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

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>

diff --git a/src/image/png/writer.go b/src/image/png/writer.go
index df23270ee975cdcb8b5b38b986a902a02a8b2f40..dd87d81629196002f12a6a4a83476cf9ee0c7c09 100644 (file)
--- a/src/image/png/writer.go
+++ b/src/image/png/writer.go
@@ -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)
                }