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These clouds were generated by using a three dimensional Perlin noise function. "Noise" means random information, like the static on a TV screen. When noise is added to more noise at successively smaller scales, it creates a new kind of noise that, while still random, has an overall structure to it (see below). This is called Perlin noise, pink noise, or fractal noise. The three dimensional version is useful for simulating rocky landscapes, vegetation, cornflakes, and clouds.
I thought of trying to use noise to create clouds about a year ago. Since I knew that clouds were fractal, it was only natural to use noise at many different scales added together. I worked on it for a few days to achieve the results above. Later I happened across Ken Perlin's homepage and found that he had already done it. He even went one dimension further to create clouds that realistically evolve over time using a four-dimensional cross section of five-dimensional Perlin noise.
I am currently looking at ways to use fractal noise in simulating brittle fracture. The program would parametrically deform an object based on the forces acting on it. It would use engineering techniques to calculate when the stress and strain were too high for the material at a particular point. Then the object would be replaced with its two halves, with an identical break surface on each of them generated using Perlin noise. One advantage to this is that the geometry could be modeled for physics purposes at a low level of detail but shown at a higher level of detail quite naturally. By altering a few parameters, the noise could look like broken stone, ceramic, metal, or even splintered wood.
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