Drawing a prism with CoreAnimation

Question

I'm trying to draw a prism and animate it with CoreAnimation. But I can't find a good solution to draw the fan of rainbow colors.

There a a couple of ways to do that, each of them has disadvantages:

• I could simply use an image. But I allow the user to zoom into the scene and I'd like to avoid any artefacts.
• Using a CAGradientLayer produces a nice rainbow. Unfortunately there is now skew (or squeeze) transformation available to transform the rectangular rainbow into a triangle.
• A simple skew effect of the layer can be obtained by modifying the m34 matrix element of the layer's transform matrix. But I wasn't able to obtain a triangle shape (because the layer would have to be of infinite size).
• OpenGL. That'll work, but maybe there's a simpler way.

Any suggestions?