Depth Test Demo with Cube

About this demo...

This demo draws a cube in OpenGL's default coordinate system, where the visible x, y, and z coordinates are limited to the range minus one to one. When the demo loads, you can only see one face of the cube. You can rotate the cube by dragging with the mouse to see it from other directions. (On a touch screen, you can drag using your finger.)

Try turning off the depth test, and rotate the cube. You won't get a physically correct image. The sides are actually drawn in the order red, cyan, green, magenta, blue, yellow -- does that help you understand what you see?

Clearing the depth buffer has no effect if the depth test is not enabled. Try turning off "Clear Depth Buffer", but leave "Depth Test Enabled" turned on. Rotate the cube for a while. What happens? What happens if you switch to a bigger cube after a while? Think about what's left in the depth buffer when it is not cleared, and about how it is used the next time something is drawn.

If you turn on the "Use Bigger Cube" option, and if you rotate the cube, then it's possible for part of the cube to be cut off because it is outside the visible range of z-values. See how it looks! The effect is called "clipping."

One point of this demo is that you will probably encounter similar problems with your own programs. It's a good idea to see what can go wrong and what the results can look like.