Clouds

Another integral part of any natural scene is the presence of clouds. In Horng-Shyang et al.^[13] a method is proposed using cellular automata as in Dobashi et al.^[14] which has been modified to allow simulation at run-time.

Modeling

Clouds can be modeled using particle systems^[9], metaball volumes^[14] or image-based modeling^[15]. The metaball approach is used in [13].

Rendering

The two common techniques of rendering clouds are ray-tracing (Kayija et al.^[16]) and procedural texturing (Ebert et al.^[17]) but both are time consuming methods. Most rendering methods use two-pass rendering schemes in which illumination and light-dynamics are taken into account on the first render and the final image is created on the second pass. In [13] a preprocessing scheme is utilized to compute shadow relation tables (STR) and metaball lighting texture databases (MLTDB). The clouds are finally rendered in a back-to-front order by traversing the octree. The texture for each voxel is obtained by multiplying the correct entry from the MLTDB and the voxel density.

Animation

Clouds in [13] implement algorithms for calculating cloud-to-vapor and extinction probabilities. The main facet of the simulation of the cloud relies on the rules as defined in the cellular automata.

Atmospheric phenomena