VRay RT and AO

[Jiri.Matys]

Will be a big problem to add a support for ambient occlusion ? I mean support for AO from VRay GI.

[vlado]

Why would you need that? AO is a means of adding detail to an otherwise blurry GI solution; however V-Ray RT always does path tracing, so no detail is lost in the GI solution.

Best regards,
Vlado

[Jiri.Matys]

Yeah, you right. But sometimes AO produces more depth like images.

[duke2]

Yeah AO is also an artistic/style choice.

[vlado]

Yep, however it is a problem to add this to the RT engine. The issue is that AO is multiplied by the GI. For this to be done, one first needs to calculate AO and GI separately, and then multiply the result. However the RT is a path tracer and calculates a single GI ray at a time. So even if this was done, the result would look different than the normal renderer.

To be more precise, in the normal V-Ray, this works as follows:
(result1)=[(gi1+gi2+...+giN)/N]*[(ao1+ao2+...+aoM)/M] where N and M are the number of samples taken for GI and AO respective.

If this is translated to the RT engine, it would look like this:
(result2)=(gi1*ao1+gi2*ao2+...+giK*aoK)/K where K is the number of samples V-Ray RT has traced for a pixel. In the general case, this would be a different color than the first result.

This is just one example of a "cheating" effect that cannot be calculated by a path tracer in the same way as other methods. (Luckily, the rendering equation itself is not one of these )

You can actually notice this in the regular V-Ray as well, in some combinations of settings, where different settings produce different results because of the different number of samples.

Best regards,
Vlado