Windows Phone 7 Game Development : The World of 3D Graphics - Hidden Surface Culling

You probably won't be surprised to hear that XNA has a solution for this problem—and it's nice and easy to use, too.

XNA can work out whether each triangle is facing toward us (as the front face of the cube is) or away from us (as the back face of the cube is). It does this based on how the triangle is actually rendered, not just how its vertices were defined, so that, as a triangle rotates, the direction in which it is facing will change. Those triangles that are found to be facing away from us are culled and are not considered for inclusion in the depth or color buffers, saving all the work that would otherwise have been involved in checking and updating them.

In order for XNA to be able to determine the direction in which the triangles are facing, we need to give our triangle vertices to it in a particular way. When we define the triangles, we ensure that, when the front of the triangle is facing toward us, the vertices are provided such that they appear in a clockwise direction. Figure 1 shows two triangles, one whose vertices are defined in clockwise order (on the left) and the other in counterclockwise order.

Figure 1. Triangles with vertices defined in clockwise (left) and counterclockwise (right) order

Given these two triangles to render, XNA will by default display the one on the left, not the one on the right. If we rotate them around so that their backs are toward us, the triangle on the left would not display, and the triangle on the right would appear.

If you look back at the vertices defined for the front face of the cube in Figure 2 in this article, you will see that both of the triangles have their vertices defined in clockwise order. This is by design, of course. With our triangles specified correctly, XNA will automatically ignore those triangles that face away from us.

The HiddenSurfaceCulling project draws yet another cube, but this time it omits the final triangle from the cube (it tells DrawUserPrimitives to draw 11 triangles instead of 12), resulting in a triangular hole. We can look through this hole to the interior of the cube to see exactly what XNA is drawing for the reverse sides of the cube triangles.

If you run this project, you will see that the orange side of the cube that contains the missing triangle shows nothing behind it whatsoever (see Figure 2). Clearly, XNA shows the triangles only when they are actually facing toward us.

Figure 2. The rear faces of the cube's triangles have been culled.

Sometimes it is useful to draw the rear surfaces of triangles, too. If you have objects that are completely flat and need to be viewed from in front or behind, or if you have objects that are hollow and whose interior can be seen, we can configure XNA to render both surfaces of each triangle.

To instruct XNA to render in this way, we need to disable surface culling. Culling is controlled by the GraphicsDevice.RasterizerState.CullMode property, which accepts one of these values: CullCounterClockwiseFace (the default), CullClockwiseFace (the reverse; culls faces that are defined in clockwise order), or None (the value we need here; none of the faces is culled).

The RasterizerState object's properties all become read-only once the object has been set into the GraphicsDevice. We therefore can't update the properties of the existing object. Instead, we create a new object, configure it as required, and then set the whole object into the GraphicsDevice, as shown in Listing 1.

Example 1. Disabling hidden surface culling

// Create and activate a new RasterizerState with a different cull mode.
        RasterizerState rs = new RasterizerState();
        rs.CullMode = CullMode.None;
        GraphicsDevice.RasterizerState = rs;

You will find this code present but commented out within the HiddenSurfaceCulling project's Initialize function. Uncomment it so that it becomes active and then run the project again. Now you will find when the orange face is toward you that you can see through the missing triangle into the fully rendered interior of the cube, as shown in Figure 3.

Figure 3. The rear faces of the cube's triangles have not been culled.

The important thing to remember with hidden surface culling disabled is that the interior faces are being calculated and compared against the depth buffer even when they are completely hidden from view. When hidden surface culling was enabled, the interior faces were discarded immediately as XNA knew that they faced away and therefore didn't need to be checked against the depth buffer at all.

The final culling mode, culling clockwise faces, can be useful if you are importing model files  that have been defined with their faces counterclockwise instead of clockwise. Some modeling applications will define triangles in this way, and this is in fact the default culling mechanism used by OpenGL. The easiest way to deal with such objects is to swap the culling mode.

When the culling mode doesn't match the order with which the vertices have been defined, XNA will render just the inside of the object, hiding all the triangles that face toward you. This can result in potentially useful effects, though they can be visually disorientating. Try modifying the example project code so that it culls clockwise faces and running it again. You will now see the inside of the cube, as shown in Figure 4.

Figure 4. The front faces of the cube's triangles have been culled.