Trees

Excluding water, trees are probably the most intricate objects to create realistically. Let us look at the three main tasks involved in creating trees.

Modeling

The tree modeling process can be subdivided to separate the trunk and the leaves. The trunk, including all the branches, may be modeled with one of three methods: cellular automata, fractals and L-Systems. L-Systems, as introduced by Lindenmayer^[1], are probably the most widely used now. The two major methods of modeling leaves are spline definition and polygon definition. Spline definition as in Lintermann et al.^[2] allows easier definitions of curved surfaces but lacks the ability to insert the jagged edges one is capable of producing with polygons as in Deussen et al.^[3].

Rendering

The most important part of rendering trees has to do with illuminating it. This is also the most intricate facet thereof on account of the countless thousands of leaves and the ridged textures of barks. María et al.^[4] proposes a polar-plane based method which delivers excellent results at relatively low computational times. The sky is rendered as triangle-mesh hemisphere as in Preetham et al.^[5] in which a light source is placed at each vertex. Different (but not all) leaf orientations and their maximum illuminations are pre-computed and orientations that have not been calculated are then estimated by Shepard-like interpolation of the four closest neighbors. The luminance depends on the maximum luminance, which is calculated by establishing which sky-sector projects onto that orientation, and the occlusion factor, which is calculated by finding the amount of light that is blocked by other leaves.

Animation

Basic tree animation is simulated by modeling a branch and its leaves as a billboard which may sway according to external forces. This method, while physically incorrect, fools the eye extremely well from a medium distance whilst still looking relatively realistic from up close.

Water