Zoompf's Web Performance Blog

Top PNG Optimizers Don’t Use zlib

 Billy Hoffman on January 5, 2010. Category: Optimization

Oleg Kikin has an interesting chart comparing the performance multiple different PNG optimizing tools. The tools tested are:

Go take a look at the PNG comparison chart. I can wait.

So what do these results mean? Well I believe it shows how far image optimization has come in the last 2 years. Tools that just manipulate the parameters for the stock DEFLATE compressor code that is included in the zlib compression library and remove extra PNG chunks no longer produce the smallest optimized image. PNGOut and AdvanceCOMP produce the smallest PNGs because they use custom DEFLATE compressors that achieve better compression than zlib’s implementation. PNGOut’s deflate compressor was written from scratch and AdvanceCOMP uses the custom DEFLATE compressor written for 7Zip. We’ve talked about 7Zip and DEFLATE before in the Rezipping Web Resources for Fun and Profit post. I used 7Zip for my rezipping work because it’s optimized DEFLATE compressor compresses data better than the DEFLATE compressor in zlib. This in turn produces smaller ZIP files but the logic applies to image formats that use DEFLATE.

Unfortunately I cannot find any information about the command line options Oleg used with each tool.

It is interesting to note the difference between Smush.it and PNGCrush. According to Smush.it’s information page it is using PNGcrush under the covers. Any difference in the output of Smush.it and PNGCrush is entirely from the command line options that we know nothing about. It would be possible to reverse engineer what Smush.it is doing by using the service and comparing the output. I image they are using the -m option instead of the -brute option to reduce the number of rounds of PNGCrush and improve the response speed of the Smush.it web service.

What we really need is a web service that accepts images and tries several different optimization tools. Smush.it has hinted at this for a while now in their FAQ but improvements to the tool seem to have stalled since Yahoo took it over (to say nothing of the un-sexy-fying of the Smush.it UI). Hopefully something like this will appear.

Want to see what performance problems you have? Unoptimized PNG images are just one of the 200+ performance issues Zoompf detects while assessing your web applications. You can sign up for a free mini web performance assessment at Zoompf.com today!


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