Neat filter.
For those interested in the source code, it appears to be
here.
The dither seems to be error diffusion with a twist. r1 and l1 flip sign on each line. This is fine for l1 because it applies to the rerr2[] pointers of the next line, but r1 applies to the rerr1[] pointers of the current line, so the accumulating error is added to an already processed pixel
