[Bf-committers] Cycles + AO + Transparency

[Jason Wilkins]

I would tend to agree with François, ambient occlusion is function of
3-space where ao(x,y,z) is the proportion of the solid angle around
that point that contains geometry that blocks ambient light.  All
geometry, even transparent geometry, blocks ambient light.  So, if you
are seeing darkening on a refracting or reflecting surface it is
because you are not combining ambient occlusion correctly, not because
the AO has been calculated incorrectly.  A refracting or reflecting
object really does block ambient light 100% and only lights other
surfaces through caustics (or itself through transmission).

If a surface is partially refractive or reflective, but also diffuse,
then only the diffuse part should be affected by AO.

If it is because the object has alpha transparency then that is not a
physically based kind of lighting and then there really isn't a
"correct" answer.  People might give different answers if you asked
them should the object give AO to itself vs contributing to other
objects in the scene.  From Brecht's explanation it sounds like Cycles
just ignores this kind of transparency for global illumination
purposes.

On Sun, Apr 22, 2012 at 5:21 PM, Brecht Van Lommel
<brechtvanlommel at pandora.be> wrote:
> Augustin is right, it works, but only for the Transparent BSDF. In
> Cycles, Transparent is defined to work as, "render as if there was no
> geometry there", which also affects AO and shadow rays.
>
> The case could be made that the Glass BSDF should also work this way
> for AO rays, but it gets fuzzy then, where depending on the particular
> settings of a BSDF it may seem a good/bad idea to make it transparent.
> Right now there's a clear and consistent rule at least, and you can
> control it with the light path node.
>
> Brecht.
>
> On Sun, Apr 22, 2012 at 11:32 PM, François T. <francoistarlier at gmail.com> wrote:
>> As AO is geometry based, as far as I know, it is not supposed to make
>> difference on its transparency, as at least the basic algorithm only check
>> for mesh data, but not the material.
>>
>>
>> 2012/4/22 Agustin Benavidez <agustinbenavidez at gmail.com>
>>
>>> I am pretty sure it does work with transparent BSDF, and not with caustics
>>> i.e. using glass bsdf , so if you are using glass bsdf, use light path node
>>> to switch to transparent bsdf at least for shadow/diffuse rays or whatever
>>> ray you need.
>>> Cheers
>>>
>>>
>>> 2012/4/22 Tobias Oelgarte <tobias.oelgarte at googlemail.com>
>>>
>>> > I made some experiments with Cycles today and I noticed that AO sees all
>>> > faces as opaque, like the internal Renderer did. That means that a face
>>> > with glass material (entirely or nearly entirely transparent) will be
>>> > dark (itself) at parts close to other geometry. Should AO not have much
>>> > less impact on such surfaces? Otherwise it can be hardly combined with
>>> > transparent materials.
>>> >
>>> > Do you see room for improvement or should it stay this way?
>>> >
>>> > Greetings from
>>> > Tobias Oelgarte
>>> > _______________________________________________
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>>> >
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>>
>>
>>
>> --
>> ____________________
>> François Tarlier
>> www.francois-tarlier.com
>> www.linkedin.com/in/francoistarlier
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