Datorseende

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Datorseende
TexPoint fonts used in EMF AA
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What is computer vision?

• Image Understanding (AI, behavior)
• Computer emulation of human vision
• A sensor modality for robotics
• Inverse of Computer Graphics

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Intersection of vision and graphics
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Image-based rendering

• What is image-based rendering?
• The synthesis of new views of a scene from
  pre-recorded pictures
• Why?
• Many applications

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Example Panoramic mosaics

• 
  

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Image-based rendering

• How?
• General pipeline

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Image-based rendering

• Three approaches
• 3D model construction from image sequences
• Transfer-based image synthesis
• Light field

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Approach 1 3D model construction from image
sequences

• Techniques that first recover a three dimensional
  scene model from a sequence of pictures, then
  render it with classical computer graphics tools
• Scene modelling from
• Registered images
• Unregistered images

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Scene modellingfrom registered images

• All images are registered in the same global
  coordinate system
• What kinds of reconstruction?
• Volumetric reconstruction
• Surface reconstruction
• Depth maps

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Surfaces and their outlines
Occluding contour
Camera centre
Image contour
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Surfaces and their outlines
Shadow boundary
The viewing cone
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Volumetric reconstruction

• It is impossible to uniquely reconstruct an
  object from its image contours. Why?
• Two main constraints imposed on a solid shape by
  its image contours
• The shape should lie in the intersection of all
  viewing cones
• The cones should be tangent to its surface
• Techniques
• Voxel carving
• Polyhedral approximation
• Smooth surface fitting

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Smooth surfaces from image contours

• Example by Ponce Spline parametrization which
  minimizes the energy

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Virtualized RealityTM

• Capture synchronized video from a full hemisphere
  of views.
• Perform new view generation

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Virtualized RealityTM

• Spatio-Temporal View InterpolationS. Vedula, S.
  Baker, and T. KanadeEurographics Workshop on
  Rendering, June, 2002.

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Virtualized RealityTM

• Build 3D model and compute 3D scene flow,
  interpolate view and time.

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FILM!
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Scene modellingfrom unregistered images

• Not necessary to reconstruct all images into one
  global coordinate system
• A priori model of the scene

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Image-based modeling
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Façade

• Select building blocks
• Align them in each image
• Solve for camera poseand block parameters(using
  constraints)

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View-dependent texture mapping

• Determine visible cameras for each surface
  element
• Blend textures (images) depending on distance
  between original camera and novel viewpoint

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FILM!
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Model-based reconstruction from one image
J-E Solem, F. Kahl, 2005
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Approach 2 Transfer-based image synthesis
This example is based on computing consistent homo
graphies between all planes (B. Johansson, 2003)
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View Morphing

• Morph between pair of images using epipolar
  geometry Seitz Dyer, SIGGRAPH96

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Affine view synthesis

• På tavlan!

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Approach 3 The light field
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What is light?

• Electromagnetic radiation (EMR) moving along rays
  in space
• R(l) is EMR, measured in units of power (watts)
• l is wavelength

• Light field
• We can describe all of the light in the scene by
  specifying the radiation (or radiance along all
  light rays) arriving at every point in space and
  from every direction

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Ray

• Constant radiance
• time is fixed
• 5D
• 3D position
• 2D direction

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Line

• Infinite line
• 4D
• 2D direction
• 2D position
• non-dispersive medium

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Image

• What is an image?
• All rays through a point
• Panorama

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Panoramic Mosaics

• Convert panoramic image sequence into a
  cylindrical image
• 
  

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Image

• Image plane
• 2D
• position in plane

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Object

• Light leaving towards eye
• 2D
• just dual of image

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Object

• All light leaving object

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Object

• 4D
• 2D position (on surface)
• 2D direction

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Object

• All images

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The light field

• Summary
• Capture as many images as possible
• Store them in a smart way
• Discretize rays to synthesize new images

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Complex Light Field acquisition

• Digital Michelangelo Project
• Marc Levoy, Stanford University
• Lightfield (night) assembled by Jon Shade

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Surface Light Fields

• Wood et al, SIGGRAPH 2000

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Sammanfattning

• Vysyntes och bildbaserad modellering
• Nära relationer till datorgrafik
• Tre strategier
• Först 3D modell, sedan använd datorgrafik
• Transfer-baserad vysyntes
• Light field