Shading and Inverse Shading from Direct Illumination

An understanding of light and its interaction with matter is essential to produce images. As the modeling of light sources, light transport and light re ection improves, it becomes possible to render images with increasing realism. The central motivation behind this thesis is to improve realism in computer graphics images by more accurate local shading models and to assist the user to obtain the desired lighting e ects with these more complex models. The rst part of the thesis addresses the problem of rendering surfaces illuminated by extended (linear and area) light sources. To compute the light re ected by a surface element in a given direction, one needs to determine the unoccluded regions (shadowing) of each light source and then to compute the light re ection (shading) from each of these regions. Traditionally, point light sources are distributed on the lights to approximate both the shadowing and the shading. Instead, an e cient analytical solution is developed for the shading. Shadowing from extended light sources is a fairly expensive process. To give some insights on the complexity of computing shadows, some properties of shadows and algorithms are presented. To reduce the cost of computing shadows from linear light sources, two acceleration schemes, extended from ray tracing, are introduced and evaluated. The second part of this thesis addresses the problem of achieving a desired shading e ect by building up systems of constraints. This approach is called inverse shading. It allows a user to obtain a desired lighting e ect by interacting directly with the scene model. An interactive modeling system has been built to study the introduction of rendering issues into the modeling process itself. Two rendering aspects are considered: shading and shadows. By specifying a highlight position and size, the unique direction of a directional light source as well as the surface glossiness are determined. Interactively moving this highlight on the surface rede nes the light direction while changing its size rede nes the surface glossiness. By manipulating shadow volumes (the volume within which a particular light cannot be completely seen), the light source de nition and position can be modi ed. By assigning colours to various points in a 3D scene, information about the colour of a surface as well as other surface characteristics can be deduced. This relatively new concept of de ning the causes by manipulating the e ects is expected to have a major impact on the design of future computer graphics modeling systems. It begins a new family of tools for the design of more intuitive user interfaces.