antialias matrix-image-filter to get smooth diagonals

src/core/SkMatrixImageFilter.cpp

 /*
  * Copyright 2014 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 "SkMatrixImageFilter.h"
 #include "SkBitmap.h"
 #include "SkCanvas.h"
@@ skipping 58 matching lines @@
     matrix.postConcat(fTransform);
     matrix.postConcat(ctx.ctm());
     matrix.mapRect(&dstRect, srcRect);
     dstRect.roundOut(&dstBounds);
 
     SkAutoTUnref<SkBaseDevice> device(proxy->createDevice(dstBounds.width(), dstBounds.height()));
     if (NULL == device.get()) {
         return false;
     }
 
+    SkPaint paint;
+    paint.setFilterQuality(fFilterQuality);
+    paint.setAntiAlias(true);   // since we may be rotated, we definitely want a smooth edge
+    
     SkCanvas canvas(device.get());
     canvas.translate(-SkIntToScalar(dstBounds.x()), -SkIntToScalar(dstBounds.y()));
     canvas.concat(matrix);
-    SkPaint paint;
-
-    paint.setXfermodeMode(SkXfermode::kSrc_Mode);
-    paint.setFilterQuality(fFilterQuality);
     canvas.drawBitmap(src, srcRect.x(), srcRect.y(), &paint);
 
     *result = device.get()->accessBitmap(false);
     offset->fX = dstBounds.fLeft;
     offset->fY = dstBounds.fTop;
     return true;
 }
 
 void SkMatrixImageFilter::computeFastBounds(const SkRect& src, SkRect* dst) const {
     SkRect bounds = src;
@@ skipping 43 matching lines @@
                  fTransform[SkMatrix::kMPersp2]);
 
     str->append("<dt>FilterLevel:</dt><dd>");
     static const char* gFilterLevelStrings[] = { "None", "Low", "Medium", "High" };
     str->append(gFilterLevelStrings[fFilterQuality]);
     str->append("</dd>");
 
     str->appendf(")");
 }
 #endif