[Bf-cycles] Dithering in preview mode

[David]

Hi,

the "Dither" setting is ignored during preview rendering,
which leads to unattractive banding artifacts in smooth
surfaces.  In an attempt to improve the situation I came
up with the following patch, but I am not sure I like the
solution: it reuses the sobol RNG for dithering, and therefore
has to pass the rng_state to the tonemap function.

Any RNG would need some sort of state to be passed, but alternatively
dithering could be done with error diffusion. A parallel implementation
is described here:
http://people.zoy.org/~sam/HybridDithering.pdf

This would make the code more complex, to put it mildly.

I am unsure what the best solution is, maybe nothing should
be done (for many scenes banding is not a problem).

till then, David.

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