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How does IBL and Translucency play together?

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Hello guys,

I have a fully working PBR and IBL implementation and now want to integrate translucency into my engine.
I read up on translucency and decided to go with: https://colinbarrebrisebois.com/2011/03/07/gdc-2011-approximating-translucency-for-a-fast-cheap-and-convincing-subsurface-scattering-look/.
I don't understand how this works with IBL; A pointlight okay. But what do you do with an environment map?

My understanding of a BSSRDF is that a fragment is shaded according to many light rays coming from various points on the surface and scattering through the medium + diffuse and specular part; I can see how the thickness map is a approximation of that using only a pointlight and a constant ambient term; An envmap however has to be sampled multiple times because now the ambient term is no more constant and so the light rays that scatter through the surface are of different wavelengths everytime.

I think the described method doesn't fit a PBR implementation, does it?
So my question: How can I implement this method with IBL and are there better approaches to translucency that fit PBR better?

Thanks

 

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What implementation of IBL are you using? The most common one is Karis' preconvolved environment maps. These pre-filter the env-map using a simplified specular BDRF as the filter, and then constructing a look-up table that lets you somewhat correct for the error between the simplification and the 'real' result. If you're using this method, you will have to pre-filter your env-maps twice -- once for your "normal" surfaces, and once for your translucent surfaces.

You can also implement importance sampling of the non-filtered environment maps as an option in your engine -- it will run like 100x slower, but shows you the accurate result that Karis' method is attempting to approximate.
Importance sampling also works with any BRDF/BSSRDF, so it will let you see the 'correct' results of IBL with your translucent surfaces too.

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Thank you,

I use that method. However the problem I face is that with a BSSRDF you have to integrate over the area and the incoming rays from the scattering part. So this changes the rendering equation (for IBL) and in this way I can't use the method described by Karis right?

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