A followup on Go2Webcrap codec details

I didn’t have much interest in this codec to start with. So I’ll just leave it here.

I suspect that both newer and older versions of the codec divide it into sharp details overlay over smooth image a bit like DjVu format does.

For compression method 2 (G2M2/G2M3) sharp details and filled regions are coded with ELS (weird binary coder). There’s also a special transparent pixel value of course. Smooth image seems to JPEG scan data without any other headers. They just use standard codes and default quantisation matrix from libjpeg6b with default quality setting 75.
I’d name this scheme JPEG-Binary Coder or J-B_K for short.

Here’s an example of something I hacked this morning to somehow decode baseline JPEG:
Would you be able to recognize any details here? I guess not.

And just think how wonderful to decode RGB mask and JPEG data in YUV format and combine them in the same image.

Compression scheme 3 talks about natural and synthetic image layers. It uses zlib to code some chunks inside tile data. And something else in addition to it.

Here’s an example of data there:

I remind that everybody is welcome to RE this codec (and I have more interesting codecs to deal with; I’m not VLC after all).

One Response to “A followup on Go2Webcrap codec details”

  1. compn says:

    let those french guys reverse engineer their own codecs! 😛