Anti-aliased and accelerated ray tracingReadingAliasing in renderingAnti-aliasingPolygon anti-aliasingAntialiasing in a ray tracerSpeeding it upAntialiasing by adaptive samplingFaster ray-polyhedron intersectionHierarchical bounding volumesUniform spatial subdivisionNon-uniform spatial subdivisionUniversity of Texas at Austin CS384G - Computer Graphics Fall 2008 Don FussellAnti-aliased and accelerated ray tracingUniversity of Texas at Austin CS384G - Computer Graphics Fall 2008 Don Fussell 2ReadingRequired:Watt, sections 12.5.3 – 12.5.4, 14.7Further reading:A. Glassner. An Introduction to Ray Tracing. Academic Press, 1989. [In the lab.]University of Texas at Austin CS384G - Computer Graphics Fall 2008 Don Fussell 3Aliasing in renderingOne of the most common rendering artifacts is the “jaggies”. Consider rendering a white polygon against a black background:We would instead like to get a smoother transition:University of Texas at Austin CS384G - Computer Graphics Fall 2008 Don Fussell 4Anti-aliasingQ: How do we avoid aliasing artifacts?1. Sampling:2. Pre-filtering:3. Combination:Example - polygon:University of Texas at Austin CS384G - Computer Graphics Fall 2008 Don Fussell 5Polygon anti-aliasingUniversity of Texas at Austin CS384G - Computer Graphics Fall 2008 Don Fussell 6Antialiasing in a ray tracerWe would like to compute the average intensity in the neighborhood of each pixel. When casting one ray per pixel, we are likely to have aliasing artifacts.To improve matters, we can cast more than one ray per pixel and average the result.A.k.a., super-sampling and averaging down.University of Texas at Austin CS384G - Computer Graphics Fall 2008 Don Fussell 7Speeding it upVanilla ray tracing is really slow!Consider: m x m pixels, k x k supersampling, and n primitives, average ray path length of d, with 2 rays cast recursively per intersection.Complexity = For m=1,000,000, k = 5, n = 100,000, d=8…very expensive!!In practice, some acceleration technique is almost always used.We’ve already looked at reducing d with adaptive ray termination. Now we look at reducing the effect of the k and n terms.University of Texas at Austin CS384G - Computer Graphics Fall 2008 Don Fussell 8Antialiasing by adaptive samplingCasting many rays per pixel can be unnecessarily costly. For example, if there are no rapid changes in intensity at the pixel, maybe only a few samples are needed.Solution: adaptive sampling.Q: When do we decide to cast more rays in a particular area?University of Texas at Austin CS384G - Computer Graphics Fall 2008 Don Fussell 9Let’s say you were intersecting a ray with a polyhedron:Straightforward method intersect the ray with each trianglereturn the intersection with the smallest t-value.Q: How might you speed this up?Faster ray-polyhedron intersectionUniversity of Texas at Austin CS384G - Computer Graphics Fall 2008 Don Fussell 10Hierarchical bounding volumesWe can generalize the idea of bounding volume acceleration with hierarchical bounding volumes.Key: build balanced trees with tight bounding volumes.Many different kinds of bounding volumes.Note that bounding volumes can overlap.University of Texas at Austin CS384G - Computer Graphics Fall 2008 Don Fussell 11Uniform spatial subdivisionAnother approach is uniform spatial subdivision.Idea:Partition space into cells (voxels)Associate each primitive with the cells it overlapsTrace ray through voxel array using fast incremental arithmetic to step from cell to cellUniversity of Texas at Austin CS384G - Computer Graphics Fall 2008 Don Fussell 12Non-uniform spatial subdivisionStill another approach is non-uniform spatial subdivision.Other variants include k-d trees and BSP trees.Various combinations of these ray intersections techniques are also possible. See Glassner and pointers at bottom of project web page for
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