Particle systems are a very common technique used when rendering something with a shape that's either too complex to render using single mesh or when there are many simple shapes that move in a complex pattern. Fire is a good example for the first case, because the flames produce a random shape that can't be modeled beforehand. On the other hand, rainfall is made out of many simple streaks which move in a complex pattern, which is an example for the second case.

In this recipe, we will concentrate on rain because it requires a relatively simple simulation which is mostly effected by gravity. With a slight modification, the same simulation can be extended to support snow and steroid showers. Other types of particle effects such as smoke and ...

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