Following the description of general rendering methods, we will now discuss special algorithms for the production of photorealistic landscape images. First we must determine what kind of visual effects are really necessary for the rendering of landscapes. For example, with the tools available today, it would be impossible to calculate the correct global light exchange in a scene with millions or maybe billions of leaves. On the other hand, amazingly realistic images can be rendered without the complex methods we have discussed so far. In the case of light exchange, for the exchanged amount of light simply a constant ambient factor IA can be included in the luminance calculation.
A significant factor, however, is the shadow computation. Since the leaves of a plant create effective shadowing, most of them are situated in deep shadow, and thereby create a high contrast in the overall image. Another important effect is the translucency. A leaf that is illuminated on one side shines on the other side a light green, just as if it was a light source. Although it is not necessary to apply this glow as an indirect light in the lighting calculation, the effect must still be included in the rendering of the leaf.
Figure 9.4 shows the most important effects. The model in (a) was computed without shadow, the appearance is relatively flat. In (b) the shadows are computed and naturally the contrast is much more pronounced. If the transluminance is added, a progression of reality is realized; in (c) this last step is implemented over a simple extension of the lighting model. In the following chapter
Chapter 9 we describe a model that simulates such effects in an improved way, and that Rendering also deals with the subsurface scattering within the leaf.
Other effects often present are the atmospheric scattering of dust in the air, as well as the so-called bluish fading of distant areas. In the global light exchange, all effects are generated with relatively simple rendering algorithms. More and more can be computed using graphics hardware found in most computers, which, of course, speeds up rendering significantly.
