Clouds, like smoke, have an amorphous structure without well defined surfaces and boundaries. In recent times, computationally intensive physical modelling techniques have given way to simplified mathematical models which are both computationally tractable and aesthetically pleasing [16, 13].
The main idea behind these techniques involves generating a
realistic 2D or 3D texture function t using a fractal or spectral based
function. Gardner suggests a Fourier-like sum of sine waves with phase shifts
with the relationships
Care must be taken using this technique to choose values
to avoid a regular pattern in the texture.
Alternatively, texture generation techniques described in Section 16
can be used.
Either of these techniques can be used to produce a 2D texture which can be used to render a cloud layer. A cloud layer is simulated by drawing a large textured polygon in the sky at a fixed altitude. A luminance cloud texture is used to blend a white constant texture environment color into a blue sky polygon.
Some of the dynamic aspects of clouds can be simulated by vary parameters over time. Cloud development can be simulated by scaling and biasing the luminance values in the texture. Drifting can be simulated by moving the texture pattern across the sky, i.e., transforming the texture coordinates.
Gardner also suggests using ellipsoids to simulate 3D cloud structures. The texture data is generated using a 3-dimensional extension of the Fourier synthesis method outlined above and the textures are applied with increasing translucency near the boundary of the ellipsoid. These 3D textures can also be combined the volume rendering techniques described in Section 13 to produce 3D cloud images.