Advanced Ray Tracing

1. CS 535 Advanced Ray Tracing

2. CS 535 Mapping Techniques Raytracing provides a wealth of information about the visible surface point: Position, normal, texture coordinates, color� Raytracing also has great flexibility Every point is computed independently, so effects can easily be applied on a per-pixel basis Reflection and transmission and shadow rays can be manipulated for various effects Even the intersection point can be modified

3. CS 535 Texture Mapping Define texture parameters over the surface Determine texture parameters at each hit point Use them to look up a texture map, just as with OpenGL But� Can also use them to look up other things�

4. CS 535 Bump Mapping Vary the surface normal vector according to some map Influences shading and reflection and refraction Variation may be random - for effects like stucco or structured - for effects like tiles

5. CS 535 Shadow Sharp shadows caused by point light sources Real light sources extend over an area (area light sources) and cast soft-edged shadows Some points see all the light - fully illuminated Some points see none of the light source - the umbra Some points see part of the light source - the penumbra

6. CS 535 Soft Shadows

7. CS 535 Soft Shadow Implementation To ray-trace area light sources, cast multiple shadow rays Each one to a different point on the light source Weigh illumination by the number that get through

8. CS 535 Aliasing Raytracing can alias badly Each ray is a single point sample multiple objects may cover portions of a pixel domain The ray hits the red object and thus the entire pixel is colored red

9. CS 535 Cone Tracing Amanatides SIGGRAPH 84 Replace rays with cones Cone samples pixel area Intersect cone with objects Analytic solution of cone-object intersection similar to ray-object intersection Expensive

10. CS 535 Anti-aliasing Solution Send out multiple rays per pixel; determine their respective colors, then average the results

11. CS 535 Anti-Aliasing Eye is extremely sensitive to patterns Remove pattern from sampling Randomize sampling pattern

12. CS 535 Accelerated Shadow Calculation Enclose each light source with a cube Subdivide each face of the cube and determine the potentially visible objects that could projected into each sub-region Keep a sorted list of objects at each subdivision cell Now shadow ray need only be intersected with appropriate list!

13. CS 535 Shadow Caching Any interloper between surface point x and the light source s will cast a shadow Doesn�t matter how many Doesn�t matter which is closest Stop ray intersections once any intersection found Neighboring shadowed surface points x and x� probably shadowed by the same object Start shadow ray intersection search with object intersected in last shadow search

14. CS 535 Accelerated Ray Tracing The rendering time for a ray tracer depends on The number of rays cast which is proportional to the number of pixels. the number of ray intersection tests that are required for each ray. This is roughly dependent on the number of primitives. We need to reduce the number of intersection tests.

15. CS 535 Bounding Volumes

16. CS 535 Hierarchical Bounding Volume Initial objects in the scene

17. CS 535 Hierarchical Bounding Volume Group objects into bounding volumes

18. CS 535 Hierarchical Bounding Volume Group groups

19. CS 535 Hierarchical Bounding Volume This can be represented as a tree structure Root is the entire volume Each internal bounding volume has sub-volumes If ray misses a node�s bounding volume, then no need to check any node beneath it If ray hits a node�s BV, then replace it with its children�s BV�s (or geometry)

20. CS 535 Spatial Subdivision Idea: Divide space in to sub-regions Place objects within a sub-region into a list Only traverse the lists of sub-regions that the ray passes through Must avoid performing intersections twice if an object falls into more than one region

21. CS 535 Tagging Ray-object intersection test valid for ray with entire object not just portion of object inside current cell Need only intersect object once for each ray In cell A � list = {#1} Intersect r with #1? Yes Miss ? Tag #1 with no-intersection In cell B � list = {#2} Intersect r with #2? Yes ray r hits object #2 but later in cell C Tag object #2 with intersection-at-D In cell C � list = {#1,#2} Intersect r with #1? No (no-intersection) Intersect r with #2? No (intersection-at-D) In cell D � list = {#2} Intersect r with #2? No (intersection-at-D)

22. CS 535 Other Partitioning Structures Octree Ray can parse through large empty areas Requires less space than grid Subdivision takes time Binary Space Partition (BSP) Tree Planes can divide models nearly in half Trees better balanced, shallower Added ray-plane intersections