modify nothingToDraw to notice filters

src/core/SkPaint.cpp

 /*
  * Copyright 2006 The Android Open Source Project
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkPaint.h"
 #include "SkAnnotation.h"
 #include "SkAutoKern.h"
@@ skipping 2359 matching lines @@
         if (xpos) {
             *xpos = fXPos;
         }
         return true;
     }
     return false;
 }
 
 ///////////////////////////////////////////////////////////////////////////////
 
+// return true if the filter exists, and may affect alpha
+static bool affects_alpha(const SkColorFilter* cf) {
+    return cf && !(cf->getFlags() & SkColorFilter::kAlphaUnchanged_Flag);
+}
+
+// return true if the filter exists, and may affect alpha
+static bool affects_alpha(const SkImageFilter* imf) {
+    // TODO: check if we should allow imagefilters to broadcast that they don't affect alpha
+    // ala colorfilters
+    return imf != NULL;
+}
+
 bool SkPaint::nothingToDraw() const {
     if (fLooper) {
         return false;
     }
     SkXfermode::Mode mode;
     if (SkXfermode::AsMode(fXfermode, &mode)) {
         switch (mode) {
             case SkXfermode::kSrcOver_Mode:
             case SkXfermode::kSrcATop_Mode:
             case SkXfermode::kDstOut_Mode:
             case SkXfermode::kDstOver_Mode:
             case SkXfermode::kPlus_Mode:
-                return 0 == this->getAlpha();
+                if (0 == this->getAlpha()) {
+                    return !affects_alpha(fColorFilter) && !affects_alpha(fImageFilter);
+                }
+                break;
             case SkXfermode::kDst_Mode:
                 return true;
             default:
                 break;
         }
     }
     return false;
 }
 
 uint32_t SkPaint::getHash() const {
     // We're going to hash 10 pointers and 7 32-bit values, finishing up with fBitfields,
     // so fBitfields should be 10 pointers and 6 32-bit values from the start.
     SK_COMPILE_ASSERT(offsetof(SkPaint, fBitfields) == 10 * sizeof(void*) + 6 * sizeof(uint32_t),
                       SkPaint_notPackedTightly);
     return SkChecksum::Murmur3(reinterpret_cast<const uint32_t*>(this),
                                offsetof(SkPaint, fBitfields) + sizeof(fBitfields));
 }