Some archive programs can use information about one file to help compress the next file better. In other words, each file is separately compressed, then the compressed files are concatenated into an archive. Some archive programs - including zip - compress each file independently, with no memory from one file to another file. There are actually two different effects happening here:Įach file compressed independently. Larger dictionaries mean larger code-word size, and even if two images are exactly identical you may have to encode the second one using multiple code-words (which point into the first). I don't know how the PNG compression works you may want to check the hex representations of the images you have for shared substrings manually.Īlso note that even with changed parameters and redundancy to exploit, you won't get down to the size of one image. If there are differences, compressed images may not look anything alike in memory. Note that the above only really applies if you have identical images or almost identical uncompressed images. block/chunk size (later methods) are at least as large as two images, you will probably see further compression. You can try and manually change the parameters of the compression method if window size (LZ77) resp. The details differ between the methods, but the bottom line is that by the time the algorithm reaches the second image, it has already "forgotten" the beginning of the first. At least those in the Lempel-Ziv family ( gzip uses LZ77, zip apparently mostly does as well, and xz uses LZMA) compress somewhat locally: Similarities that lie far away from each other can not be identified. Make sure that the button for constraining the proportions is selected.Have a look at how compression algorithms work.
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