RayBias

From Odwiki

When rays are sent from a surface for either reflections, refractions, or raytraced shadow tests, there are often slight precision errors which cause the ray to intersect the surface that it just left. This is exaggerated when you have smooth-shading (due to interpolated normals) on a low-resolution polygon mesh when the ray leaves the surface obliquely and so has a greater chance of violating the numerical precision very close to the surface. To avoid this self-intersection, the ray is often moved slightly off the surface. This is often called the ray bias or sometimes "shadow bias" (in the case of raytraced shadow tests - but shadowbias artifacts can also be related to ShadowMap resolution being too low). Houdini ships with quite generous defaults which might manifest themselves in thin black edges or non-shadowed edges (bright edges) where geometry intersect. The defaults are set this way to minimize the chance of problems occurring on most surfaces at a varying scene scale but it might be needed to be decreased in smaller, more intricate scene geometry.

[edit] Fixing it

In the case of reflections and refractions, your shader will no doubt have a Ray Bias parameter. Adjust this value up until you eliminate artifacts. If you're seeing self-shadowing glitches when using raytraced shadows you will find a Bias setting on your Light somewhere. For little white edges where the bias amount is too large, it's often caused by fact that your geometry might not have any thickness to it and so you can correct the geometry by extruding a back face off it. This is particularly true for interior scenes where the corners of the room show through to the now unshadowed exterior light.

[edit] More Info

One method Mantra has is to move the ray origin off the surface in the direction the ray is being fired, and the other is to move the ray off the surface in the direction of the surface Normal. You can choose this method based on the type of scene you may be rendering - for example: rendering an ocean-scape all the way to the horizon with raytraced reflections might warrant using the normal bias method.

All the Ray Bias functions in Mantra will perform the former method unless the RenderingParameter called "vm_biasnormal" is set under Mantra 9.x > Shading > Bias Along Normal. You can also globally cause all biasing in Mantra to do this by setting the environment variable MANTRA_BIAS_NORMAL. (See hconfig)