Why is collision difficult to effectively compute in graphics engines?

Question

From the oldest games to the very modern, it seems like you can still see through walls or most often the ground in some camera positions. Why is collision difficult to effectively compute in graphics engines? Is it rounding/loss of precision accumulating leading to a mis-rendered view?

Answer 1

This is not actually collision in the explicit sense. The camera position is probably not actually "inside" the wall or the ground in those situations, but it is simply very close to it.

In computer 3D graphics the camera has a concept of a near plane and a far plane. Only geometry located between these two planes will be visible, while the rest will be clipped. If you are too close to something and align the camera correctly, then chances are that some parts of the geometry will be too close to the camera as defined by the near plane and as a result that geometry will not be rendered.

Now, the distance to this near plane can be set by the developers, and it can be set to be very short - short enough to ensure that situations like these cannot occur. However, the depth buffer or z buffer that is used to determine which objects are closest to the camera during rendering, and thus which objects to render and which not to render, is closely related to the near and far plane distances.

In graphics hardware the depth buffer is represented using a fixed amount of bits for each pixel, for example 32 bits. These 32 bits must be enough to accurately represent the entire span between the near plane and the far plane. It is also not linear, but will use more precision closer to the camera. As a result, choosing a very small near plane distance will greatly reduce the overall precision of the depth buffer. This can cause annoying flickering throughout the entire scene wherever two objects are very close to each others.

You can read more about this issue here as well as section 12.040 here.

Answer 2

It's not about difficulty (of course, it's not easy to compute collision/clipping of non-convex object), but you still have only like ~33ms to compute whole frame, so some compromise have to be made (collision mesh is not the same like mesh you really see). If there is no time for precise solution (to fulfill all conditions - camera distance, object which have to be seen, collision avoidance), you have to fallback to some "easy" solution like see through the wall.