CG for Vision Research

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CG for Vision Research

• Lecture 11
• Winter Term 05-06
• Quoc Vuong, Franck Caniard, MPI

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OpenGL Projections Transformations Graphics
Primitives Textures Lighting Material Properties
Vision Research Viewpoint Effect Structure from X
(Motion, Shading, Stereo) Active
Exploration Navigation
GLUT Window Management Input Devices Event
Loop Timing
Fun/Advanced Modeling Animations Mapping (bumps,
terrains) Physics Simulation Sound (OpenAL)
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Todays Menu

• Stereo and shading
• Dissociating position, normal, and disparity
• Bump mapping
• cheap realism
• Stereo and shading integration

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Stereo - Shading
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Escher, 3 Spheres
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// Normals and z from Teapot // Disparity from
Rabbit! // Right Eye glBegin(GL_TRIANGLES) for
(i0 iltn i) glNormal3f(nxT, nyT, nzT)
glVertex3f(xRR,y,zT) glEnd() // Left
Eye glBegin(GL_TRIANGLES) for (i0 iltn i)
glNormal3f(nxT, nyT, nzT)
glVertex3f(xLR,y,zT) glEnd()
We can start mixing and matching position,
normal, and disparity!
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Limitations

• Shading does not provide good disparity signal
  because of smooth gradient
• Correspondence problem in vision
• Need textures (features)
• Position and disparity not completely
  independent disparity distorts projected image
• ok if disparity is small
• Humans have high stereoacuity
• Complexity and differences of 3d models will make
  a big difference!

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Template Code 11

• Stereo Shading

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Bump Mapping
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What do you see?
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2d images

• Most of the time, we think of color
• red, green, blue channels
• or grayscale intensity
• But can encode anything!
• e.g. intensity height
• Normals!
• nx, ny, nz -gt R, G, B

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Mark Kilgard, 2000, NVIDIA
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Components ofbump mapping

• Normal map
• Extension of texture mapping
• Just an image
• So can do interpolation, sampling, mipmapping,
  etc.
• Perturbs normal at each vertex on a per pixel
  basis
• Tangent space
• Tangents at local surface
• Normal

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Simple idea Perturb normal with normal map

• Original idea by Jim Blinn
• Change shape of the surface by changing its
  normal
• Affects shading!
• Cheap realism
• Easier than actually rendering bumpy surface
• Less memory intensive for higher quality
• Uses texture mapping techniques to smoothly
  interpolate shading per pixel

Mark Kilgard, 2000, NVIDIA
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Normals ? Normal maps

• Normals have 3 components
• Nx, Ny, Nz
• Components form unit length vector
• Nx2 Ny2 Nz2 1
• Lie within 1 to 1
• Arbitrarily convert to color space!
• R (Nx1)/2, G (Ny1)/2, B (Nz1)/2 (get
  into 0-1 range)
• Any arbitrary RGB image can be treated as a
  normal map
• Take picture of real bumpy surface, and use that
  image as a normal map

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Tangent space

• In texture mapping, image is used to change color
  per pixel of a model within object space
• In bump mapping, image is used to perturb normal
  to affect shading (and shape) within tangent
  space
• Space formed by the normal vector, and the 2
  tangent vectors
• Displacement mapping Actually perturb 3d vertex
  position using similar ideas

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Micro-textures

• Psychophysics and vision?
• Shading gradients provide a weak disparity signal
• Use bump mapping to add micro-textures
  (high-contrast features) to enhance signal
  strength
• Does not affect global shading across shape
• Possibly more controllable than just adding noise
  to the normals (as done in Stereo-Shading
  template code)
• Efficient method to add realism to stimuli by
  adding micro-textures

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Template Code 11

• Bump mapping
• (modified from www.paulsproject.net)

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