Add a comment to explain clip intersection in crop rects.

src/core/SkImageFilter.cpp

 /*
  * Copyright 2012 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 "SkImageFilter.h"
 
 #include "SkBitmap.h"
@@ skipping 69 matching lines @@
         if (fFlags & kHasTop_CropEdge) {
             cropped->fTop = devICropR.fTop;
         }
         if (fFlags & kHasWidth_CropEdge) {
             cropped->fRight = cropped->fLeft + devICropR.width();
         }
         if (fFlags & kHasHeight_CropEdge) {
             cropped->fBottom = cropped->fTop + devICropR.height();
         }
     }
+    // Intersect against the clip bounds, in case the crop rect has
+    // grown the bounds beyond the original clip. This can happen for
+    // example in tiling, where the clip is much smaller than the filtered
+    // primitive. If we didn't do this, we would be processing the filter
+    // at the full crop rect size in every tile.
     return cropped->intersect(ctx.clipBounds());
 }
 
 ///////////////////////////////////////////////////////////////////////////////////////////////////
 
 static int32_t next_image_filter_unique_id() {
     static int32_t gImageFilterUniqueID;
 
     // Never return 0.
     int32_t id;
@@ skipping 499 matching lines @@
     }
     return dev;
 }
 
 bool SkImageFilter::DeviceProxy::filterImage(const SkImageFilter* filter, const SkBitmap& src,
                                        const SkImageFilter::Context& ctx,
                                        SkBitmap* result, SkIPoint* offset) {
     return fDevice->filterImage(filter, src, ctx, result, offset);
 }