# COMP Introduction to Computer Graphics

Document Sample

```					   CAP4730: Computational
Structures in Computer Graphics

3D Concepts
Outline
• Basic Idea of 3D
• Projections
• What are some things we didn’t have to
• 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

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
– Shape info. Shadows are included here
• Size
– Usually, the larger object is closer
• Linear Perspective
– parallel lines converge at a single point
– 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
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
position and
object being
viewed.
• Objects nearer to
the eye move a
greater distance
Pulfrich Effect
• Neat trick
• Different levels of illumination require
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
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!)
What are some new things we
can do?