Add subsetting to SkScaledCodec

src/codec/SkCodecPriv.h

 /*
  * Copyright 2015 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.
  */
 
 #ifndef SkCodecPriv_DEFINED
 #define SkCodecPriv_DEFINED
 
@@ skipping 13 matching lines @@
     uint8_t zeroAlpha = 0;                      \
     uint8_t maxAlpha = 0xFF;
 
 #define UPDATE_RESULT_ALPHA(alpha)              \
     zeroAlpha |= (alpha);                       \
     maxAlpha  &= (alpha);
 
 #define COMPUTE_RESULT_ALPHA                    \
     SkSwizzler::GetResult(zeroAlpha, maxAlpha);
 
+inline bool is_valid_subset(const SkIRect* subset, const SkISize& imageDims) {

[scroggo, 2015/10/02 18:27:03]

Why not:

return SkIRect::MakeSize(imageDims).contains(subset):

?

Also, I think it makes more sense to make subset a const reference (you never
want the caller to pass nullptr, and it's not optional.

+    if (0 > subset->left() || subset->left() >= imageDims.width() ||
+            0 > subset->top() || subset->top() >= imageDims.height() ||
+            0 > subset->width() || subset->left() + subset->width() > imageDims.width() ||
+            0 > subset->height() || subset->top() + subset->height() > imageDims.height()) {
+        return false;
+    }
+    return true;
+}
+
+inline float get_scale(uint32_t sampleSize) { return 1.0f / (float) sampleSize; }
+
 /*
  * returns a scaled dimension based on the original dimension and the sampleSize
  * NOTE: we round down here for scaled dimension to match the behavior of SkImageDecoder
  */
 inline int get_scaled_dimension(int srcDimension, int sampleSize) {
     if (sampleSize > srcDimension) {
         return 1;
     }
     return srcDimension / sampleSize;
 }
@@ skipping 17 matching lines @@
 inline int get_dst_coord(int srcCoord, int sampleFactor) { return srcCoord / sampleFactor; };
 
 /*
  * When scaling, we will discard certain y-coordinates (rows) and
  * x-coordinates (columns).  This function returns true if we should keep the
  * coordinate and false otherwise.
  * The inputs may be x-coordinates or y-coordinates.
  *
  * This does not need to be called and is not called when sampleFactor == 1.
  */
-inline bool is_coord_necessary(int srcCoord, int sampleFactor, int scaledDim) {
+inline bool is_coord_necessary(int srcCoord, int sampleFactor, int scaledDim, int subsetOffset) {

[scroggo, 2015/10/02 18:27:03]

I assume scaledDim now means scaled version of subsetWidth?

Is srcCoord scaled? (or subsetOffset, for that matter)

     // Get the first coordinate that we want to keep
-    int startCoord = get_start_coord(sampleFactor);
+    int startCoord = get_start_coord(sampleFactor) + subsetOffset;
 
     // Return false on edge cases
-    if (srcCoord < startCoord || get_dst_coord(srcCoord, sampleFactor) >= scaledDim) {
+    if (srcCoord < startCoord ||
+            get_dst_coord(srcCoord - subsetOffset, sampleFactor) >= scaledDim) {
         return false;
     }
 
     // Every sampleFactor rows are necessary
     return ((srcCoord - startCoord) % sampleFactor) == 0;
 }
 
 inline uint32_t ceil_div(uint32_t a, uint32_t b) {
     return (a + b - 1) / b;
 }
@@ skipping 175 matching lines @@
 #endif
 }
 
 #ifdef SK_PRINT_CODEC_MESSAGES
     #define SkCodecPrintf SkDebugf
 #else
     #define SkCodecPrintf(...)
 #endif
 
 #endif // SkCodecPriv_DEFINED