How does Google's Page Speed lossless image compression work?

Question

When you run Google\'s PageSpeed plugin for Firebug/Firefox on a website it will suggest cases where an image can be losslessly compressed, and provide a link to download th

Answer 1

It's a matter of trading encoder's CPU time for compression efficiency. Compression is a search for shorter representations, and if you search harder, you'll find shorter ones.

There is also a matter of using image format capabilities to the fullest, e.g. PNG8+a instead of PNG24+a, optimized Huffman tables in JPEG, etc.

Photoshop doesn't really try hard to do that when saving images for the web, so it's not surprising that any tool beats it.

See

• ImageOptim (lossless) and
• ImageAlpha (lossy) for smaller PNG files (high-level description how it works) and
• JPEGmini/MozJPEG (lossy) for better JPEG compressor.

Answer 2

For windows there are several drag'n'drop interfaces for easy access.

https://sourceforge.net/projects/nikkhokkho/files/FileOptimizer/

For PNG files I found this one for my enjoyment, apparently 3 different tools wrapped in this GIU. Just drag and drop and it does it for you.

https://pnggauntlet.com/

It takes time though, try compressing a 1MB png file - I was amazed how much CPU went into this compression comparison which has to be what is going on here. Seems the image is compressed a 100 ways and the best one wins :D

Regarding the JPG compression I to feel its risky to strip of color profiles and all extra info - however - if everyone is doing it - its the business standard so I just did it myself :D

I saved 113MB on 5500 files on a WP install today, so its definately worth it!

Answer 3

There's a very handy batch script that recursively optimizes images beneath a folder using OptiPNG (from this blog):

FOR /F "tokens=*" %G IN ('dir /s /b *.png') DO optipng -nc -nb -o7 -full %G

ONE LINE!