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PNG + xz ?

Posted Mar 3, 2013 22:13 UTC (Sun) by tialaramex (subscriber, #21167)
In reply to: PNG + xz ? by endecotp
Parent article: Google releases a better compression algorithm

Filter type 0, method 0 is the unfiltered data. Yes, pngcrush tries this, it tries each of the methods in type 0 filters (the only type so far defined) one at a time.

There's almost certainly an error in either your methodology or your line of reasoning from your results. Since you said you worked with OSM-like map data I'd guess you've fed a bunch of more or less totally blank map tiles (oceans, unmapped regions) in and what you're left measuring is solely the overhead from PNG's metadata.

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PNG + xz ?

Posted Mar 5, 2013 12:21 UTC (Tue) by endecotp (guest, #36428) [Link]

> Filter type 0, method 0 is the unfiltered data. Yes,
> pngcrush tries this

You are right; I was confused. I was misled by the fact that the pngcrush output lists a whole series of "methods" that start with method 1:

IDAT length with method 1 (fm 0 zl 4 zs 0) = 38507
IDAT length with method 2 (fm 1 zl 4 zs 0) = 51462
IDAT length with method 3 (fm 5 zl 4 zs 1) = 71769
IDAT length with method 4 (fm 0 zl 9 zs 1) = 34941
IDAT length with method 7 (fm 0 zl 9 zs 0) = 32868

but this "method" is not the same as the "filter method", which is the "fm" number inside the ().

It remains true that for these images the PNG pre-processing does not help. If you're curious, I have put an example tile here: http://chezphil.org/tmp/maptile.png .

> I'd guess you've fed a bunch of more or less totally blank map tiles

No.

PNG + xz ?

Posted Mar 7, 2013 18:34 UTC (Thu) by huftis (subscriber, #58900) [Link]

Pngcrush just isn’t a very good PNG optimizer. Using OptiPNG or TruePNG, I managed to reduce the file size of your example PNG file by about 26% (from 33.5 KiB to 24.6 KiB).

PNG + xz ?

Posted Mar 7, 2013 18:50 UTC (Thu) by huftis (subscriber, #58900) [Link]

And running PNGZopfli on the result of the TruePNG output gives a file size of only 23.4 KiB. Note that this is with the various max settings of both TruePNG and PNGZopfli, and running this takes a long time, but even with the (very fast) default settings we get a comparable file size – 23.5 KiB.