8.1.2.1 Shading and ray tracing

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8.1.2.1 Shading and ray tracing

A shaded image of a surface usually gives a more realistic visual representation in comparison to a wireframe model [118]. A simple illumination model, based on Lambert's cosine law and incorporating specular reflection, gives the reflected intensity as a function of the incident intensity $I_{i}$ from a point light source, the ambient intensity $I_{a}$ , the diffuse reflection constant $k_{d}$ , the specular reflection constant $k_{s}$ , the ambient diffuse reflection constant $k_{a}$ , the angle $\theta$ between the unit surface normal ${\bf N}$ and a light direction vector, and of the angle $\alpha$ between a viewpoint direction vector and a vector in the direction of reflection:

$\displaystyle I = I_{a} k_{a} + \frac{I_{i}}{r} ( k_{d} \cos \theta + k_{s} \cos \alpha )\;,$

(8.1)

for $0 \leq k_{a}, k_{d}, k_{s} \leq 1$ and $0 \leq \theta \leq \frac{\pi}{2}$ where represents the distance from the perspective viewpoint to the point on the surface. More complicated shading models, which take into account the properties of the material, the angle of incidence, and the wavelength of the incidence light also exist [118].

The ray tracing technique [188,118] gives more realistic images than a simple shaded image method, but is much slower. The intensity for each pixel is determined using a ray from the viewpoint through the pixel into the object. Ray tracing is in fact an intersection problem. The ray to surface intersection is computed for every ray and surface in a scene. Using the ray tracing technique, the hidden surface problem is solved during such computation and also other attributes, for example multiple reflections and shadowing, can be included in the model.

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December 2009