Use new jpeg_crop_scanlines() API to optimize jpeg subset decodes

dm/DMSrcSink.cpp

Index: dm/DMSrcSink.cpp
diff --git a/dm/DMSrcSink.cpp b/dm/DMSrcSink.cpp
index 7ccaf223fc3a32b8e2baa867dcd575454332b6ec..bd881e8381739a1c723f8014ed0859fa0a747473 100644
--- a/dm/DMSrcSink.cpp
+++ b/dm/DMSrcSink.cpp
@@ -522,6 +522,31 @@ Error CodecSrc::draw(SkCanvas* canvas) const {
             canvas->drawBitmap(bitmap, 0, 0);
             break;
         }
+        case kCroppedScanline_Mode: {
+            const int width = decodeInfo.width();
+            const int height = decodeInfo.height();
+            // This value is chosen because, as we move across the image, it will sometimes
+            // align with the jpeg block sizes and it will sometimes not.  This allows us

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

Is this because it is not a multiple of 8? (But for after the second tile we'll
have a multiple of 8?)

Doesn't this mean that we'll always end up expanding the width (and every other
tile expanding the start) and only cover the case where we needed the swizzler?

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

The block size is generally 8 or 16.  And the requirement from libjpeg-turbo is
that startX be aligned to the block size.

There is no alignment requirement for the width - I think this wasn't clear.

So we should have:
startX = 0: Aligned no swizzler
startX = 36: Not-aligned
startX = 72: Maybe aligned (depending on if block size is 8 or 16)
startX = 108: Not-aligned
startX = 144: Aligned no swizzler
... 

[scroggo, 2016/02/22 19:20:20]

Ah, so if width changes, it will be to add originalStartX - alignedStartX. I
just looked back at the code in onStartScanlineDecode, and you actually do make
this clear in your comments before calling jpeg_crop_scanline

+            // to test interestingly different code paths in the implementation.
+            const int tileSize = 36;
+
+            SkCodec::Options opts;
+            SkIRect subset;
+            for (int x = 0; x < width; x += tileSize) {
+                subset = SkIRect::MakeXYWH(x, 0, SkTMin(tileSize, width - x), height);
+                opts.fSubset = &subset;
+                if (SkCodec::kSuccess != codec->startScanlineDecode(decodeInfo, &opts,
+                        colorPtr, colorCountPtr)) {
+                    return "Could not start scanline decoder.";
+                }
+
+                codec->getScanlines(bitmap.getAddr(x, 0), height, bitmap.rowBytes());
+            }
+
+            premultiply_if_necessary(bitmap);
+            canvas->drawBitmap(bitmap, 0, 0);
+            break;
+        }
         case kSubset_Mode: {
             // Arbitrarily choose a divisor.
             int divisor = 2;