Add scaled subset API to SkCodec

src/codec/SkCodecPriv.h

Index: src/codec/SkCodecPriv.h
diff --git a/src/codec/SkCodecPriv.h b/src/codec/SkCodecPriv.h
index 0442625dd20b858ee7e9cd84877f3417124fcbd1..766b222e6bee2976f05aa0f923f19df7692735ae 100644
--- a/src/codec/SkCodecPriv.h
+++ b/src/codec/SkCodecPriv.h
@@ -31,6 +31,10 @@
 #define COMPUTE_RESULT_ALPHA                    \
     SkSwizzler::GetResult(zeroAlpha, maxAlpha);
 
+inline bool is_valid_subset(const SkIRect& subset, const SkISize& imageDims) {
+    return SkIRect::MakeSize(imageDims).contains(subset);
+}
+
 /*
  * 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
@@ -64,21 +68,28 @@ inline int get_dst_coord(int srcCoord, int sampleFactor) { return srcCoord / sam
  * 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.
+ *
+ * The inputs may be x-coordinates or y-coordinates, but for clarity, it helps
+ * to name variables using y-coordinate naming conventions.

[scroggo, 2015/10/12 20:47:07]

Was y picked arbitrarily over x? I understand it makes sense to be less
abstract, but I don't see why y is more clear.

  *
  * This does not need to be called and is not called when sampleFactor == 1.
+ *
+ * @param srcY               Unscaled, original y-coordinate
+ * @param sampleY            Sampling factor used in the y-dimension
+ * @param scaledSubsetHeight Height of the final output, after any scaling or subsetting
+ * @param subsetY            Top of the subset as an unscaled, original y-coordinate
  */
-inline bool is_coord_necessary(int srcCoord, int sampleFactor, int scaledDim) {
+inline bool is_coord_necessary(int srcY, int sampleY, int scaledSubsetHeight, int subsetY) {
     // Get the first coordinate that we want to keep
-    int startCoord = get_start_coord(sampleFactor);
+    int startY = get_start_coord(sampleY) + subsetY;
 
     // Return false on edge cases
-    if (srcCoord < startCoord || get_dst_coord(srcCoord, sampleFactor) >= scaledDim) {
+    if (srcY < startY || get_dst_coord(srcY - subsetY, sampleY) >= scaledSubsetHeight) {
         return false;
     }
 
-    // Every sampleFactor rows are necessary
-    return ((srcCoord - startCoord) % sampleFactor) == 0;
+    // Every sampleY rows are necessary
+    return ((srcY - startY) % sampleY) == 0;
 }
 
 inline bool valid_alpha(SkAlphaType dstAlpha, SkAlphaType srcAlpha) {