code review 106230044: vp8: skip filtering for all-zero-DC macroblock residuals.

vp8: skip filtering for all-zero-DC macroblock residuals.

This makes the Go code match the libwebp C code's output on
blue-purple-pink-large.*-filter.lossy.webp

Also make the various WEBP benchmarks all decode a similar image,
the image at http://blog.golang.org/gophercon/image01.jpg, to make
it more meaningful to e.g. compare the simple filter's numbers with
the normal filter's numbers.

Also fix a "go vet" warning in webp/decode.go.

The test data was generated by:
wget http://blog.golang.org/gophercon/image01.jpg -O blue-purple-pink-large.jpeg
convert blue-purple-pink-large.jpeg blue-purple-pink-large.png
cwebp -lossless blue-purple-pink-large.png -o blue-purple-pink-large.lossless.webp
cwebp -q 80 -f 0 blue-purple-pink-large.png -o blue-purple-pink-large.no-filter.lossy.webp
cwebp -q 80 -strong blue-purple-pink-large.png -o blue-purple-pink-large.normal-filter.lossy.webp
cwebp -q 80 -nostrong blue-purple-pink-large.png -o blue-purple-pink-large.simple-filter.lossy.webp
dwebp -pgm blue-purple-pink-large.no-filter.lossy.webp -o tmp.pgm && convert tmp.pgm blue-purple-pink-large.no-filter.lossy.webp.ycbcr.png && rm tmp.pgm
dwebp -pgm blue-purple-pink-large.normal-filter.lossy.webp -o tmp.pgm && convert tmp.pgm blue-purple-pink-large.normal-filter.lossy.webp.ycbcr.png && rm tmp.pgm
dwebp -pgm blue-purple-pink-large.simple-filter.lossy.webp -o tmp.pgm && convert tmp.pgm blue-purple-pink-large.simple-filter.lossy.webp.ycbcr.png && rm tmp.pgm

[nigeltao, 2014-06-27 07:17:56]

Hello r@golang.org (cc: golang-codereviews@googlegroups.com),

I'd like you to review this change to
https://code.google.com/p/go.image

[r, 2014-06-27 15:01:49]

LGTM

[nigeltao, 2014-06-30 00:36:46]

*** Submitted as
https://code.google.com/p/go/source/detail?r=58506d3a4ba4&repo=image ***

vp8: skip filtering for all-zero-DC macroblock residuals.

This makes the Go code match the libwebp C code's output on
blue-purple-pink-large.*-filter.lossy.webp

Also make the various WEBP benchmarks all decode a similar image,
the image at http://blog.golang.org/gophercon/image01.jpg, to make
it more meaningful to e.g. compare the simple filter's numbers with
the normal filter's numbers.

Also fix a "go vet" warning in webp/decode.go.

The test data was generated by:
wget http://blog.golang.org/gophercon/image01.jpg -O blue-purple-pink-large.jpeg
convert blue-purple-pink-large.jpeg blue-purple-pink-large.png
cwebp -lossless blue-purple-pink-large.png -o
blue-purple-pink-large.lossless.webp
cwebp -q 80 -f 0 blue-purple-pink-large.png -o
blue-purple-pink-large.no-filter.lossy.webp
cwebp -q 80 -strong blue-purple-pink-large.png -o
blue-purple-pink-large.normal-filter.lossy.webp
cwebp -q 80 -nostrong blue-purple-pink-large.png -o
blue-purple-pink-large.simple-filter.lossy.webp
dwebp -pgm blue-purple-pink-large.no-filter.lossy.webp -o tmp.pgm && convert
tmp.pgm blue-purple-pink-large.no-filter.lossy.webp.ycbcr.png && rm tmp.pgm
dwebp -pgm blue-purple-pink-large.normal-filter.lossy.webp -o tmp.pgm && convert
tmp.pgm blue-purple-pink-large.normal-filter.lossy.webp.ycbcr.png && rm tmp.pgm
dwebp -pgm blue-purple-pink-large.simple-filter.lossy.webp -o tmp.pgm && convert
tmp.pgm blue-purple-pink-large.simple-filter.lossy.webp.ycbcr.png && rm tmp.pgm

LGTM=r
R=r
CC=golang-codereviews
https://codereview.appspot.com/106230044