what is a "bent normal" ??

[Mauritius]

I only use this term in the context of ambient occlusion. Ambient occlusion refers to the exposure of each point on a surface to its surrounding ambient environment -- that is all objects/lights which cast ambient light into the scene. Usually this is a sphere enclosing the scene which has some HDR texture map casting light mapped onto it. All geometry present in the scene (including the object being shaded itself) acts as a blocker to light arriving from there. Additionally a certain distance metric is used to weight this exposure as this affects the 'contrast' and 'brightness' of the ambient occlusion and therefore the look of the shading.
Here (http://www-viz.tamu.edu/students/bmoyer/617/ambocc/) is a good explanation including pictures.

The surface normal is used for several shading calculations. Changing the surface normal hence changes the result of this calculation. A 'bent normal' is a normal that points into the average direction of light arriving at the surface point. All rays fired to calculate ambient occlusion are averaged together to get this new vector which is the (usually) used to look up the environment light texture.
Look at the pictures on the page linked above. The rays which are longest will contribute more to the average direction.

Cheers,

.mm

[xteapot]

thanks!!
this can really help me understand, so if I use this bent normal to find which color on the env sphere I can get an env lighting pass...:thumbsup:

[Mauritius]

Yes. Actually, you can also use the real normal but the bent normal gives much better looking results and it comes for free if you calculate an ambient occlusion pass.

.mm

[marcusss]

then, is this ambient occlusion something only done with Renderman code, or other renderers offer this also? Is it by default calculated in any GI calculation?

thx

Mark'Huss

[Mauritius]

Ambient occlusion just determines some sort of occlusion. This naturally is a part of calculating GI (but only a part). Using bent normals obtained in this pass to index into an evironment map for lighting is an additional thing which has nothing to do with occlusion per se.
Notice the term "occlusion"? Occlusion calculates occlusion from light, not contribution of light.

.mm

[xteapot]

I think part of Mentalray's Final Gather do the ambient oclusion
and for Maya the DiffuseLight plugin will do it...

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