Could be considered OT, but front end devs are usually chopping images in
their day to day, and this trick has become standard practice for me.

Using the posterize layer effect (in Photoshop) will let you constrain the
number of unique colour values.
For example instead of an image having 200 different unique colour values,
you can limit it to 100 or whatever, and still save as a PNG-24.

Eg
http://www.kacevisual.com/files/img-test/not-posterized.png - 14.19KB (256+
colours)
http://www.kacevisual.com/files/img-test/posterized.png - 5.94KB (40
colours)
http://www.kacevisual.com/files/img-test/compare.tif - both, can you tell
which is which?

There are certainly many other tricks to reduce image file size, but this is
just so easy and almost always makes a huge difference.

Another step can be to duplicate the layer, posterize 1, and use the other
with a mask to fill in the areas where you can see steps between the colours
(often in a gradient).

Further reading:
http://www.smashingmagazine.com/2009/07/15/clever-png-optimization-techniques/

Kevin

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