Christopher Howard
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Jay 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?

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Christopher Howard
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