Use possible read_partial_scanlines() API in SkJpegCodec

src/codec/SkJpegCodec.cpp

Index: src/codec/SkJpegCodec.cpp
diff --git a/src/codec/SkJpegCodec.cpp b/src/codec/SkJpegCodec.cpp
index 7db772da6386ad9a3b0002d5334598a625fd6fa2..34b5b5a826ce953b631704563e98bcbe2e751a4e 100644
--- a/src/codec/SkJpegCodec.cpp
+++ b/src/codec/SkJpegCodec.cpp
@@ -348,7 +348,13 @@ void SkJpegCodec::initializeSwizzler(const SkImageInfo& dstInfo, const Options&
         }
     }
 
-    fSwizzler.reset(SkSwizzler::CreateSwizzler(srcConfig, nullptr, dstInfo, options));
+    Options swizzlerOptions = options;
+    if (options.fSubset) {
+        // Use fSwizzlerSubset if this is a subset decode.  This is necessary in the case
+        // where libjpeg-turbo provides a subset and then we need to subset it further.
+        swizzlerOptions.fSubset = &fSwizzlerSubset;
+    }
+    fSwizzler.reset(SkSwizzler::CreateSwizzler(srcConfig, nullptr, dstInfo, swizzlerOptions));
     fStorage.reset(get_row_bytes(fDecoderMgr->dinfo()));
     fSrcRow = fStorage.get();
 }
@@ -387,11 +393,50 @@ SkCodec::Result SkJpegCodec::onStartScanlineDecode(const SkImageInfo& dstInfo,
         return kInvalidInput;
     }
 
+    if (options.fSubset) {
+        fSwizzlerSubset = *options.fSubset;
+    }
+
+#ifdef TURBO_HAS_SUBSET
+    if (options.fSubset) {
+        uint32_t startX = options.fSubset->x();
+        uint32_t width = options.fSubset->width();
+
+        // libjpeg-turbo may need to align startX to a multiple of the IDCT
+        // block size.  If this is the case, it will decrease the value of
+        // startX to the appropriate alignment and also increase the value
+        // of width so that the right edge of the requested subset remains
+        // the same.
+        jpeg_set_partial_scanline(fDecoderMgr->dinfo(), &startX, &width);
+
+        SkASSERT(startX <= (uint32_t) options.fSubset->x());
+        SkASSERT(width >= (uint32_t) options.fSubset->width());
+        SkASSERT(startX + width >= (uint32_t) options.fSubset->right());
+
+        // Instruct the swizzler (if it is necessary) to further subset the
+        // output provided by libjpeg-turbo
+        fSwizzlerSubset.setXYWH(options.fSubset->x() - startX, 0,
+                options.fSubset->width(), options.fSubset->height());
+
+        // We will need a swizzler if libjpeg-turbo cannot provide the exact
+        // subset that we request.  Or if we are converting from CMYK.
+        if (startX != (uint32_t) options.fSubset->x() ||
+                width != (uint32_t) options.fSubset->width()) {
+            this->initializeSwizzler(dstInfo, options);
+        }
+    }
+
+    // Make sure we have a swizzler if we are converting from CMYK.
+    if (!fSwizzler && JCS_CMYK == fDecoderMgr->dinfo()->out_color_space) {
+        this->initializeSwizzler(dstInfo, options);
+    }
+#else
     // We will need a swizzler if we are performing a subset decode or
     // converting from CMYK.
     if (options.fSubset || JCS_CMYK == fDecoderMgr->dinfo()->out_color_space) {
         this->initializeSwizzler(dstInfo, options);
     }
+#endif
 
     return kSuccess;
 }