1. Volume Clipping via Per-Fragment Operations in Texture-Based Volume Visualization Visualization and Interactive Systems Group University of Stuttgart Germany

2. Approach Currently –Primitive clip planes Clip object specified –Clip geometry using a polygon mesh –Volumetric 3d textures as a voxel mask

3. 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

4. Depth-based volume probing – Render front faces of geometry to depth buffer to get z front -Store depth buffer to a texture (gfx-mem) -Fragment shader shifts all depth by –z front -Render back faces -Render slices with depth shift/test

5. 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

6. 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??)