Chapters 5

1
Chapters 5

• 2 March 2004

2
Classical Computer Viewing

• Same elements
• objects
• viewer
• projectors
• projection plane

3
Classical and Computer Viewing

• Perspective views
• fixed Center of Projection (COP)
• Parallel Views
• COP at infinity

4
Viewing in OpenGL

• Perspective
• Parallel - Orthogonal

5
Perspective viewing

• glMatrixMode(GL_PROJECTION)
• gluPerspective(40.0, 1.0, 1.0, 40.0)
• // field of view, aspect ratio, z near, z far
• // 0-180 w/h
• glMatrixMode(GL_MODELVIEW)
• gluLookat(0.0, 0.0, 30.0, / eye /
• 0.0, 0.0, 0.0, / center /
• 0.0, 1.0, 0.0) / up /

6
Camera Analogy and Transformations

• Projection transformations
• adjust the lens of the camera
• Viewing transformations
• tripoddefine position and orientation of the
  viewing volume in the world
• Modeling transformations
• moving the model
• Viewport transformations
• enlarge or reduce the physical photograph

7
Coordinate Systems and Transformations

• Steps in Forming an Image
• specify geometry (world coordinates)
• specify camera (camera coordinates)
• project (window coordinates)
• map to viewport (screen coordinates)
• Each step uses transformations
• Every transformation is equivalent to a change in
  coordinate systems (frames)

8
Affine Transformations

• Want transformations which preserve geometry
• lines, polygons, quadrics
• Affine line preserving
• Rotation, translation, scaling
• Projection
• Concatenation (composition)

9
Specifying Transformations

• Programmer has two styles of specifying
  transformations
• specify matrices (glLoadMatrix, glMultMatrix)
• specify operation (glRotate, glOrtho)
• Programmer does not have to remember the exact
  matrices

10
Programming Transformations

• Prior to rendering, view, locate, and orient
• eye/camera position
• 3D geometry
• Manage the matrices
• including matrix stack
• Combine (composite) transformations

11
Transformation Pipeline
normalized device
eye
object
clip
window
v e r t e x
Modelview Matrix
Projection Matrix
Perspective Division
Viewport Transform
Modelview
Projection

• other calculations here
• material è color
• shade model (flat)
• polygon rendering mode
• polygon culling
• clipping

Modelview
l l l
12
Applying Projection Transformations

• Typical use (orthographic projection)
• glMatrixMode( GL_PROJECTION )
• glLoadIdentity()
• glOrtho( left, right, bottom, top, zNear, zFar )

13
Viewing Transformations

• Position the camera/eye in the scene
• place the tripod down aim camera
• To fly through a scene
• change viewing transformation andredraw scene
• gluLookAt( eyex, eyey, eyez, aimx,
  aimy, aimz, upx, upy, upz )
• up vector determines unique orientation
• careful of degenerate positions

14
Projection Tutorial
15
Transformation Tutorial
16
Connection Viewing and Modeling

• Moving camera is equivalent to moving every
  object in the world towards a stationary camera
• Viewing transformations are equivalent to several
  modeling transformations
• gluLookAt() has its own command
• can make your own polar view or pilot view

17
Projection is left handed

• Projection transformations (gluPerspective,
  glOrtho) are left handed
• think of zNear and zFar as distance from view
  point
• Everything else is right handed, including the
  vertexes to be rendered

y
y
z
left handed
right handed
x
x
z
18
resize() Perspective Translate

• Same effect as previous LookAt
• void resize( int w, int h )
• glViewport( 0, 0, (GLsizei) w, (GLsizei) h )
• glMatrixMode( GL_PROJECTION )
• glLoadIdentity()
• gluPerspective( 65.0, (GLfloat) w/h,
• 1.0, 100.0 )
• glMatrixMode( GL_MODELVIEW )
• glLoadIdentity()
• glTranslatef( 0.0, 0.0, -5.0 )

19
Hidden Surface Removal

• Modeling a cube
• what causes only the 3 front facing sides to be
  visible?
• Hidden surface removal algorithms
• object space algorithms
• image space algorithms
• z buffer algorithm (requires DEPTH buffer and the
  GL_DEPTH_TEST to be enabled)

20
Hidden Surface Removal

• Optimize the process by rendering only front
  facing polygons
• glEnable(GL_CULL_FACE)
• what is a front facing polygon?
• One with its normal facing the viewer

21
Homework

• Begin Presentation.
• No class Thursday. Go to the library or online.
  Browse Computer Graphics articles. Find a topic
  that interests you. You will need to cite a
  minimum of two sources for your 10 minute
  presentation.
• Computer Graphics Quarterly - SIGGRAPH
• Computer Graphics World

22
Program 2 due 3/18

• Logic