Particles

Left 4 Dead 2 & The Source Engine provide an incredibly good particle fx system which can be added into the game real-time. Although you can not edit the particle fx real-time you can apply them however you wish.

Particle fx are heavy memory wise and L4D2 is built on the 32 bit Source Engine which likely means there are memory constraints. By finding ways to reuse particles more creatively it is possible to achieve high visual fidelity without increasing the memory footprint too much.

Some of the most heavily reused particle fx in this mod are fog, fire and flying paper. Others include cockroaches and flies, smoke and water. Some particle fx are triggered automatically when the player comes within range of them like the cockroaches, most don't though.

To further appreciate the presence of particle fx there have been complimentary sounds added to them like fire sounds to fire fx and engine sounds to generator exhaust fx. Other ways to reuse particle fx are adding them to the skybox, this is a cheat method of changing their scale without being able to edit the scale of the particle system directly. Some particle fx are activated by the destruction of property around them to create a sense of damage to the environment.