image - Why is saving a picture twice in a row producing different binary in Paint -


i thought little odd. open paint on windows (i'm using windows 7) , draw (anything). save .png example called 1.png. save 'n' number of other copies straight away without modifying image (2.png, 3.png,..etc).

i notice 1.png has different checksum 2/3/4/../n.png. 1.png varies in such (sometimes smaller , other times bigger) compared other images.

what going on?

the difference in filesize due choice of scanline filters used compressor. don't have idea why application use different set of filters when compressing image multiple times, it's harmless.

there's no time stamp in 2 images mohammad posted. according "pngcheck -v", difference in content of idat chunk. image signatures computed imagemagick identical. neither image contains time chunk.

"pngcrush" produces 2 identical images smaller filesize (11493 bytes).

according "pngtest -mv" (pngtest included in libpng distribution), 1 image uses png "none" filter while other uses "none", "sub", , "up" filters.


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