Ray epsilon and shadows

[Yambo]

Ray epsilon is the distance for offsetting new rays so they wont intersect with the geometry. As far as i know the epsilon value suppose to solve artifacts that may appear from scale issues.
But sometimes it acts a bit like a shadow bias in vray (even though is totally different). And can approach some "floating" geometry or shadow issues.

I can use my eyes of course to see how between 0.000001 and 0.1 it looks correct, but any ideas how to approach this value without guessing?

Thanks.

[abstrax]

The ray epsilon exists to avoid "self shadowing" due to limited numerical precision during path tracing, i.e. surfaces shadowing themselves because a reflection ray intersects immediately with the surface it reflected off. To avoid this problem a reflected ray doesn't start exactly at the point it bounced off, but with some distance from the surface. The ray epsilon is specifying that distance and is in meters.

E.g. if the ray epsilon is 0.1, then the offset is 0.1m (= 10cm). If your scene is modeled to scale, the shadow ray would start very close to the bottom of the chair causing these strong shadows, because the bottom of the chair is maybe just 15-20cm above the ground. During rendering it appears as if the chair would only sit 5-10cm above the floor - making those shadows quite strong. -> In general you want to make the ray epsilon so small that the offset isn't relevant anymore. As a rule of thumb, you can make it maybe ~1/100000th - 1/1000th of the camera <-> object distance, but is of course scene dependent. In most scenes that are in the "human" scale range (with objects of size 0.1m - 100m), the default (0.0001) should work just fine.

As a side effect, the ray epsilon actually behaves like the shadow bias in VRay, if I understand its function correctly, but currently it can't be controlled per light source, which makes it hard to misuse it to avoid "terminator acne".

I hope that makes some sense

[Yambo]

This i a great answer. Thank you very much.