SkPDF: with opaque draws, treat SRC mode as SRC_OVER (cherrypick a681433 and 3098a75)

src/core/SkXfermodeInterpretation.cpp

Index: src/core/SkXfermodeInterpretation.cpp
diff --git a/src/core/SkXfermodeInterpretation.cpp b/src/core/SkXfermodeInterpretation.cpp
new file mode 100644
index 0000000000000000000000000000000000000000..1b2c8e32e6315cef7ceefafd9f11379585f8a6ec
--- /dev/null
+++ b/src/core/SkXfermodeInterpretation.cpp
@@ -0,0 +1,51 @@
+/*
+ * Copyright 2015 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+#include "SkXfermodeInterpretation.h"
+#include "SkPaint.h"
+

[reed1, 2015/06/08 17:30:09]

Does this function need to return true only if there is a single color, or can
it return true if the "src" is opaque? Perhaps it should have a diff name.

[hal.canary, 2015/06/08 17:40:36]

Good question, but outside the scope of this CL.  Notice that this code is
logically the same as the code I removed (with the same name) from SkBlitter. 
For a cherry-pick, I don't want to change the SkBlitter functionality at all.

+static bool just_solid_color(const SkPaint& p) {

[reed1, 2015/06/08 17:30:09]

We can inspect shaders and colorfilters as well to catch more cases.

[hal.canary, 2015/06/08 17:40:36]

same.

+    return SK_AlphaOPAQUE == p.getAlpha()
+        && !p.getColorFilter() && !p.getShader();
+}
+
+SkXfermodeInterpretation SkInterpretXfermode(const SkPaint& paint,
+                                             bool dstIsOpaque) {
+    const SkXfermode* xfer = paint.getXfermode();
+    SkXfermode::Mode mode;
+    if (!SkXfermode::AsMode(xfer, &mode)) {
+        return kNormal_SkXfermodeInterpretation;
+    }
+    switch (mode) {
+        case SkXfermode::kSrcOver_Mode:
+            return kSrcOver_SkXfermodeInterpretation;
+        case SkXfermode::kSrc_Mode:
+            if (just_solid_color(paint)) {
+                return kSrcOver_SkXfermodeInterpretation;
+            }
+            return kNormal_SkXfermodeInterpretation;
+        case SkXfermode::kDst_Mode:
+            return kSkipDrawing_SkXfermodeInterpretation;
+        case SkXfermode::kDstOver_Mode:
+            if (dstIsOpaque) {
+                return kSkipDrawing_SkXfermodeInterpretation;
+            }
+            return kNormal_SkXfermodeInterpretation;
+        case SkXfermode::kSrcIn_Mode:
+            if (dstIsOpaque && just_solid_color(paint)) {
+                return kSrcOver_SkXfermodeInterpretation;
+            }
+            return kNormal_SkXfermodeInterpretation;
+        case SkXfermode::kDstIn_Mode:
+            if (just_solid_color(paint)) {
+                return kSkipDrawing_SkXfermodeInterpretation;
+            }
+            return kNormal_SkXfermodeInterpretation;
+        default:
+            return kNormal_SkXfermodeInterpretation;
+    }
+}