progressive coding

[skal65535]

jpegrescan can sometimes shrink the size further. There should be an option to have progressive-coded output.

Starting a branch 'prog' for that.

[jzern]

Drive by!

I'm wondering why the output needs to be progressive to see the benefit or if it's just improvement on matrices.

Also:
https://github.com/kud/jpegrescan
NB: MozJPEG has the same optimisation built-in and is faster, so we recommend using MozJPEG when possible.

Not sure if MozJPEG was tried.

cc: @vrabaud

[skal65535]

So, jpegrescan, through jpegtran, adds a dedicated Huffman table for each scan. This has a ~40 bytes cost per scan, so is sometimes worth it (as opposed to a unique Huffman table).
Quantization tables can't be changed between scans (paragraph B.2.4: "The quantization table specification shall not be altered between progressive DCT scans of a given component.").