COMP Introduction to Computer Graphics

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COMP Introduction to Computer Graphics Powered By Docstoc
					   CAP4730: Computational
Structures in Computer Graphics


         3D Concepts
                 Outline
• Basic Idea of 3D
• Projections
• What are some things we didn’t have to
  worry about before?
• What are some new things we can do?
     Right Handed Coordinate System

           +Y           +Y



                          +Z

                  +X           +X


+Z
     Viewing a 3D world
        +Y
              We have a model in this world
              and would like to view it from a
              new position.


                  +X

+Z
              We’ll call this new position the
              camera or eyepoint. Our job is to
              figure out what the model looks
              like on the display plane.
Parallel Projection
              +Y




+Z
                      +X
 Perspective Projection
              +Y




+Z
                          +X
What are some new things to
       think about?

               Hidden Surface Removal
               Visibility
               Depth Cueing
How to make a 2D image appear as
              3D!
• Output is typically 2D Images
• Yet we want to show a 3D world!
• How can we do this?
  – We can include ‘cues’ in the image that give
    our brain 3D information about the scene
  – These cues are visual depth cues
           Visual Depth Cues
•   Monoscopic Depth Cues (single 2D image)
•   Stereoscopic Depth Cues (two 2D images)
•   Motion Depth Cues (series of 2D images)
•   Physiological Depth Cues (body cues)
               Monoscopic Depth Cues
• Interposition
    – An object that occludes another is closer
• Shading
    – Shape info. Shadows are included here
• Size
    – Usually, the larger object is closer
• Linear Perspective
    – parallel lines converge at a single point
• Surface Texture Gradient
    – more detail for closer objects
• Height in the visual field
    – Higher the object is (vertically), the further it
      is
• Atmospheric effects
    – further away objects are blurrier
• Brightness
    – further away objects are dimmer
     Stereoscopic Display Issues
•   Stereopsis
•   Stereoscopic Display Technology
•   Computing Stereoscopic Images
•   Stereoscopic Display and HTDs.
•   Works for objects < 5m. Why?
                   Stereopsis views of the
The result of the two slightly different
external world that our laterally-displaced eyes
receive.
    Time-parallel stereoscopic
            images
• Image quality may also be affected by
  – Right and left-eye images do not match in
    color, size, vertical alignment.
  – Distortion caused by the optical system
  – Resolution
  – HMDs interocular settings
  – Computational model does not match viewing
    geometry.
                  Disparity
• If an object is closer than the fixation point, the
  retinal disparity will be a negative value. This
  is known as crossed disparity because the two
  eyes must cross to fixate the closer object.
• If an object is farther than the fixation point,
  the retinal disparity will be a positive value.
  This is known as uncrossed disparity because
  the two eyes must uncross to fixate the farther
  object.
• An object located at the fixation point or whose
  image falls on corresponding points in the two
  retinae has a zero disparity.
          Convergence Angles
              f1

                        a+a+c+b+d = 180
              a
                        b+c+d = 180
D1
              f2
                        a-b = a+(-b) = 1+2 =
                          Retinal Disparity
          a   b    b        D2


          c        d



     1       i        2
      Stereoscopic Display
• Stereoscopic images are easy to do badly,
       hard to do well, and impossible to do
                                  correctly.
        Stereoscopic Displays
• Stereoscopic display systems create a three-
  dimensional image (versus a perspective
  image) by presenting each eye with a
  slightly different view of a scene.
  – Time-parallel
  – Time-multiplexed
     Time Parallel Stereoscopic
             Display
Two Screens                 Single Screen
• Each eye sees a           • Two different images
  different screen            projected on the same
• Optical system directs      screen
  each eye to the correct   • Images are polarized at
  view.                       right angles to each
• HMD stereo is done          other.
  this way.                 • User wears polarized
                              glasses (passive glasses).
 Passive Polarized Projection Issues

• Linear Polarization
  – Ghosting increases when you tilt head
  – Reduces brightness of image by about ½
  – Potential Problems with Multiple Screens (next
    slide)
• Circular Polarization
  – Reduces ghosting but also reduces brightness
    and crispness of image even more
Problem with Linear Polarization
                • With linear polarization,
                  the separation of the left
                  and right eye images is
                  dependent on the
                  orientation of the glasses
                  with respect to the
                  projected image.
                • The floor image cannot be
                  aligned with both the side
                  screens and the front
                  screens at the same time.
     Time Multiplexed Display
• Left and right-eye views of an image are
  computed and alternately displayed on the
  screen.
• A shuttering system occludes the right eye
  when the left-eye image is being displayed
  and occludes the left-eye when the right-eye
  image is being displayed.
Stereographics Shutter Glasses
          Motion Depth Cues
• Parallax created
  by relative head
  position and
  object being
  viewed.
• Objects nearer to
  the eye move a
  greater distance
             Pulfrich Effect
• Neat trick
• Different levels of illumination require
  additional time (your frame rates differ base
  of amount of light)
• What if we darken one image, and brighten
  another?
• http://dogfeathers.com/java/pulfrich.html
• www.cise.ufl.edu/~lok/videos/pulfrich.avi
     Physiological Depth Cues
• Accommodation – focusing adjustment
  made by the eye to change the shape of the
  lens. (up to 3 m)
• Convergence – movement of the eyes to
  bring in the an object into the same location
  on the retina of each eye.
                Summary
• Monoscopic – Interposition is strongest.
• Stereopsis is very strong.
• Relative Motion is also very strong (or
  stronger).
• Physiological is weakest (we don’t even use
  them in VR!)
• Add as needed
  – ex. shadows and cartoons
    What are some new things we
              can do?
•   Lighting and Shading!
•   Texturing!
•   Stereo
•   Surfaces
    – Normals
    – Materials

				
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