modify nothingToDraw to notice filters

BUG=skia:

Review URL: https://codereview.chromium.org/717753002

2 files changed, 43 insertions, 1 deletions

diff --git a/src/core/SkPaint.cpp b/src/core/SkPaint.cpp
index ebdc866d65..4ccfce235a 100644
--- a/src/core/SkPaint.cpp
+++ b/src/core/SkPaint.cpp
@@ -2377,6 +2377,18 @@ bool SkTextToPathIter::next(const SkPath** path, SkScalar* xpos) {
 
 ///////////////////////////////////////////////////////////////////////////////
 
+// 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;
@@ -2389,7 +2401,10 @@ bool SkPaint::nothingToDraw() const {
             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:
diff --git a/tests/PaintTest.cpp b/tests/PaintTest.cpp
index 4c45eed761..c307aa94bc 100644
--- a/tests/PaintTest.cpp
+++ b/tests/PaintTest.cpp
@@ -344,3 +344,30 @@ DEF_TEST(Paint_getHash, r) {
     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());
+}
+