Use new jpeg_crop_scanlines() API to optimize jpeg subset decodes

src/codec/SkJpegCodec.cpp

Index: src/codec/SkJpegCodec.cpp
diff --git a/src/codec/SkJpegCodec.cpp b/src/codec/SkJpegCodec.cpp
index e1440af5c3b6c79651361e1e20e61660258c7f1c..b54871b9a6d83f05a3c1385d2bd0ab96decf43ae 100644
--- a/src/codec/SkJpegCodec.cpp
+++ b/src/codec/SkJpegCodec.cpp
@@ -371,7 +371,13 @@ void SkJpegCodec::initializeSwizzler(const SkImageInfo& dstInfo, const Options&
         srcConfig = SkSwizzler::kRGB;
     }
 
-    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;

[scroggo, 2016/02/22 15:55:03]

Should we assert here that fSwizzlerSubset is initialized logically? It looks
like we should never hit a case where we didn't initialize it, but it requires
knowing the callers of this method, and the fact that one of them early exits if
options.fSubset is non-NULL, and the other one initializes fSwizzlerSubset
before calling this method.

[msarett, 2016/02/22 17:38:16]

Yes that is a bit confusing.  I'll add an assert.

+    }
+    fSwizzler.reset(SkSwizzler::CreateSwizzler(srcConfig, nullptr, dstInfo, swizzlerOptions));
     SkASSERT(fSwizzler);
     fStorage.reset(get_row_bytes(fDecoderMgr->dinfo()));
     fSrcRow = fStorage.get();
@@ -411,12 +417,51 @@ SkCodec::Result SkJpegCodec::onStartScanlineDecode(const SkImageInfo& dstInfo,
         return kInvalidInput;
     }
 
+    if (options.fSubset) {
+        fSwizzlerSubset = *options.fSubset;
+    }
+
+#ifdef TURBO_HAS_CROP
+    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_crop_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,

[scroggo, 2016/02/22 15:55:03]

So if I understand correctly, this is set here (rather than inside the if
statement below) so that even when we do not need to do our own subsetting, but
we still need a swizzler for CMYK, initializeSwizzler can blindly use
fSwizzlerSubset?

Should we test CMYK images with subsets that do and do not require further
subsetting?

[msarett, 2016/02/22 17:38:16]

Yes that's correct.
This will be covered by my new implementation of kCroppedScanline_Mode in
DMSrcSink.

[scroggo, 2016/02/22 18:46:47]

Maybe add a comment?

[msarett, 2016/02/22 19:07:43]

Done.

+                options.fSubset->width(), options.fSubset->height());

[scroggo, 2016/02/22 15:55:03]

It seems odd that we ever care about the startY and height here. These don't
actually affect anything, do they?

[msarett, 2016/02/22 17:38:16]

No they don't.  They are ignored by SkSwizzler.  The base class implementation
of startScanlineDecode() requires that they are (0, height), since we don't
need/want to specify vertical subsets here.

[scroggo, 2016/02/22 18:46:47]

Maybe add a comment?

[msarett, 2016/02/22 19:07:43]

Done.

+
+        // We will need a swizzler if libjpeg-turbo cannot provide the exact
+        // subset that we request.  Or if we are converting from CMYK.

[scroggo, 2016/02/22 15:55:03]

This part of the comment seems unnecessary here. It doesn't really affect this
code, and we already have a comment down below where it's relevant.

[msarett, 2016/02/22 17:38:16]

Done.

+        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.
     J_COLOR_SPACE colorSpace = fDecoderMgr->dinfo()->out_color_space;
     if (options.fSubset || JCS_CMYK == colorSpace || JCS_RGB == colorSpace) {
         this->initializeSwizzler(dstInfo, options);
     }
+#endif
 
     return kSuccess;
 }