A new member of the gimp-print development team, Mark Tomlinson, is working on integrating raph's EvenTone dither algorithm into the project. It's still only on the mainline (development), but it's showing absolutely spectacular promise. I thought our Adaptive Hybrid dither algorithm was pretty good (and it certainly is very good, when compared against a lot of others that I've seen), but with the exception of a few specific problems it looks like we're going to have a new flagship dither algorithm fairly soon.
The big improvement is in smoothness. This is particularly noticeable in solid color midtones, but it's also noticeable in some line art, such as the 1 degree spaced radial lines in the CUPS test page. With adaptive hybrid, the lines look somewhat rough; with EvenTone, the lines are absolutely smooth to within the limits of the printer's resolution. The results at 1440x1440 on my Epson Stylus C80 are astounding -- at 1440x720, the 720 DPI vertical resolution is perceptible as very fine stairstepping in the almost-horizontal lines; the 1440 DPI resolution is not. Shame on Epson for underselling the true capability of that printer! They only advertise it as 2880x720. It's really capable of 2880x1440, and there's a real use for it!
There are still a few problems. There's still some waterfall effect in very pale areas (very pale regions near a dark boundary have some separation between the boundary and the printing), but it's much better than most error diffusion I've seen. There are some odd artifacts at 720 DPI on the CUPS test page; that's probably a discrete logic bug somewhere that should be easy to fix. And finally, there's some roughness in some light midtone range (I think about 5-10% density). Those issues notwithstanding -- and I have every confidence that they'll get fixed -- this is really something.
I don't want Gimp-print to just be the best quality free printer driver package. I want it to be the best, period. That means better than the OEM drivers. Quite a few people think that Gimp-print's quality is better than Epson's own drivers, especially in terms of color quality, but judging by what I'm seeing there's still room for improvement. If the way to get the very highest quality output is to use free software, then we suddenly have a much stronger position on the desktop. And that's good for everyone.
This probably isn't going to make 4.2.1; that's going to just fix some bugs and probably improve the situation with Canon printers. It's probably a few months of hard testing and such away from beta release; currently it only supports one kind of output (CMYK from CMY input; it also needs to support grayscale, CMY, and raw CMYK). However, if you want to try it, it's on our development CVS. Maybe I'll do 4.3.0 around the time we do 4.2.1, for people who want to experiment.
Look at the CMYK printing in my previous diary entry. That's something that can't be done with the OEM driver.