint);
 

tools/fiddle/all_examples.cpp

@@ -208,6 +208,7 @@
 #include "../../docs/examples/Canvas_saveLayerPreserveLCDTextRequests.cpp"
 #include "../../docs/examples/Canvas_saveLayer_2.cpp"
 #include "../../docs/examples/Canvas_saveLayer_3.cpp"
+#include "../../docs/examples/Canvas_saveLayer_4.cpp"
 #include "../../docs/examples/Canvas_scale.cpp"
 #include "../../docs/examples/Canvas_setMatrix.cpp"
 #include "../../docs/examples/Canvas_skew.cpp"