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Could someone explain what a "repacker" is? What I know: In Photoshop, when I save an image as JPEG, I can decide the "quality" (Low, Medium, High, etc). The l

Could someone explain what a "repacker" is? What I know: In Photoshop, when I save an image as JPEG, I can decide the "quality" (Low, Medium, High, etc). The lower it is, the smaller the file size but the image will have (more) artifacts. The resulting image can then be opened in any image viewer including browsers. Also, I was told to save the "master" copy in a lossless format (e.g. TIFF or PNG) because JPEG is a lossy format (like MP3 to WAV). So how does a "repacker" come to play?

detaro 7y ago
> Brunsli allows for a 22% decrease in file size while allowing the original JPEG to be recovered byte-by-byte. It's a lossless file compression application that's specialized on compressing a specific file format, so it can beat generic compression tools like gzip.
mkl 7y ago
JPEG compression works in two phases: 1. Discrete Cosine Transform, discarding insignificant information. 2. Compression of bitstream. Step 1 is lossy in practice because it throws away things that seem insignificant. The "quality" control determines what counts as insignificant. Step 2 is lossless, and just tries to make the data coming out of Step 1 take up as little space as possible. A repacker redoes Step 2 better: it makes the file smaller without reducing the quality, by changing how the data is compressed, not changing which parts are kept.