问题
I have looked at the documentation on MSDN about these 2 functions. However, I don't exactly understand the difference between these 2 functions, other than one is for setting camera view for 3D, and the other one is for setting camera view for 2D. It would be great if it can be answered. Thanks in advance for comments.
回答1:
An orthographic projection is basically a 3D projection that does not have perspective. Essentially it means that a given position does not get closer to the centre of projection the further it gets from the viewer. Perspective is obviously the opposite. Due to the fact that you divide by w after projecting it means that a value with a larger W (One that is further from the centre of projection in world terms) will "appear" closer to the centre of projection post w-divide. It is this perspective projection and w-divide that gives us the sense of depth in 3D graphics.
If you recall drawing a cube in early maths lessons you will recall that if you draw each of the 2 squares that make up the end of the cube as the same size then the back end of the cube will look larger. This is an orthographic projection. It looks weird because our eyes are used to seeing things with perspective.
IF you shrink that second square then you get perspective and hence the perspective projection.
Wikipedia has some good images demonstrating the difference as well as a good explanation.
Parallel (or Othographic) projection
Perspective (or 3D) projection
a decent explanation of perspective in general
来源：https://stackoverflow.com/questions/1401326/gluperspective-vs-gluortho2d