Another Look at Camera Control

1
Another Look at Camera Control

• Karan Singh, Cindy Grimm, Nisha Sudarsanan
• Media and Machines Lab
• Department of Computer Science and Engineering
• Washington University in St. Louis
• University of Toronto

2
What is camera control?

• Manipulation of camera parameters
• Projection of 3D geometry into 2D plane
• Applications
• Interactively navigate around a scene (games)
• Create a fixed camera path (movies)
• Use different viewpoints to understand structure
  (visualization)
• Take pictures

3
Cameras in different fields

• Computer graphics
• From, at, up, field of view
• View matrix

4
Cameras in different fields (cont.)

• Film, photography
• Camera is physical object
• Describe movement of camera itself
• Dolly in/out, tilt, pan, roll
• Zoom in/out
• Graphics systems use same vocabulary
• Through-the-lens
• Exterior views
• Camera-centric
• Reposition the camera
• Object-centric
• Reposition the camera relative to an object

5
Cameras in different fields (cont.)

• Mathematics
• 4X4 matrix
• 11 degrees of freedom
• Straight lines to straight lines
• Computer vision
• 3X4 matrix
• Dont keep depth
• Graphics maps 11 dof to useful parameters
• J. C. Michener, I. B. Carlbom, Natural and
  efficient viewing parameters, SIGGRAPH 80
• Six extrinsic parameters (position, orientation)
• 5 intrinsic parameters (center-of-projection,
  focal length, skew, aspect ratio)

6
Cameras in different fields (cont.)

• Artists have a qualitative vocabulary
• Describe relationship of camera to object in the
  scene
• Perspective effects

Vanishing point
One point
Horizon line
Three point
Two point
7
Goals

• Visualization of camera parameters in the 2D
  image
• E.g., feed back on perspective distortion
• Manipulate current projection
• New position indicates desired projection change
• Changes appropriate camera parameters
• May change multiple parameters
• Minimal mouse, keyboard use
• Current approaches use entire right button
• Click-through interface

8
Related work

• Original camera paper
• J. C. Michener, I. B. Carlbom, Natural and
  efficient viewing parameters, SIGGRAPH 80
• Trackball manipulation
• Michael Chen, S. Joy Mountford, and Abigail
  Sellen,
• A Study in Interactive 3D Rotation using 2D Input
  Devices , SIGGRAPH
• K. Henriksen, J. Sporring, and K. Hornbaek
• Virtual trackballs revisited, IEEE TVCG

9
Related work

• Through-the-lens camera control
• Use image constraints to change camera parameters
• Not very stable
• Jim Blinn, Where am I? What am I Looking at?,
  IEEE CGA, 1988
• Michael Gleicher and Andrew Witkin,
  Through-the-lens camera control, SIGGRAPH 92

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The IBar

• A cube centered along the look vector
• Changing the rendering of the cube changes the
  camera in a corresponding way
• Different segments move limbs simultaneously
• Cyan top and bottom left limbs
• Red left and right bottom limbs
• Rendering of cube reflects projection parameters

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Demo (traditional)
Allows framing
12
Demo (Perspective change)
13
Demo (Just Weird)
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Camera- versus object-centric

• Camera-centric
• Allows framing of objects
• Position cube in relation to scene
• Object-centric
• Traditional camera-moves-with-mouse
• Nice to have both
• Map different limbs
• E.g., zoom using the left handle is
  camera-centric, the right handle is object-centric

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Some implementation details

• Camera parameters
• T Eye position
• L Look vector
• V Up vector
• W,H width, height
• f focal length
• u0,v0 center of projection
• d distance to object

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Drawing the IBar

• Cube edge is centered on look vector
• Adjust for center of projection
• Size of cube adjusted so is always sc high on
  screen

17
Drawing the IBar (cont.)
Draw horizontal bar at horizon line
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Drawing the IBar (cont.)

• Feedback
• IBar highlights when mouse is over active part
• Indicate selected segment with circle

19
Manipulating the IBar

• Relative, not absolute
• Determine which limb, and which segment selected
• Determine ratio/percentage moved
• Change corresponding parameter(s) by ratio
• E.g.,
• Multiply zoom by ratio of length change
• Pan by mouse movement

20
Manipulating the IBar

• Limb movement constrained to vertical (or
  horizontal)
• Shift key unconstrains
• Left-right movement rotates up or down
• Up-down movement changes center of projection
• Pan unconstrained
• Shift key constrains
• Stem
• Shift chooses aspect ratio or skew

21
Changing parameters simultaneously

• Dolly plus zoom
• Calculate dolly in
• Find zoom that keeps everything at a distance d
  away the same size
• Center of projection
• Translate in reverse direction

22
Camera- versus object-centric

• Camera-centric
• Render cube with new camera, scene with original
  camera
• Object-centric
• Render both cube and scene with new camera
• Changing parameters
• Invert operation (i.e., pan in the opposite
  direction)

23
Centering the IBar on an object

• Allows rotation around arbitrary point
• User selects point p in scene
• Determines d
• Render at p
• Rotation
• Translate p to origin (and camera)
• Rotate
• Translate back

24
In practice

• In use in short animated film, Ryan
• Used for dramatic perspective changes

25
Summary

• Visualization of COP, horizon line, perspective
  distortion
• Also at arbitrary points in the scene
• One mouse button for all 11 parameters
• Shift key chooses less-common action
• Click through interface
• Toggle key for disabling
• Usable perspective manipulation
• Simultaneous editing of parameters

26
Drawbacks

• Remembering which parameters go where
• Visual clutter
• User study comparing IBar to Maya camera
• Primary conclusion camera manipulation is hard
  for both naïve and knowledgeable users
• Unable to manipulate camera to match a target
  scene
• IBar helped people to learn camera manipulation

27
Future work

• Reducing number of parameters on widget
• Multiple widgets, quick swap between them
• Similar handles
• Pre-viewing of manipulation effects
• What does this handle do?
• Bookmarking and camera paths
• Visualizing in scene
• May be out of scene