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
• 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


                  +X           +X

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


              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

 Perspective Projection

What are some new things to
       think about?

               Hidden Surface Removal
               Depth Cueing
How to make a 2D image appear as
• 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
• 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
    Time-parallel stereoscopic
• 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
• 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
• An object located at the fixation point or whose
  image falls on corresponding points in the two
  retinae has a zero disparity.
          Convergence Angles

                        a+a+c+b+d = 180
                        b+c+d = 180
                        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
        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
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
• 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
• 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
• 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
     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.
• Monoscopic – Interposition is strongest.
• Stereopsis is very strong.
• Relative Motion is also very strong (or
• 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