DirectX 10 Ray Tracing

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DirectX 10 Ray Tracing

• Tad Litwiller

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Background

• Technique for image generation that traces the
  path of light through pixels of an image plane
• Produces very realistic images, does well with
  reflection, refraction, accurate shadows, etc.
• Computationally expensive

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Algorithm

• Trace a path from the viewpoint through each
  pixel in an image plane and into the scene
• Test for intersection with each object in the
  scene
• If there is a ray-object collision, perform
  shading and generate reflection, refraction, and
  shadow rays
• Composite results of shaded object material with
  additional rays

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Algorithm
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Algorithm

• Ray can be defined by direction vector, and
  origin
• My implementation supports spheres as objects
  defined by center point and radius
• Objects have material with emissive, ambient,
  diffuse, specular, specular power, reflection,
  and refraction
• Light source defined by position and color

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Algorithm

• Use DirectX to set up a Draw of points (ex.
  1024x1024)
• The vertex shader will be run once per vertex
  (point in this case)
• Set up the initial eye ray origin and direction
• The geometry shader is run once per primitive,
  points in this case

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Algorithm Geometry Shader

• Eye ray is sent from the VS
• Test eye ray for intersection with each sphere in
  the scene
• Use the eye position and sphere center to get the
  distance, multiply by the ray direction, use
  quadratic equation to test intesection
• If there is an intersection cast additional
  rays and use blinn-phong shading

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Algorithm Additional Rays

• Shadow
• Trace a ray from the intersection point to each
  light source if this ray hits any other object
  its in a shadow
• Reflected
• Angle of incident ray matches angle of outgoing
  ray if reflected ray hits an object, use color
  from that object
• Refracted
• Ray passes through object and ray is bent
  depending on properties of object - if refracted
  ray hits an object, use color from that object

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Algorith Blinn-Phong Shading

• Use light vector, view vector, and the normal
• Ambient
• Constant doesnt depend on vectors
• Diffuse
• Light and normal vectors intensity is the same
  when viewed from any angle
• Specular
• Depends on all highlights vary depending on
  viewing angle
• Emissive
• Constant object emits light

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Testing

• Implemented same algorithm for both a CPU and GPU
• Tested resolutions of 512x512, 1024x1024, and
  2048x2048
• CPU AMD Phenom X4
• 2.5 GHz
• 4GB RAM
• GPU ATI Radeon HD 4670
• 750 MHz Core
• 512 MB
• 320 Stream Processors

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CPU
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GPU - Video
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Results
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Conclusion

• This is a very simplistic algorithm only
  spheres and no acceleration structure
• Can be much more optimized on both CPU and GPU,
  so speed up will likely change, but still be very
  high because of the high level of data parallelism

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Future Work

• Add ray-triangle intersection so much more robust
  objects could be loaded
• Add an acceleration structure such as uniform
  grid, bounding volume, or k d-tree
• Use geometry shader more effectively for
  additional ray bounces stream out and DrawAuto

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References

• Timothy John Purcell, Ray Tracing on a Stream
  Processor, 2004.
• Martin Christen, Ray Tracing on GPU, 2005.
• Timothy J. Purcell, Ray Tracing on Programmable
  Graphics Hardware, SIGGRAPH
• Colleagues Vineet Goel, Mangesh Nijasure and
  Marcos Zini

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Questions?