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186

PNG is compressed, but lossless The compression level is a tradeoff between file size and encoding/decoding speed. To overly generalize, even non-image formats, such as FLAC, have similar concepts. Different compression levels, same decoded output Although the file sizes are different, due to the different compression levels, the actual decoded output ...


157

PNG is lossless. GIMP is most likely just not using the best word in this case. Think of it as "quality of compression", or in other words, "level of compression". With lower compression, you get a bigger file, but it takes less time to produce, whereas with higher compression, you get a smaller file that takes longer to produce. Typically you get ...


22

PNG compression happens in two stages. Pre-compression re-arranges the image data so that it will be more compressible by a general purpose compression algorithm. The actual compression is done by DEFLATE, which searches for, and eliminates duplicate byte-sequences by replacing them with short tokens. Since step 2 is a very time/resource intensive task, ...


13

A primary motivation for the PNG format was to create a replacement for GIF that was not only free but also an improvement over it in essentially all respects. As a result, PNG compression is completely lossless - that is, the original image data can be reconstructed exactly, bit for bit - just as in GIF and most forms of TIFF. PNG uses a 2-stage ...


4

OK, I am too late for the bounty, but here is my answer anyway. PNG is always lossless. It uses Deflate/Inflate algorithm, similar to those used in zip programs. Deflate algorithm searches repeated sequences of bytes and replaces those with tags. The compression level setting specifies how much effort the program uses to find the optimal combination of ...


3

Compression level in lossless stuff is always just trading encode resources (usually time, sometimes also RAM) vs. bitrate. Quality is always 100%. Of course, lossless compressors can NEVER guarantee any actual compression. Random data is incompressible, there's no pattern to find and no similarity. Shannon information theory and all that. The whole ...


1

Found a solution, download any command line compression utility, for example: xz (it gives good compression ration: from 1.2GB database size to just 100MB), put it on mysql directory using smb://, then run cmd.exe, change directory to the mysql directory using cd, then execute: mysqldump -u user -p --all-databases | xz > all.sql.xz then just copy the ...


1

Using a 'for' loop will definitely work - and is a good general technique - but you almost certainly have more than 1 processor on your machine, so why do just one conversion at a time? You can get things moving a lot quicker if you do: find *.jpg | xargs -n1 -P8 -I{} convert -resize 20% "{}" "opt-{}" The arguments to xargs are: n1 - Only give ...



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