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image/png: improve compression by skipping filter for paletted images

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Compression of paletted images is more efficient if they are not filtered.
This patch skips filtering for cbP8 images.
The improvements are demonstrated at https://github.com/olt/compressbench

Fixes #16196

Change-Id: Ie973aad287cacf9057e394bb01cf0e4448a77618
Reviewed-on: https://go-review.googlesource.com/29872
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
This commit is contained in:
Oliver Tonnhofer 2016-09-27 15:24:00 +02:00 committed by Brad Fitzpatrick
parent 590f3f0c9d
commit 7de7d20e9f
1 changed files with 4 additions and 1 deletions
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5
src/image/png/writer.go
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@ -420,8 +420,11 @@ func writeImage(w io.Writer, m image.Image, cb int, level int) error {
}
// Apply the filter.
// Skip filter for NoCompression and paletted images (cbP8) as
// "filters are rarely useful on palette images" and will result
// in larger files (see http://www.libpng.org/pub/png/book/chapter09.html).
f := ftNone
if level != zlib.NoCompression {
if level != zlib.NoCompression && cb != cbP8 {
f = filter(&cr, pr, bpp)
}