6 Mar 2002 rlk   » (Journeyer)

EvenTone screening

Seriously good stuff. Mark has been doing a lot of work tuning it. The combinations I've tried have all been significantly smoother than adaptive hybrid; this is actually most noticeable at high resolutions (I'd have expected the opposite).

The one serious problem we have left to solve is "waterfalling". This is a common problem with error diffusion algorithms in general; it takes a while to build up sufficient accumulated error to print anything at all. The result is that there's a strip or small region where there should be a low density of ink that has none at all. I tried eliminating it last night by seeding the error with pseudo-random values to try to perturb some points over the threshold. It worked, but it ruined the smoothness of the texture.

The next thought I have is to perturb the threshold value in a similar pseudo-random fashion (specifically, using a carefully constructed dither matrix). The amount of the perturbation can vary, depending upon the density at the point in question. Waterfalling appears to only be an issue when the ink coverage is less than about 1%; perhaps we can find a way to work with that.

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