MedicalVisualization

DVR Shading

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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
Illu-Example1 Illu-Example2

Disadvantage of shading:

  • transparency lessens the contrast of shaded surfaces
  • rendering artefacts due to non-linear transfer functions → pre-integration

Literatur:

  • S. Röttger, M. Kraus et al. Pre-Integrated Volume Rendering. IEEE Vis 2000.
  • Klaus Engel, Martin Kraus, Thomas Ertl, High-quality pre-integrated volume rendering using hardware-accelerated pixel shading, 2001.
  • S. Röttger and Thomas Ertl. A two-step approach for interactive pre-integrated volume rendering of unstructured grids, VolVis 2002.
  • S. Röttger, S. Guthe, D. Weiskopf, W. Strasser, T. Ertl. Smart hardware-accelerated volume rendering. VisSym 2003.


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