Use possible read_partial_scanlines() API in SkJpegCodec

dm/DMSrcSink.cpp

Index: dm/DMSrcSink.cpp
diff --git a/dm/DMSrcSink.cpp b/dm/DMSrcSink.cpp
index e27628ad043aeedc9676dfd1a00187c843ad2fa8..1ee838e6c753fb29b4effbfbf4dab45a28750458 100644
--- a/dm/DMSrcSink.cpp
+++ b/dm/DMSrcSink.cpp
@@ -373,6 +373,7 @@ Error CodecSrc::draw(SkCanvas* canvas) const {
             break;
         }
         case kStripe_Mode: {
+            const int width = decodeInfo.width();
             const int height = decodeInfo.height();
             // This value is chosen arbitrarily.  We exercise more cases by choosing a value that
             // does not align with image blocks.
@@ -380,7 +381,10 @@ Error CodecSrc::draw(SkCanvas* canvas) const {
             const int numStripes = (height + stripeHeight - 1) / stripeHeight;
 
             // Decode odd stripes
-            if (SkCodec::kSuccess != codec->startScanlineDecode(decodeInfo, NULL, colorPtr,
+            SkCodec::Options opts;
+            SkIRect subset = SkIRect::MakeXYWH(0, 0, width / 2, height);
+            opts.fSubset = ⊂
+            if (SkCodec::kSuccess != codec->startScanlineDecode(decodeInfo, &opts, colorPtr,
                                                                 colorCountPtr)
                     || SkCodec::kTopDown_SkScanlineOrder != codec->getScanlineOrder()) {
                 // This mode was designed to test the new skip scanlines API in libjpeg-turbo.
@@ -403,7 +407,7 @@ Error CodecSrc::draw(SkCanvas* canvas) const {
             }
 
             // Decode even stripes
-            const SkCodec::Result startResult = codec->startScanlineDecode(decodeInfo, nullptr,
+            SkCodec::Result startResult = codec->startScanlineDecode(decodeInfo, &opts,
                     colorPtr, colorCountPtr);
             if (SkCodec::kSuccess != startResult) {
                 return "Failed to restart scanline decoder with same parameters.";
@@ -420,6 +424,48 @@ Error CodecSrc::draw(SkCanvas* canvas) const {
                     codec->skipScanlines(linesToSkip);
                 }
             }
+
+            // Begin duplicated code
+            SkIRect nextSubset = SkIRect::MakeXYWH(width / 2, 0, width / 2 + width % 2, height);
+            opts.fSubset = &nextSubset;
+            startResult = codec->startScanlineDecode(decodeInfo, &opts, colorPtr, colorCountPtr);
+            if (SkCodec::kSuccess != startResult) {
+                return "Failed to restart scanline decoder with same parameters.";
+            }
+
+            for (int i = 0; i < numStripes; i += 2) {
+                // Skip a stripe
+                const int linesToSkip = SkTMin(stripeHeight, height - i * stripeHeight);
+                codec->skipScanlines(linesToSkip);
+
+                // Read a stripe
+                const int startY = (i + 1) * stripeHeight;
+                const int linesToRead = SkTMin(stripeHeight, height - startY);
+                if (linesToRead > 0) {
+                    codec->getScanlines(bitmap.getAddr(width / 2, startY), linesToRead,
+                            bitmap.rowBytes());
+                }
+            }
+
+            // Decode even stripes
+            startResult = codec->startScanlineDecode(decodeInfo, &opts, colorPtr, colorCountPtr);
+            if (SkCodec::kSuccess != startResult) {
+                return "Failed to restart scanline decoder with same parameters.";
+            }
+            for (int i = 0; i < numStripes; i += 2) {
+                // Read a stripe
+                const int startY = i * stripeHeight;
+                const int linesToRead = SkTMin(stripeHeight, height - startY);
+                codec->getScanlines(bitmap.getAddr(width / 2, startY), linesToRead, bitmap.rowBytes());
+
+                // Skip a stripe
+                const int linesToSkip = SkTMin(stripeHeight, height - (i + 1) * stripeHeight);
+                if (linesToSkip > 0) {
+                    codec->skipScanlines(linesToSkip);
+                }
+            }
+
+
             canvas->drawBitmap(bitmap, 0, 0);
             break;
         }