Presentaci

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CS6998 - Topics on Computational Vision and
Graphics Apr 20th, 2004
Real-Time Rendering Using CUReT BRDF Materials
with Zernike Polynomials
Sebastian Enrique Columbia University senrique_at_cs.
columbia.edu
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Real-Time Rendering Using CUReT BRDF Materials
with Zernike Polynomials
CS6998
Apr 20th, 2004
Problem

• As far as I know, real-time rendering of objects
  using its BRDF was not achieved yet.
• I decided to deal with this problem, with the
  goal to create a system with the following
  initial requirements
• Allow rendering of any kind of mesh.
• Allow real-time viewpoint changes.
• Allow real-time lighting changes.
• Allow selection of material from a set of BRDF
  samples.
• Compute final colors per pixel.

Sebastian Enrique - Columbia University -
senrique_at_cs.columbia.edu
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Real-Time Rendering Using CUReT BRDF Materials
with Zernike Polynomials
CS6998
Apr 20th, 2004
Decisions and Simplifications

• Meshes should be made of triangles. Topology
  information of the mesh (vertices positions and
  indices of each triangle) should be completed
  with vertex normals.
• Viewpoint can be rotated and zoomed in and out,
  but not translated. Change of camera target the
  origin of the coordinate system- is also not
  allowed. Perspective projection will be used.
• Illumination will be limited to a white distant
  point light source. This means that every point
  in the surface will be receiving light from the
  same direction. The user should be able to change
  this direction.
• CUReT public BRDF materials databse will be
  used. It contains 61 material samples.
• Final colors should be computed in the GPU using
  fragment shaders.
• We will not deal with shadows and
  interreflections.

Sebastian Enrique - Columbia University -
senrique_at_cs.columbia.edu
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Real-Time Rendering Using CUReT BRDF Materials
with Zernike Polynomials
CS6998
Apr 20th, 2004
CUReT

• 205 measurements of 61 materials under different
  light and view directions.
• A representation or interpolation should be used
  to get the BRDF of each material under novel
  light and view directions Zernike polynomials
  (mapping of points on a hemisphere over the unit
  disk).
• It is a good representation for smooth BRDFs and
  scattered data. It is not so good for materials
  with high specular lobes.
• 55 coefficients for polynomials of order 8.
• 5 coefficients for polynomials of order 2, which
  is good for most of the materials in the database.

Sebastian Enrique - Columbia University -
senrique_at_cs.columbia.edu
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Real-Time Rendering Using CUReT BRDF Materials
with Zernike Polynomials
CS6998
Apr 20th, 2004
Per Vertex Implementation

• Colors are computed for each vertex in software
  and then passed to the graphics hardware using
  OpenGL.
• Pixel values in between vertices are
  interpolated using Gouraud shading.
• Zernike polynomials are evaluated with
  corresponding material coefficients to get the
  correct color depending on light and viewing
  directions every frame.
• Some costly operations on angles are precomputed
  and stored in a table to speed up real-time
  processing.
• Extra features of the application such as to
  show surface normals and N dot L type of
  rendering are done using pixel and vertex shaders.

Sebastian Enrique - Columbia University -
senrique_at_cs.columbia.edu
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Real-Time Rendering Using CUReT BRDF Materials
with Zernike Polynomials
CS6998
Apr 20th, 2004
Per Pixel Implementation

• Same operations done per vertex could be done
  per pixel using a fragment shader.
• Precomputed values can be passed as textures.
• Current implementation involves two 2D-textures
  (Zernike coefficients and precomputed operations
  on angles) and one 3D-texture (OpenGL 1.2
  extensions required).
• Color computation for Zernike polynomials of
  order 2 (5 coefficients) requires only 1
  rendering pass (1 pixel shader).
• To compute order 5 (55 coefficients) 8 passes
  are required with current Cg implementation
  (quite slow) in current graphics hardware.
• I havent finished software part debugging of
  per pixel implementation.

Sebastian Enrique - Columbia University -
senrique_at_cs.columbia.edu
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Real-Time Rendering Using CUReT BRDF Materials
with Zernike Polynomials
CS6998
Apr 20th, 2004
Results

• Happy Buddha model 32,328 vertices 69,451
  triangles Per Vertex 5 coefficients.
• Materials Plant, Orange Peel, and Insulation,
  using same light direction / pose.

Sebastian Enrique - Columbia University -
senrique_at_cs.columbia.edu
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Real-Time Rendering Using CUReT BRDF Materials
with Zernike Polynomials
CS6998
Apr 20th, 2004
Results (cont.)

• Dragon model 22,998 vertices 47,794 triangles
  Per Vertex 5 coefficients.
• Material Rabbit Fur changing light direction
  (top) and view direction (bottom).

Sebastian Enrique - Columbia University -
senrique_at_cs.columbia.edu
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Real-Time Rendering Using CUReT BRDF Materials
with Zernike Polynomials
CS6998
Apr 20th, 2004
Results (cont.)

• Stanford Bunny model 35,947 vertices 69,451
  triangles Per Vertex 5 coefficients.
• Material Sponge changing light direction and
  view direction in every image.

Sebastian Enrique - Columbia University -
senrique_at_cs.columbia.edu
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Real-Time Rendering Using CUReT BRDF Materials
with Zernike Polynomials
CS6998
Apr 20th, 2004
Results (cont.)

• Stanford Bunny model 35,947 vertices 69,451
  triangles Per Vertex 5 coefficients.
• Left rendered with Rug B material.
• Middle showing surface normals.
• Right wireframe using N dot L.

Sebastian Enrique - Columbia University -
senrique_at_cs.columbia.edu
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Real-Time Rendering Using CUReT BRDF Materials
with Zernike Polynomials
CS6998
Apr 20th, 2004
Conclusions

• Real-time rendering of BRDF materials using any
  mesh was implemented and possible.
• Images shown before were rendered per vertex
  with Zernike polynomials of order 2 for order 8,
  vertices and triangles should be reduced 4 times
  for real-time rendering in test machine (Pentium
  4 3Gz 1Gb RAM nVidia GeForce FX5900).
• Other BRDF representations should be analyzed to
  render in real-time more specular BRDFs
  materials.
• Next step is to finish per pixel implementation.
• Natural extension for this is to use environment
  lighting.
• Shadows could be added using traditional
  techniques like shadow mapping.
• First, I must correct some problems like
  artifacts in grazing angles and not matching
  final colors with original CUReT rendered spheres
  (coefficients are not in good shape?).

Sebastian Enrique - Columbia University -
senrique_at_cs.columbia.edu
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Real-Time Rendering Using CUReT BRDF Materials
with Zernike Polynomials
CS6998
Apr 20th, 2004
The End

• Aknowledgments
• Ravi Ramamoorthi for CUReT materials Zernike
  coefficients and some Zernike polynomials code.
• The Stanford 3D Scanning Repository for models
  used.
• Georgia Institute of Technology for tools to
  manipulate PLY files.
• Questions?
• Thanks for listening...

Sebastian Enrique - Columbia University -
senrique_at_cs.columbia.edu
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