CS 248: Introduction to Computer Graphics

Pat Hanrahan

FAQ

Questions

Answers

When you cut-and-paste a body part in your scene file, make sure you do so only after the body part is completely designed, and right before you begin animation (during which, most probably, you will animate differently the right and left copies). If you plan to make further edits in the design, you would have to maintain two copies in sync if you replicated the part first.

As concerns lighting, scales, whether uniform or not, may have strange effects, as they change the length of unit normals, which are the key elements of all lighting calculations. Enabling GL_NORMALIZE asks OpenGL to re-normalize the length of normals prior to making the lighting calculations. However, if a scale transformation has managed to turn a unit normal into a zero (or near zero, due to floating point errors) vector, GL_NORMALIZE does not help: this happens only when a scale is degenerate (or nearly so), i.e. fully "squashing" one dimension, such as when it turns a cone into a disc. Avoid such scales! Can you see now why MOAN offers a separate disc primitive?

Group Begin
  Name: foo
  Primitive
    Line
Group End

For more information on trees and their metrics, see Cormen, T.H., C.E. Leiserson, and R.L. Rivest, Introduction to Algorithms.

• is simple to implement, as it only takes into account the two adjacent keyframes, and yet
• provides smooth transitions in velocity between keyframes.

1. (5 points) Your (almost) stationary figure should always look at the moving one, and
2. (5 points) the moving figure should look at the stationary one for as long as it is natural for it to do so. "natural" depends on whether your figure is a human, who can barely twist his/her neck, or an owl or sea lion, who can easily turn their heads 180 degrees.

/usr/sbin/SceneViewer

/usr/share/data/models/simple/tri.iv 

• view the triangle shown on the screen from some angle other than sideways (which makes it look like a long, skinny rectangle),
• modify the material properties of the triangle,
• create light sources, and
• much more...

Yoy are on your own using SceneViewer. The on-line documentation should be adequate, though. If you really need help, come see us/call us during office hours (not via email).

This means that if a joint has two or three degrees of freedom in its orientation, then you need to use two or three rotations (each about a constant, different axis, such as the coordinate axes). Thus, you are wasting space, since in theory, an arbitrary orientation can be represented by a single rotation. However, specifying the orientation of a joint using the four parameters of a single joint rotation is much harder than using the three angles of three successive rotations; so, what you lose in efficiency you gain in user-friendliness.

If you decide to animate the rotation axis, bear in mind that you risk making it (0,0,0): this is illegal (see glRotate*()) and produces totally unexpected, ill-defined warps of your scene.