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For a website I control that sells consumer goods, fast page performance is important, especially for potential customers on mobile devices. Therefore, we want our images to be as small as possible without showing any noticeable degradation of image quality. Because of that, we use PNG files when providing graphic content such as infographics on how a product helps a customer.

The website TinyPNG.com compresses PNG files by stripping metadata and converting from 24 bit to 8 bit depth, so we are using this site to compress PNGs served up from our site. I found that you can run it repeatedly on the same file and it keeps getting smaller. Why would the file keep getting smaller and will the result be noticeable? Do you just get fewer and fewer colors?

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That's exactly it - the file gets smaller because the colors are combined into fewer, more prominent colors. For example, multiple shades of blue living next to each other will combine to display as fewer, more uniform shades, in the process making the image file less heavy. Each time you run your software, it'll find places to optimize this process, though with every run the quality will generally diminish, as will the amount of optimizations the software can perform. Find a sweet spot you can live with, and save your original. I like to use Optimizilla - I've found that in a pinch, it does a good job and gives you plenty of control: http://optimizilla.com/

  • Please note that the reduction in colors is not a feature of PNG compression, which is lossless. It is due to optimization that happens before the image is sent to the PNG compressor. – Glenn Randers-Pehrson Aug 14 '17 at 17:10

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