Viewer Movement in OpenGL

Tom Kelliher, CS 320

Apr. 1, 2011

Administrivia

Announcements

New project handout.

Assignment

Read 5.8-5.9.

From Last Time

Linear algebra basis for computer graphics transformations.

Outline

1. Understanding clipping volumes and their specifications.

2. Projections.

3. Movements in 3-D.

4. Toward a better movement model.

Coming Up

Movement through a room.

Preliminary: Viewing Volumes

Are our viewing volume coordinates relative or absolute?

Consider:

1. By default, the eye is at $(0,0,0)$ looking down the $-z$ axis.

2. What does

   glOrtho(-10.0, 10.0, -5.0, 5.0, -2.0, 2.0);
   

   mean?

3. Other viewing modes:
   1. glFrustum: same parameters as glOrtho. What's a frustum? Truncated pyramid.

   2. gluPerspective: fovy, aspect ratio, zNear, and zFar.

   znear and zfar need to be positive.

Moving and Positioning the Eye

View specification:

1. One way of specifying eye position and viewing angle:
   1. Specify position of eye.

   2. Specify center of field of view.

   3. Specify ``up.''

2. Use of gluLookAt() in cubeview.c:

   void display(void)
   {
   
      glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
   
   /* Update viewer position in modelview matrix */
   
      glLoadIdentity();
      gluLookAt(viewer[0],viewer[1],viewer[2], 0.0, 0.0, 0.0,
                0.0, 1.0, 0.0);
   
   /* rotate cube */
   
      glRotatef(theta[0], 1.0, 0.0, 0.0);
      glRotatef(theta[1], 0.0, 1.0, 0.0);
      glRotatef(theta[2], 0.0, 0.0, 1.0);
   
       colorcube();
   
      glutSwapBuffers();
   }
   

   Note order of matrix multiplications: view, then model transformations.

3. Is it really necessary to have view and model transformations?

Example Runs

1. P1: Stock viewcube using frustum. Demonstrate clipping, invisibility when up vector is parallel to line of sight, walking through the cube.

2. P2: Perspective view with fovy 45, near 2, and far 20.

3. P3: Perspective view with fovy 135, near 0.1, far 100.

A Movement Model

Problems with viewer movement in cubeview:

1. Must specify movement in global coordinate values.

2. Can't speak of left, right, forward, backward, etc.

Consider this model:

1. What should the radius of the circle be?

2. Given $x$, $y$, and $\theta$, what's $\Delta x$ and $\Delta y$?

3. How do we handle left, right, forward and backward?

4. Suppose, to see the ``big picture,'' I wanted to elevate on the Z-axis. What should I do with center? Is that easy to do?

Tom Kelliher