RailWorks 3 – Train Simulator 2012

[b:85iektek]Occlusion maps are a way to do shadows with multiple light sources without having to cast rays to surrounding geometry at runtime[/b:85iektek]. Ambient occlusion maps are a special, simplified case of this which doesn’t take light direction into account – it assumes the light comes from all directions at once, which is fine if it’s predominantly coming from the sky but not the Sun. In RW2, ambient occlusion is used on loco and asset models to make them look better in all lighting conditions, but a stencil shadow is still needed for dealing with the main shadow cast by the Sun. Unfortunately, [u:85iektek]stencil shadows are too expensive to work with many light sources in scene, ie. at night[/u:85iektek].

But occlusion maps can actually be cleverer than this and take light direction into account too, and still be small enough to fit into a texture that wraps around a model. This is usually done using spherical harmonics, which can be thought of as a sort of angular Fourier transform. Just like with most audio compressors these days, you transform it and store just the most significant coefficients that fall out, instead of having to deal with all the raw data. So instead of having to render a full geometry pass for each light, you only need to pick the most significant lights for each object being drawn, and pass these to the clever maths in the fragment shader which looks them up in the occlusion map.

To deal with moving objects (ie. trains), a shadow map can also be built for each of these lights on-the-fly (this also requires a geometry pass for each light but it’s a relatively cheap one). This allows trains to cast shadows on the landscape and other trains and other objects, and vice versa. It’s just a more efficient replacement for stencil shadows.

By the sound of it, [b:85iektek]RW3 will have two somplete graphics engine[/b:85iektek]s: one lifted from RW2 which ensures backwards compatibility with older hardware, but requires stencil shadows on models; and the new RW3 one for everyone with new hardware which can cope with it. The good news is that with any luck, the new engine will require more from the GPU but less from the CPU, especially on new routes built with the instancing assets instead of individual trees. If true, this will let everyone recommend a relatively standard gaming setup (ie. spend about twice as much on the GPU as the CPU) instead of a business machine setup (ie. spend twice as much on the CPU as the GPU).

The downside of stencil shadows is that it requires adding geometry for it to the model. (This is actually unusual among game engines, usually the stencil volumes are auto-generated from the model itself.) Since people sticking with the old RW2 engine will tend to have older hardware which struggles with any extra work, you can sensibly stop adding stencil shadows to your models now.

The downside of occlusion maps – ambient or otherwise – is that they have to be pre-calculated, which takes a lot of CPU effort. Hence the "baking" process that RSderek often refers to, which is fortunately run by authors rather than drivers. With RW3, routes will need "baking" as well as objects, so that mountains (and less extreme geography) can cast shadows. I suspect that until a route is baked, the lighting model will fall back to RW2 standard – another reason why both graphics engines are included at once.[/quote:85iektek]