[ubuntu-art] Recompressing PNGs to save space?

Mark Shuttleworth mark at canonical.com
Tue Apr 25 01:03:13 BST 2006

This is a very cool bit of work from Frank Schoep over on
ubuntu-art at l.u.c that I think might be of interest to ubuntu-devel at .
Very late for such a pervasive change BUT 15MB is not to be sniffed at.

Frank Schoep wrote:
> Hi all,
> I've been fiddling around with the Dapper beta release and I tried an 
> experiment with the graphics data included with it.
> I used a simple "find" statement and a Bash script to copy over all PNG images 
> in the default installation and recompressed all these images using a 
> combination of PNGRewrite, OptiPNG and AdvancePNG to see if there was any 
> space to be saved, the results of the recompression are as follows:
> - original PNGs apparent bytes: 53.636.628
> - recompressed apparent bytes: 38.185.898
> - apparent bytes saved: 15.450.730
> So, recompressing all distributed PNGs would save just over 15 megabytes in 
> the actual default installation. The results can also be expressed in 
> disk-blocks, since small files often take up a whole disk block the results 
> are quite surprising:
> - original PNGs disk-blocks: 86.428
> - recompressed disk-blocks: 72.012
> - disk-blocks saved: 14.416
> Looking at it this way, the recompression would save about fourteen and a half 
> thousand disk blocks. I also looked at how well the original and recompressed 
> PNGs would be archived using BZIP2:
> - original PNGs bzip2 archive in bytes: 40.028.323
> - recompressed PNGs bzip2 archive: 32.109.940
> - archived bytes saved: 7.918.383
> So, looking at the archived versions we can save eight megabytes by 
> recompressing the PNGs.
> My actual question is this: is it worth it to optimize all PNGs by 
> recompressing them? It doesn't affect the appearance or content, it only 
> makes each file as small as possible.
> By doing this, we could probably trim about seven megabytes off the install CD 
> (which amounts to 1% of the total size) and we can cut 15 megabytes off the 
> required hard disk space for installation. I think that because of the 
> smaller file sizes, there's also an actual performance benefit here (or is it 
> neglible?)
> To give an indication on the amount of time required for recompression: it 
> took my Athlon 64 3200+ about 24 hours to recompress all PNGs included in the 
> Dapper beta default desktop installation.
> Another proposition would be to build an automated PNG optimizer so that all 
> PNG in packages are automatically recompressed without any user intervention.
> I hope my research interests some of you, if anything was unclear I'd be happy 
> to explain more of the process and the results. Just ask away!
> With kind regards,
> Frank Schoep

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