Learning XNA 3.0

Backface Culling

What you're seeing when the triangle disappears is a process called backface culling. Culling is a process in 3D graphics that limits the number of things drawn on the screen to improve performance. Essentially, the goal of backface culling is to draw only the side of a primitive that is facing the camera. In this application, only one side of your triangle is drawn.

By default, XNA will cull primitives that are drawn in a counter-clockwise fashion. Remember when you created the vertices for this triangle? You drew your vertices in a clockwise fashion, as shown in Figure 9-9.

Figure 9-9. Drawing vertices in clockwise fashion

If you were to draw those vertices in a counter-clockwise order and run the game, you'd see that the triangle actually starts out culled and therefore invisible. In this case, only when it has rotated 180° will you see it.

So, what does culling do for you? It enhances the performance of your game. Imagine a soccer ball. What does the inside of the ball look like? Do you care? Does anybody care? The obvious answer is no. So, why waste valuable processor time drawing the inside of a soccer ball when nobody will ever see it?

Just as a real-life soccer ball is hollow, all objects in 3D graphics are hollow. All they really consist of is a "skin" drawn around some empty space using a series of triangles. Nobody cares what the inside of those skins looks ...