3D coordinates

You may have noticed in the previous statement that the player will move along the X-Z plane. If you have spent any time developing 2D games, you will likely be used to working with X-Y coordinates, with X running across the screen from left to right and Y running down the screen from top to bottom.

When we move into 3D, we no longer have a fixed viewing angle on our action. In a 2D game, we typically describe actions in the X-Y plane for a side-scrolling game, overhead shooter, or a puzzle game. Since there are only two dimensions to deal with, the relationship between objects on the screen is the same type of relationship they would have if you drew them on a piece of paper. The only time we even really consider a third dimension in a 2D game, we refer to it as the Z-Order, or the order in which the sprites will be drawn to make some appear on top of others.

Knowing that we have three axes to deal with, X, Y, and Z, the next step is to determine how coordinates along each axis relate to the others. XNA uses a right-handed coordinate system, meaning that you can try to contort your hand in various directions to have your palm, fingers, and thumb pointing along the positive directions for each axis. Without spraining anything, what it really means is that if you were standing in the 3D world at a point where you could see the positive Y axis shooting up into the sky and the positive X axis running off to the right as in the previous diagram, the positive Z axis would be pointing towards you.