To decide where to crop an image, how can I highlight it's most compressible areas?

Posted by Umber Ferrule on Super User See other posts from Super User or by Umber Ferrule
Published on 2009-10-20T20:55:31Z Indexed on 2010/06/10 14:43 UTC
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image

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compression

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visualization

I'm looking to get the most compression out of each of the most popular image formats, such as, JPEG, PNG, GIF, etc.

Ideally, this would be a tool, or a series of transforms that could be performed (perhaps using a macro and then discarded) in popular image editors (Paint.NET/PaintShopPro/PhotoShop/GIMP) to highlight areas which will compress less.

Alternatively, what rules of thumb can be used other than reducing colours (for PNG/GIF), reducing image dimensions, avoiding high detail areas...

I'm not asking for help deciding what format to use for a particular image type as I think this is fairly common knowledge, i.e. diagram and images with few colours = PNG/GIF, photographs = JPEG.

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