Optmize08_1 - 1 1 What we will cover • Contour Tracking...

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Unformatted text preview: 1 1 What we will cover • Contour Tracking • Surface Rendering • Direct Volume Rendering • Isosurface Rendering • Optimizing DVR • Pre-Integrated DVR 2 When we stop re-sampling along the ray? What voxels we can skip during rendering? How to improve re-sampling process? How to find the nearest object boundary? How to improve image quality? Optimizing DVR 2 3 Early ray termination Terminate resampling when the accumulated opacity reaches the threshold value. Image plane pixel Volume data 4 Using data coherency • Good for empty space skipping • Octree 1 2 3 4 5 6 • • • • • • root 2 1 3 4 5 6 7 8 3 5 Using data coherency • K-d tree – Recursively subdivide the volume along x,y, and z-axis aligned planes. • Run length encoding – encoded by first location of run and length of run 6 • Traverse a discrete representation of the ray • 3-D line scan-conversion or voxelization algorithm • Three types of connected paths Discrete ray casting 4 7 Template based rendering • Accelerating ray casting by minimizing resampling time – use inter ray coherence • Need a different template per individual displacement of a ray in a image pixel • No interactive speed because of image order processing template Image plane ray 8 Polygon assisted ray casting • Efficient empty space skipping • But the more twisted the object is, the more polygons are needed 5 9 Distance map Good for empty space skipping Good for pre-classified volume Each voxel contains the shortest distance to the boundary for visualization Need extensive pre-processing time When the distance is too big, set the maximum value for reducing memory space for distance. 10 Distance map – example Traversal time with Distance map : 2 times Without Distance map : 5 times 6 11 P. Lacroute and Marc Levoy[94] Image and object space method Very fast s/w based algorithm Need preprocessing step for encoding Pre-classification with opacity-weighted colors are common Shear-warp algorithm 12 Shear-warp algorithm 1. Transform the volume data to sheared object space by translating and resampling each slice (also scaling for perspective transformation) warp shear Project warping 7 13 Shear warp rendering 2. Composite the resampled slices together in front-to- back order using the “over” operator 3. Transform the intermediate image to image space by warping it according to warp view M S P M view warp M P S M 1 1 warp M 14 Shear warp rendering 8 15 Shear warp rendering • Run-length encoding 16 Shear-warp rendering Pros Fast : image & object space algorithm Simple perspective projections possible hardware acceleration possible Cons: Not good image quality bi-linear interpolation & warping distortion voxel/pixel=1: problems for zooming Require three sets of encoded volume 9 17 Shear-warp vs. Ray-tracing nearest neighbor interpolation linear interpolation...
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Optmize08_1 - 1 1 What we will cover • Contour Tracking...

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