-- Christopher Howard http://indicium.us http://theologia.indicium.us
--- Begin Message --------BEGIN PGP SIGNED MESSAGE----- Hash: SHA1Jay Cox wrote: > > On Oct 20, 2009, at 1:39 PM, Sven Neumann wrote: > >> Hi, >> >> On Tue, 2009-10-20 at 12:22 -0800, Christopher Howard wrote: >> >>> Though having a far from sufficient understanding of how the GIMP brush >>> painting process works, it seems to me like this is the fundamental >>> problem: In making black-and-white brushes semi-transparent, GIMP >>> somehow "levels out" the grey-scale, so that certain ranges of darkness >>> become the same. On a fuzzy brush, the result is that the fine >>> graduation is changed to appear stepped or banded. >> >> The problem here is that the brush masks are 8bit only. With such a >> limited set of values to start with, banding is basically unavoidable. >> >> >> Sven > > You should be able to get rid of most of the banding by performing error > diffusion when multiplying the opacity into the brush mask. I suspect > that a simple 1d error diffusion algorithm will be sufficient and I > wouldn't bother with a Floyd-Steinberg type algorithm unless you could > come up with an example where the simpler algorithm showed visible > artifacts. > > I believe the function that introduces the banding is > apply_mask_to_sub_region in paint-funcs.c or in the function > apply_mask_to_alpha_channel depending on your point of view. I have not > worked with that code in a long time and I could be wrong about that. > > You will need to make the starting error term effectively random or you > run the risk of letting the upper-left pixel in each tile get special > treatment. > > Good luck, > Jay Cox > jay...@gimp.org Thank you for your guidance in this matter. However, I am wondering: Wouldn't error diffusion tend to sharpen, rather than smooth out, the banding? It seems like some kind of blurring is more what we need. I played around in apply_mask_to_alpha_channel() a little bit and my results seemed to confirm that. But maybe I misunderstand error diffusion or precisely what you mean about how to use it? - -- Christopher Howard http://indicium.us http://theologia.indicium.us -----BEGIN PGP SIGNATURE----- Version: GnuPG v2.0.11 (GNU/Linux) Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org iEYEARECAAYFAksEcwwACgkQQ5FLNdi0BcWKRQCfct8rNIrRpQ9Rr6h1l7AQE1ft 9KEAn1KyfNh66uNVFpmF301gJI4zQ4tn =/vnh -----END PGP SIGNATURE----- --------- To unsubscribe, send email to <aklug-requ...@aklug.org> with 'unsubscribe' in the message body.
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