JPEGrescan: unique way to losslessly shrink any JPEG file

by DarkShikari • 17 years ago

54 11 17 years ago

Is there any other information on what he's doing in the code and why? I wouldn't mind implementing something like this in a project I'm working on.

DarkShikari3• 17 years ago

He's basically taking advantage of a trick which is almost never used for compression purposes, so it can be used after any JPEG encoder to improve compression, since nobody else seems to do this.

Specifically, it abuses the progressive scan system. It's well-known that progressive scan in JPEG (which causes the image to load starting at the lowest detail and being refined up to full quality) doesn't just add to usability for the viewer of the image, but it also slightly improves compression. However, progressive scan allows you to specify almost any splitting of the coefficients--each of which gets its own Huffman table for compression.

To make this script, Loren did an exhaustive search of all possible splitting options on a large collection of images. He then collected statistics from this and used it to devise a pretty fast and simple search of the most common best-split situations to maximize compression. You can read the comments for more specific details.

Extra note: Loren has been the primary maintainer of the x264 video encoder over the past 5 years and is also an ffmpeg developer.

omouse• 17 years ago

The GIMP takes advantage of this and lets you specify the coefficient (to an extent).

blasdel• 17 years ago

So it's the JPEG version of PNGcrush et. al.?

jacquesm• 17 years ago

The way I read the code it tries a number of scans scripts by passing them to jpegtran one by one and remembering the size of the output.

The best result is then used.

pbhj• 17 years ago

an equivalent to how pngcrush works then

johng• 17 years ago

Just ran this on some files, random files about 801 or so jpg files...

Pre utility: du -d 1 -k 39710 .

Post utility: du -d 1 -k 37074 .

So about a 6-7% reduction. These are files uploaded via wordpress, not sure if it does any type of reduction by itself in the upload wizard, i doubt it.

Time to run the utility:

      279.84 real       176.08 user        72.19 sys

csbrooks• 17 years ago

Hey! If the title is accurate, we can just keep applying it over and over, until every jpeg shrinks to 0 bytes!

Yaaaay!

JPEGrescan: unique way to losslessly shrink any JPEG file

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