Here is the discussion on why raster images should be interlaced for a slow connection: http://en.wikipedia.org/wiki/Interlacing_(bitmaps)
But I have had the experience that interlacing an image, either under LZW with GIF or Deflate with PNG seems to improve the compression.
I have attached a zip file of the files I was working with, apparently from wikimedia based on the filenames:
My question is, is the benefit from compression because interlacing "exposes" the self-similarity of the image, or is it because the interlacing imposes a sorting of the image elements that is easier to compress ?
I think it is the latter because this seems to work with a lot of line-art images that are drawn by a human. Anybody have an interesting test set of 1bpp images ?