[Bf-cycles] Need help with shaders

Brecht Van Lommel brechtvanlommel at pandora.be
Tue Sep 27 22:23:55 CEST 2011

I've added Transparent Volume and Isotropic Volume nodes now. They
don't actually do anything, it's just the code for getting them from
blender into the kernel, but might be enough to get you started.


On Sun, Sep 25, 2011 at 2:56 AM, storm <kartochka22 at yandex.ru> wrote:
> Hi all, I stuck at problem that was not look as hard from the beginning
> but now I realize that it prevent me to share first half- working
> volumetric patch.
> TL;DR: please create proper particle density node with ability to fast
> detect 3 cases from device inner loop and evaluate scalar density, keep
> all "Diffuse BSDF" logic same.
> I cannot get particle density info from shaders. For now I just
> copy-paste and rename diffuse BSDF closure, and use Color red component
> as density, hard coding phase function pdf to stop Color modify it. It
> is enough for my experiments but suxx for wide testing.
> How to add extra data stream to closure? Any closure have only 1 data
> stream, weight. If we already have Color as attenuation factor in
> diffuse bsdf node, how to add another one (density)? How to evaluate it?
> For optimization we need to get 3 cases that must be checked after any
> ray intersection test before we hit background or triangle:
>        1. volumetric turned off or constant zero particle density in
> homogeneous media (vacuum, air), fast skip any media code, use good old
> render steps. (unbiased, fastest)
>        2. homogeneous media with particle density > 0 (constant mist, fog),
> use well known fast sampling step = -log(random)/sigma (unbiased, fast
> and optimal from MontrCarlo solver point of view)
>        3. inhomogeneous media where particle density can vary around volume
> (clouds, smokes ). Slowest, in general almost unusable in practice with
> current hardware except some corner cases (tiny hull around surfaces
> like short fur or planet glowing atmosphere from far distance camera, or
> data with known max density that vary little from max value, Woodcock
> alg.) (unbiased, extremely slow).
> In my experiments density shader evaluation take ~70% of all time, so we
> really need to distinguish 3 cases as fast as possible, maybe add shader
> bitfield flag to kernel shader texture?
> >From artists point of view there is Volume shader input slot in
> property, so to provide clean and intuitive GUI we must detect that it
> empty or some shader plugged, and what class of media data it provide.
> I lost in all that triple svm/osl/native node system, it too complex for
> me.
> Maybe create 3 different closures? Null phase function, Homogeneous
> phase function, Phase function? And code them to set ShaderData flag to
> return state?
> Or create extra input, now we have Surface, Volume, Displacement. Add
> something like Volume particle Density before Displacement?
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