python - The images are different even though they are saved to the same quality -

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i used function imwrite of opencv python in order convert set of images png jpeg format, without specifying quality (the default value 95), obtained first set of jpeg images.

then used same function in order convert same set of images png jpeg format, specifying value 75 quality, obtained second set of jpeg images.

finally, tried use same function in order convert first set of jpeg images, specifying value 75 quality, obtained third set of jpeg images.

when perform binary comparison between second , third set of jpeg images, images different. why? target quality same (75), source images different.

when first compression (quality 95) first set of jpegs not going identical input pngs when uncompressed. jpeg lossy. means input quality 75 compression not same. in first case original png images, , in second case jpeg distorted version of same files. since input images not same, output not same either.


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