Forty particles move linearly at random speeds. Pixels are colored with red component, green component and blue component an increasing function of the distance to the nearest, second nearest and third nearest particle (respectively).

Made with Processing.

Fifty particles move linearly in random directions at random speeds. Color each pixel a grey level that is an increasing function of the difference of the distance to the two nearest particles.

Made with Processing.

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I had thirty particles moving linearly at constant speeds. Then, color each pixel a level of grey based on the distance to the nearest particle.