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I can export a file from PSD (photoshop) to PNG (portable network graphics) format.

With an image sized 1024x768 I usually get a PNG file that's approximately 120KB. However, if I open that exact same image with Apple's preview, and save it with no modifications, that file goes all the way down to near 40KB (one third of the original file).

Why is that? Is Preview losing some vital information? Is Photoshop adding extra information for internal use and Preview gets rid of it?

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2 Answers

There are two factors:

  1. Images can contain additional invisible/unused meta-data (such as comments, information about program that created the file and color profiles that are not used by many PNG-reading programs)

  2. Compression can be done poorly, trading off speed for efficiency (or if PNG-saving program is lousy it may be both slow and compress poorly :)

You can remove invisible junk from PNGs and/or re-compress them with heavier compression without losing any quality with image optimizers.

ImageOptim is a Mac GUI for all popular image optimizers.

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Have you considered, while you're answering some of these questions, maybe also rephrasing the titles into questions, if they aren't already, and maybe clearing up the question to reflect what the asking-person was looking for? I've been looking at quite a few recently, and fixing non-questions and crappy questions to better questions. –  killermist Mar 4 '13 at 0:24
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One can examine the difference in Photoshop via the "Image Information" command (shift+I).

Apple's preview has surely reduced the number of colors in the image, therefore gaining space and compression by reducing the number of bits taken up by the pixels.

This kind of "optimization" means of course a loss of quality.

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In the majority of cases, PNG compression is visually lossless. In other words, there will be no differences in visual perception of the files, but you will probably lose metadata and other junk that you don't need. Tip to web devs: Always compress your PNG files. –  Wk_of_Angmar Mar 4 '13 at 1:44
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