1137016 - Speed up the PNG encoder by using a lower zlib compression level

Status: RESOLVED DUPLICATE of bug 1724331

(Core :: Graphics: ImageLib, enhancement)

Description

[Glenn Randers-Pehrson]

This is a spinoff of bug #1132081.  The PNG encoder (image/encoders/png/nsPNGEncoder.cpp) uses the default zlib compression level.  By switching to a lower level such as 4 we can save about 10 milliseconds per 640x300 image that is encoded.  This mainly occurs when thumbnails of web pages are collected for later use as menu items when a new tab is opened.  It's only a one-line addition to nsPNGEncoder.cpp, but some reference tests would need to be updated too (which is why this change was dropped from bug #1132081).

Comment 1

[Glenn Randers-Pehrson]

Attachment: v00-1137016-part01-png-encoder-level-4.diff

Part 01 is the changes to code in nsPNGEncoder.cpp and media/libpng/pnglibconf.h,
OK to build now but some tests will fail.

Part 02 will contain updated tests.

Comment 2

[Glenn Randers-Pehrson]

Pngcrush results with 26 cached thumbnails found in $HOME/.cache/mozilla/firefox/*/thumbnails indicate that level 4 is a good tradeoff between time and filesize.  Command was pngcrush -force -m 1 -f 0 -l $L -e _test.png *.png

  zlib  encode    total
 level   time     bytes

   1    0.638    1991606
   2    0.660    1946166
   3    0.719    1904228
   4    0.965    1779118
   5    1.075    1758058
   6    1.333    1726207
   7    1.592    1719363

Another 150k or so can be saved by storing them as RGB (PNG24) instead of RGBA (PNG32) images, but that's for another bug.

Comment 3

[Glenn Randers-Pehrson]

For completeness I should have included the level 0 (uncompressed) result:

    0    0.667   23976550

It's slower than levels 1 and 2 because of the extra IO time expended in writing
24 megabytes of output instead of less than 2 megabytes.

Comment 4

[BugBot [:suhaib / :marco/ :calixte]]

The bug assignee didn't login in Bugzilla in the last 7 months, so the assignee is being reset.

Comment 5

[Timothy Nikkel (:tnikkel)]

I didn't know this bug existed. We ended up doing something similar in bug 1724331.