PNG files re compress?

[KB1]

Hello

Wondered when you open a png in Photoshop CS 1, and alter it then save it, does the png file get re-compressed further, each time you change and save it or does it remain the same?

Thanks
KB

[mburt]

It shouldn't. And if it does there's something wrong, because it saves by the file's width/height/detail etc.
So it should remain the same.

[KB1]

That's good news thanks...

KB

[Twey]

It would be nice if it did, but no :)

PNG is a lossless algorithm; its compression does not affect the image's quality.

[ItsMeOnly]

Actually, there can be some truth in "recompressing", as newer algorithms (never versions of libbz2) have improved strenght due being more optimised- you can get PNG recompressed to lesser filesize with newer software (but of coure there's sometimes an absolute limit- usually 92-94 bytes in my test cases for files that had more than one pixel)

[Twey]

pngcrush (http://www.google.com/search?q=pngcrush&btnI=true) is always worth looking into. It doesn't have much of an effect on GIMP-created PNGs (I guess they're already optimised to the limit), but I hear it has quite an effect on Photoshop-engineered ones.