Particles can obscure other particles behind them, can be transparent, and can cast shadows on other particles. They can also interact with other, conventionally modeled primitives. In this system the authors made two assumptions. The first was that the particle systems do not intersect with other primitives (so the rendering system only has to handle particles). The other objects in a scene are rendered separately and then composited with the particle system images. If the particles do interact with other objects, e.g., go behind them, then the images are divided into sub-images which are composited.