modify nothingToDraw to notice filters

tests/PaintTest.cpp

 /*
  * Copyright 2011 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkBlurMask.h"
 #include "SkBlurMaskFilter.h"
 #include "SkLayerDrawLooper.h"
@@ skipping 326 matching lines @@
     REPORTER_ASSERT(r, paint.getHash() != defaultHash);
     paint.setTypeface(NULL);
     REPORTER_ASSERT(r, paint.getHash() == defaultHash);
 
     // This is part of fBitfields, the last field we hash.
     paint.setHinting(SkPaint::kSlight_Hinting);
     REPORTER_ASSERT(r, paint.getHash() != defaultHash);
     paint.setHinting(SkPaint::kNormal_Hinting);
     REPORTER_ASSERT(r, paint.getHash() == defaultHash);
 }
+
+#include "SkColorMatrixFilter.h"
+
+DEF_TEST(Paint_nothingToDraw, r) {
+    SkPaint paint;
+
+    REPORTER_ASSERT(r, !paint.nothingToDraw());
+    paint.setAlpha(0);
+    REPORTER_ASSERT(r, paint.nothingToDraw());
+
+    paint.setAlpha(0xFF);
+    paint.setXfermodeMode(SkXfermode::kDst_Mode);
+    REPORTER_ASSERT(r, paint.nothingToDraw());
+
+    paint.setAlpha(0);
+    paint.setXfermodeMode(SkXfermode::kSrcOver_Mode);
+
+    SkColorMatrix cm;
+    cm.setIdentity();   // does not change alpha
+    paint.setColorFilter(SkColorMatrixFilter::Create(cm))->unref();
+    REPORTER_ASSERT(r, paint.nothingToDraw());
+
+    cm.postTranslate(0, 0, 0, 1);    // wacks alpha
+    paint.setColorFilter(SkColorMatrixFilter::Create(cm))->unref();
+    REPORTER_ASSERT(r, !paint.nothingToDraw());
+}
+