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Perception and Virtual Reality MONT 104S, Spring 2008 Lecture 1 Introduction, Light Course webpage:

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Understanding how the brain processes visual information is hard. ... This allows us to construct interesting visual illusions such as... 12. The Ames Room ... – PowerPoint PPT presentation

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Title: Perception and Virtual Reality MONT 104S, Spring 2008 Lecture 1 Introduction, Light Course webpage:


1
Perception and Virtual Reality MONT 104S, Spring
2008Lecture 1Introduction, LightCourse
webpagehttp//mathcs.holycross.edu/croyden/mont
104S
2
What's the Big Deal?
  • Vision seems easy. It is effortless for us.
  • Building machine vision systems is hard.
    Machines still cannot see.
  • Understanding how the brain processes visual
    information is hard. We still understand only
    the most basic computations.
  • To understand why it is so difficult, we must
    examine the problem.

3
Light behaves like both particles and waves
  • Light sometimes acts as individual particles
  • Photons are individual particles of light.
  • Photons are detected by photoreceptor cells in
    the retina.
  • Light sometimes behaves as a wave
  • Light travels in waves, that have a frequency or
    wavelength.
  • The wavelength of light affects the perceived
    color.

The electromagnetic spectrum
4
The properties of light
  • Light is emitted from one or more sources. These
    may be point sources or more distributed sources
    of light.
  • The light hits surfaces and interacts with them,
    with some being reflected, some absorbed and some
    transmitted.
  • The reflected light may bounce off multiple
    surfaces before reaching the eye.
  • Some of the light rays will eventually reach the
    eye and be focused on the retina.
  • We will diagram this in class.

5
The Eye as a Pinhole Camera
  • We can approximate the image formation performed
    by the lens of the eye as a pinhole camera.
  • Light rays from an object project through a
    single point (the center of projection) onto the
    retina (or image plane).
  • This is perspective projection.
  • We will diagram this in class.
  • Objects that are more distant form smaller
    images. We will work out the equation in class.

6
Ill-posed problems
  • It is not so hard to compute a 2D image from a 3D
    scene.
  • It is very difficult to compute the original 3D
    scene from the 2D image.
  • Many aspects of a 2D scene are consistent with
    multiple (or even infinite) possible arrangements
    of the 3D scene.
  • Because there is no single solution, the problem
    is ill-posed.

7
Visual Difficulties
  • An image of a given size can represent a small,
    nearby object or a large, distant object.
  • Reflectance changes occur at corners of a room or
    of objects. How do we see the color as constant?
  • How do we know which edges belong to which
    objects?
  • When you move your eyes, the image moves. How do
    you know the world is not moving?

8
How do we solve it?
  • To solve ill-posed problems, we must make
    assumptions about the scene and the objects in
    it.
  • These assumptions are also known as heuristics or
    constraints.
  • We can determine which assumptions are made by
    the human visual system by performing
    psychophysical experiments.
  • Psychophysical experiments use a known physical
    stimulus to test what information is important
    for perception and what the limits of the visual
    system are.

9
Height in the Visual Field
In normal images, things that are farther away
often have images that are higher in the visual
field. The visual system uses this height in the
visual field as a cue to distance. We then
adjust our interpretation of the size of the
object based on this distance.
10
A small man
When we remove the height cue, the second man
looks small indeed!
11
Assumption of Right Angles
The visual system has a tendency to interpret
angles as right angles.
We see this as a right angle
This allows us to construct interesting visual
illusions such as...
12
The Ames Room
Ever wonder how the hobbits are made to look so
short in "The Lord of the Rings" movies?
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