Iray Programmer's Manual

Tessellating curved surfaces

To appear smooth when rendered, smooth curved surfaces must be tessellated with enough detail. Especially in areas of high curvature shading artifacts may occur if the geometry is not sufficiently tessellated. Shadow edges appear to be blocky; faces are shaded when they shouldn't be. This phenomenon is called shadow terminator artifact. If the silhouette looks under-tessellated, it is likely that shadow terminator problems will occur as well. Increase the tessellation of the geometry to avoid shadow terminator artifacts.

Optionally, the per-object attribute on the mi::neuraylib::IAttribute_set class can be set to avoid most of such visible artifacts:

bool shadow_terminator_offset = false

If set to true, the automatic handling of most common shadow terminator artifacts is enabled. Note that the effect is very limited if the Caustic sampler is also enabled.

Note though that this automatic handling can create new kinds of problems: One example are dark spots if geometry is close-by (like in concave corners) or artifacts on refractive objects. Thus a sufficient tesselation is still recommended in all cases.

Figure 1. Poorly tessellated geometry showing bad shadow artifacts at the edge of the nose.
File: images/nose_tessellation_coarse.jpg
Figure 2. Same object with finer tessellation.
File: images/nose_tessellation_fine.jpg
Figure 3. Poorly tessellated objects do not look good in general.
File: images/knot_tessellation_coarse.jpg
Figure 4. Same object with proper tessellation.
File: images/knot_tessellation_fine.jpg