Incremental decode: only use subset for subsetting

src/codec/SkSampledCodec.cpp

Index: src/codec/SkSampledCodec.cpp
diff --git a/src/codec/SkSampledCodec.cpp b/src/codec/SkSampledCodec.cpp
index 46286cff9da6f868d181e3250c6bdbe08fe7e41d..22b5cfae5aad31e3d40877fd76c08976fa55fd5e 100644
--- a/src/codec/SkSampledCodec.cpp
+++ b/src/codec/SkSampledCodec.cpp
@@ -186,7 +186,7 @@ SkCodec::Result SkSampledCodec::sampledDecode(const SkImageInfo& info, void* pix
         // We will need to know about subsetting in the y-dimension in order to use the
         // scanline decoder.
         // Update the subset to account for scaling done by this->codec().
-        SkIRect* subsetPtr = options.fSubset;
+        const SkIRect* subsetPtr = options.fSubset;
 
         // Do the divide ourselves, instead of calling get_scaled_dimension. If
         // X and Y are 0, they should remain 0, rather than being upgraded to 1
@@ -210,7 +210,7 @@ SkCodec::Result SkSampledCodec::sampledDecode(const SkImageInfo& info, void* pix
 
     const int samplingOffsetY = get_start_coord(sampleY);
     const int startY = samplingOffsetY + subsetY;
-    int dstHeight = info.height();
+    const int dstHeight = info.height();
 
     const SkImageInfo nativeInfo = info.makeWH(nativeSize.width(), nativeSize.height());
 
@@ -218,22 +218,15 @@ SkCodec::Result SkSampledCodec::sampledDecode(const SkImageInfo& info, void* pix
         // Although startScanlineDecode expects the bottom and top to match the
         // SkImageInfo, startIncrementalDecode uses them to determine which rows to
         // decode.
-        // Note: We *could* use "subsetY" and "subsetHeight" (calculated above) for
-        // incrementalSubset, but this code gives us a tighter bounds on the subset,
-        // meaning that we can start with the first row actually needed by the output,
-        // and stop when we've decoded the last row needed by the output.
+        SkCodec::Options incrementalOptions = sampledOptions;
         SkIRect incrementalSubset;
-        incrementalSubset.fTop = startY;
-        incrementalSubset.fBottom = startY + (dstHeight - 1) * sampleY + 1;
         if (sampledOptions.fSubset) {
+            incrementalSubset.fTop = subsetY;
+            incrementalSubset.fBottom = subsetY + subsetHeight;
             incrementalSubset.fLeft = sampledOptions.fSubset->fLeft;
             incrementalSubset.fRight = sampledOptions.fSubset->fRight;
-        } else {
-            incrementalSubset.fLeft = 0;
-            incrementalSubset.fRight = nativeSize.width();
+            incrementalOptions.fSubset = &incrementalSubset;
         }
-        SkCodec::Options incrementalOptions = sampledOptions;
-        incrementalOptions.fSubset = &incrementalSubset;
         const SkCodec::Result startResult = this->codec()->startIncrementalDecode(nativeInfo,
                 pixels, rowBytes, &incrementalOptions, options.fColorPtr, options.fColorCount);
         if (SkCodec::kSuccess == startResult) {