How can a volume be shaded?
Problem: We do not have an explicit triangle mesh that could supply normals for shading operations.
Solution: Lighting of the volume by modulation of emission according to the Blinn Phong shading-model: The surface normal is substituted for the normalized gradient vector of the volume. This effectively simulates the shading of iso surfaces without explicitly extracting those.
Head-light with ambient, diffuse, and specular terms of the Blinn-Phong shading model:
images by Christop Rezk-Salama and Peter Hastreiter
Advantage of shading:
- specular lighting reveals surface structures:
- still interactive compared to iso surface extraction
Disadvantage of shading:
- transparency lessens the contrast of shaded surfaces
- rendering artefacts due to non-linear transfer functions → pre-integration