Ray Tracing The Final Chapter

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Ray TracingThe Final Chapter
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Administrivia

• Reminder No class next week

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Topics

• CSG
• Ray tracing optimizations
• Utah work on parallelization
• Frameless rendering

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CSG

• Constructive Solid Geometry
• CSG is a way to make complex solids from simple
  primitives and operations

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Basics on 2D Examples

• Consider two simple objects below left
• Position as shown on right

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Union (A C)
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Intersection (A C)
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Difference (A - C)
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CSG Trees

• Operations can be thought of as tree
• Primitives are leaves
• Result is at root

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Ray Tracing CSG Models

• Pretty easy using the method of Roth (Ray
  Casting for Modeling Solids, Computer Graphics
  and Image Processing 18, 1982
• I put a copy in class notebook).

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Example

• Trace ray through our two objects
• Call arch object A and circle B

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Operations

• This shows how Ill illustrate in parametric
  coordinates on ray

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Algorithm

• Trace ray to each component and log the
  intersections into in-out lists.
• Perform the Boolean operations on these lists as
  follows
• Merge the lists of intersections.
• Classify the merged points as in or out (see
  table next slide).
• Combine adjacent segments.

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Table of Boolean Combinations
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Example with Substeps

• Add

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Ray Tracing Optimizations

• Whitted wrote 95 of time spent computing
  intersections
• Exhaustive ray tracing intersect each ray with
  all objects.
• Too Slow
• Presentation mostly after Arvo and Kirk, A Survey
  of Ray Tracing Acceleration Techniques, in
  Glassner, ed., An Introduction to Ray Tracing

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Arvo Kirk Present a Taxonomy of Acceleration
Approaches

• Faster intersections
• Faster ray/object intersections
• Fewer of them
• Fewer rays
• Generalized rays

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Faster intersections

• Bounding volumes for objects
• Test ray on volume first
• Sphere is simple used by Whitted

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Hierarchical Bounding Volumes

• Still a topic of interest, mostly for polygonal
  rendering.
• Bound the ray length and a tree of volumes.
• Only follow branches that the ray intersects and
  if a ray hits, you can cut the length (dont care
  about further intersections).

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Bounding Primitives

• What should the bounding primitives be?
• boxes, spheres?
• and how should they be aligned?
• axis aligned or oriented to the objects.
• bounding sphere, axis-aligned box, oriented box.
• See Stefan Gottschalk publications and thesis on
  oriented bounding boxes

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Spatial Subdivision

• Instead of building a hierarchy bottom-up, divide
  space top-down.
• The former selects volumes based on sets of
  objects.
• This one selects objects based on volumes.

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Octrees

• Built so that list of objects intersecting (or
  contained in) a node is kept at each node.
• Test for intersection with faces
• Follow ray through octrees, node by node, and
  only test with objects that are in nodes that ray
  traverses.

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Non-uniform subdivision

• BSP trees.
• Axis aligned planes instead of Henrys
  arbitrarily planes for visibility determination.
• Rays are cut into segments by crossings of
  partitioning planes and if theres a hit the
  farther ones are not searched.

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Uniform Subdivision

• Not adapted to objects, but may be easier.
• Problem if primitives are sparsely distributed
• Can use DDA to trace ray through cells

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Watch Out For

• Repeated tests with same object. Can keep
  intersection info with object.
• Missing an object
• In cell 3 test B
• Notice hit
• Will you ignore C?
• Make sure hit in cell

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Generalized Rays

• Trace volumes instead of rays
• cones, beams, pencils.
• Seems awfully complicated
• Here just to stimulate thought

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Beam Tracing

• Beams are polygonal and intersect polygonal
  objects only. As a polygon is intersected, parts
  of the end of the beam are clipped away.

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Parallelization

• Ray tracing very parallelizable, if you can
  replicate database or keep memory network traffic
  down
• Recent papers from Utah
• Parallel on SGI
• Groups of eye rays given to processors as tasks.

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Load Balancing

• Cant predict the amount of work required for
  each group
• they use a work queue and each processor takes
  items from queue.
• To reduce load imbalance and idle time at end of
  frame, they have different task granularities and
  start out with larger ones.

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Illustration

• Note that size of tasks (granularity) directly
  affects load balancing, but small tasks incur too
  much overhead.

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Speedup

• Reasonable speedup.
• Load balancing and synchronization time limits
  final speedup.

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Frameless Rendering

• Dont update all pixels at once.
• They update a pseudo-random set every frame.
  Screen updated constantly.
• New samples are blended. They can use 4 jittered
  samples and update only some of the samples.
• Frameless rendering not as useful for
  conventional polygon rendering because coherence
  is what makes it fast!
• They also implemented progressive refinement.

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Ray Tracing Machines

• One at Duke for CAD work
• ART ray tracing hardware
• http//www.art-render.com/index.ihtml
• Not for interactive work

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Next Time

• No class next week
• 9/10 Three paper presentations
• Wald paper on ray tracing at interactive rates on
  a cluster
• Purcell, Ray Tracing on Programmable Graphics
  Hardware
• Carr, The Ray Engine