CG for Vision Research

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CG for Vision Research

• Lecture 8
• 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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Binocular Vision(Stereopsis)
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For fixated object, there is no retinal disparity
( F F ), i.e., light rays fall on fovea.
?
FD
FD focal distance IOD interocular distance ?
vergence angle FD IOD / ( 2tan(?) )
F
F
IOD
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?
F
F
IOD
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Objects on horopter also has no retinal disparity
( ?L ?R ). Radius of horopter depends on FD
FD
Horopter
?L
?R
P
P
F
F
IOD
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Crossed (negative) disparity for objects in
front of horopter.
FD
P
P
F
F
IOD
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Uncrossed (positive) disparity for objects
behind horopter.
FD
P
P
F
F
IOD
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Positive parallax (Uncrossed)
Zero parallax
Negative parallax (Crossed)
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Retinal Disparity

• Horizontal retinal disparity depends on
• Fusion distance
• Interocular distance
• Vergence and accommodation
• Vertical disparity scales distance
• Problems to consider
• Correspondence Problem The extent to which the
  visual system can find corresponding points in
  the 2 retinal images
• Fusion Limit limit to which observer can fuse 2
  images
• Perceived depth correlated with (horizontal)
  retinal disparity
• Can be scaled by vergence angle, vertical
  disparity, size perception, etc.

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Disparity Motion

• Disparity spatial displacement of corresponding
  points
• Motion temporal displacement of corresponding
  points
• Neurons tuned to both disparity and motion
  signals (Qian)

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Simulating Disparity Field

• Projection Method

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? ?L ?R ?L, ?R in radians
To simplify, assume FD large!
?L
?R
IOD
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? ?L ?R ?L, ?R in radians uL, uR in screen
units that project to P and P
uL
uR
Projection Plane (monitor screen)
P
?L
?R
P
IOD
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z
P(x,z)
z depth
uL
uR
Projection Plane
x
D Viewing distance
?L
?R
E IOD/2
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? ?L ?R
z
P(x,z)
z
uL
uR
x
D
?L
?R
If ( D z ) gtgt ( E x ) then ?L ? ( E x )
/ ( D z ) So can drop tan() to simplify
calculations
E
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Need to solve for uL and uR using Equivalent
Triangles Left eye ( E x ) / ( D z ) ( E
uL ) / D uL ( Dx Ez ) / ( D z )
? ? uR - uL ( 2Ez ) / ( D z )
z
P(x,z)
z
uL
uR
x
D
?L
?R
E
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Simulating Disparity Field

• Camera Methods

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Toe-In Method

• Geometrically incorrect as it produces some
  vertical disparities
• Can still be used (simpler less computational)

http//astronomy.swin.edu.au/pbourke/stereographi
cs/stereorender/
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Off-Axis Method

• Geometrically correct No vertical disparity
• Also called parallel axis asymmetric frustum
  perspective projection

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Stereo Technology
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Implementation Issues

• Ghosting/ Crosstalk
• When information for one eye seen by other
• Accommodation and Vergence angle
• Can lead to conflicts (especially if using a
  screen)
• Spatial Resolution
• Stereo acuity 3 arc sec or 1/25000th of an inch
  http//white.stanford.edu/brian/numbers/node1.htm
  l)
• Pixels are discrete so there will be rounding
  errors larger than stereo-acuity
• Fusion
• Fusion limit
• Negative (in front of screen), positive (behind
  screen) and zero parallax (disparity)

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Mirror Systems
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Stereoscope Wheatstone (1838)
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Active/Passive Viewing Systems
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True 3D Systems
http//bankslab.berkeley.edu/Projects/StereoDispla
yIntro.html
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Framebuffer

• RedBook, Chap. 10

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Simulated 3D Scene
Point
3D Transformations
Rasterization ? Fragment (color depth)
Texture mapping
Fragment tests
Pixel
(0,0)
Image on screen
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Framebuffer

• Buffer Memory storage for a single component
  (e.g., red, green, blue, alpha, depth, etc.)
• Framebuffer All available buffers on graphics
  card

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Color buffer

• Refers to the collection of RED, GREEN, BLUE and
  ALPHA buffers
• The buffer you actually see!
• Several type of color buffers (depends on
  graphics card)
• Front (screen) and back (GL_FRONT, GL_BACK)
• For front and back, there is also a left and
  right buffer for stereo-capable graphics cards
  (GL_FRONT_LEFT, GL_FRONT_RIGHT)
• Also auxiliary buffers (GL_AUX0, GL_AUX1, )
• Need to initialize the type of buffer you want
• glutInitDisplayMode( GLUT_DOUBLE GLUT_RGBA
  GLUT_STEREO )
• If double buffering is enabled,
  glutSwapBuffers() swaps front and back buffers

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Buffer Operations

• Clear buffers can be a very expensive
  operation!
• glClear(GL_COLOR_BUFFER_BIT GL_DEPTH_BUFFER_BIT)
  
• Selecting buffers
• glDrawBuffer(GL_BACK) // Put color in back
  buffer
• glReadBuffer(GL_AUX1) // Get color data from
  auxiliary 1 buffer
• Masking buffers
• glColorMask(GL_TRUE, GL_FALSE, GL_FALSE,
  GL_FALSE)
• // Only write color to RED channel

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Buffer Masking
Fragment1 R 200 G 50 B 50 A 255 Depth
3 Coord 210, 250
color
depth
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Buffer Masking
Fragment1 R 200 G 50 B 50 A 255 Depth
3 Coord 210, 250
color
depth
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Fragment Tests

• 7 tests performed sequentially to see if
  fragments makes it into framebuffer (to become a
  pixel!) by comparing value of incoming fragment
  with value in appropriate buffer
• Scissor Test
• Alpha Test
• Stencil Test
• Depth Test
• Blending
• Dithering
• Logical Operations
• These tests can be enabled/disabled e.g.
  glEnable(GL_DEPTH_TEST)

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Depth Test
Fragment1 R 255 G 0 B 0 A 255 Depth 3 Coord
200, 500
color
depth
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Depth Test
Fragment2 R 200 G 50 B 0 A 255 Depth 8 (gt
3) Coord 200, 500
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Template Code 5
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A few clarifications

• Screen refreshes
• Use glutPostRedisplay() to force the screen to
  refresh
• Put the drawing function into the idle callback
• glutIdleFunc(DrawGLScene)
• Put glutPostRedisplay() in the idle callback
• Local variable with same name as global variable
• Commands within function is affected by value of
  local variable
• The global variable (with the same name) DOES NOT
  get updated

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