Game Math - C# Game Programming: For Serious Game Creation - page 193

In a game a 3D plane might be positioned somewhere to signify a change; if the
player crosses this plane then he has finished the level, or a boss should spawn, or
he has fallen to drown in the sea. The test to see which side a player is on goes
like this.

Create a vector from the plane position to the player position. ■

■ Dot product the plane normal with the newly created vector.

If the result is zero, the player position is exactly on the plane.

■

If the result is above zero, the player is on the normal side of the plane.

■

If the result is below zero, the player is on the other side of the plane.

■

This test can be seen graphically in Figure 8.15.

Figure 8.15
Player and plane position.