image/jpeg: decode progressive JPEGs.
To be clear, this supports decoding the bytes on the wire into an
in-memory image. There is no API change: jpeg.Decode will still not
return until the entire image is decoded.
The code is obviously more complicated, and costs around 10% in
performance on baseline JPEGs. The processSOS code could be cleaned up a
bit, and maybe some of that loss can be reclaimed, but I'll leave that
for follow-up CLs, to keep the diff for this one as small as possible.
Before:
BenchmarkDecode 1000
2855637 ns/op 21.64 MB/s
After:
BenchmarkDecodeBaseline 500
3178960 ns/op 19.44 MB/s
BenchmarkDecodeProgressive 500
4082640 ns/op 15.14 MB/s
Fixes #3976.
The test data was generated by:
# Create intermediate files; cjpeg on Ubuntu 10.04 can't read PNG.
convert video-001.png video-001.bmp
convert video-005.gray.png video-005.gray.pgm
# Create new test files.
cjpeg -quality 100 -sample 1x1,1x1,1x1 -progressive video-001.bmp > video-001.progressive.jpeg
cjpeg -quality 50 -sample 2x2,1x1,1x1 video-001.bmp > video-001.q50.420.jpeg
cjpeg -quality 50 -sample 2x1,1x1,1x1 video-001.bmp > video-001.q50.422.jpeg
cjpeg -quality 50 -sample 1x1,1x1,1x1 video-001.bmp > video-001.q50.444.jpeg
cjpeg -quality 50 -sample 2x2,1x1,1x1 -progressive video-001.bmp > video-001.q50.420.progressive.jpeg
cjpeg -quality 50 -sample 2x1,1x1,1x1 -progressive video-001.bmp > video-001.q50.422.progressive.jpeg
cjpeg -quality 50 -sample 1x1,1x1,1x1 -progressive video-001.bmp > video-001.q50.444.progressive.jpeg
cjpeg -quality 50 video-005.gray.pgm > video-005.gray.q50.jpeg
cjpeg -quality 50 -progressive video-005.gray.pgm > video-005.gray.q50.progressive.jpeg
# Delete intermediate files.
rm video-001.bmp video-005.gray.pgm
R=r
CC=golang-dev
https://golang.org/cl/
6684046
