Volume Clipping via Per-Fragment Operations in Texture-Based

Volume Visualization

Visualization and Interactive Systems Group

University of StuttgartGermany

Approach

• Currently– Primitive clip planes

• Clip object specified– Clip geometry using a polygon mesh– Volumetric 3d textures as a voxel mask

Depth-based clipping

Depth structure for 1 pixel in the frame buffer

Straightforward approach-1 boundary- Render clip to depth buffer- Disable writing to depth buffer;-Enable depth test

- “Greater” or “less”- Render textures

Depth-based volume probing

– Render front faces of geometry to depth buffer to get zfront

-Store depth buffer to a texture (gfx-mem)-Fragment shader shifts all depth by –zfront

-Render back faces-Render slices with depth shift/test

Volumetric Clip Textures

• Voxelise the clip geometry as binary volume– 1: voxel is inside ; 0: voxel is clipped

• Per-fragment multiplicaton of vol*clip texture• Disadvantages

– Jaggy artifacts due to discretization

Results

Voxel Clipping-Varying alpha for smooth variation-Visible jaggy edges :trilinear interpolation doesn’t give sharp edges-Reuse clip texture – cylinder as many translated 2d circle texture

Clip Geometry-High quality (per pixel)-No aliasing-Change of geometry is trivial-slower (x4)- Convex geometry (multipass??)