Er.. not quite Tain7ed and Ant.
Every thing in directx is made up of polygons (triangles), that are in turn each made up of 3 vertices (x, y, z points).
A
normal is a perpendicular vector pointing away from the front side of a face (the polygon).
A polygon will only have a normal on its front side, a normal on the other side would be quite pointless because of culling. A normal is used for lighting calculations. Meaning that if you were to view the side with a normal with lighting enabled, it would appear correctly.. whilst the side without a normal would just appear completely black.
You might wonder what the point is in normals, but there is a good reason for them, performance for one, and more advanced things such as normal mapping can be done also.
Culling is the term used for basically not displaying polygons that can't be seen. But how would that be calculated? By the order in which the verts appear of course!
By default, directx only culls counter-clockwise faces, which is referred to as 'back-face culling'. As you can see from the picture, the polygon's verts are currently being drawn clockwise. But now imagine that we are viewing it through a camera, now move the camera so we are viewing the other side; suddenly the verts will be being drawn counter-clockwise relevant to our camera, so we see nothing for the triangle will be culled! Remember the culling is always relevant to our viewing frustum. Another example of this would be if you fell through the map and looked up; you should be looking at the otherside of the floor, but actually you'll just see right through because Bungie never meant for you to get there and so didn't waste polygons on making that side.
Obviously with many many pieces of large complicated geometry being used; culling can be quite a hit on performance because of all the math being done.. but this far out-ways the performance hit by drawing all the geometry instead, and so you would be stupid not to cull.
The Blam engine uses clockwise culling, as well as right-handed Cartesian coordinates, which is normally seen in OpenGL.
