What is really interesting here is that we still see only the original triangle. The original triangle, at z=0,
(-1.0, -1.0, 0.0) (1.0, -1.0, 0.0) (0.0, 1.0, 0.0)
is blocking the view of the new triangle at z=0.5.
(-1.0, -1.0, 0.5) (1.0, -1.0, 0.5) (0.0, 1.0, 0.5)
This tells us that z increases into the screen. This completes our picture of the coordinate system we are drawing into. Each of x, y, and z range from -1 to plus 1.
The gl_Position output from the vertex shader will always fall within these normalized device coordinates. Before the end of this presentation we will see how arbitrary scenes are mapped into this coordinate system.