05-05-2012, 01:15 PM
Illumination and Shading
Illumination and Shading.doc (Size: 54 KB / Downloads: 33)
Lighting or illumination is the deliberate application of light to achieve some practical or aesthetic effect. Lighting includes the use of both artificial light sources such as lamps and light fixtures, as well as natural illumination by capturing daylight. Daylighting (using windows, skylights, or light shelves) is often used as the main source of light during daytime in buildings. This can save energy compared with artificial lighting, which represents a major component of energy consumption in buildings. Without proper design, energy can be wasted by using too much light, or using out-dated technology. Proper lighting can enhance task performance, improve the appearance of an area, and have positive psychological effects on occupants. One of the central dogmas of proper lighting is that a uniform illumination is required in many applications, such as projection displays, LCD backlights, medical lighting, microscopy, solid-state lighting, and general lighting.
Modelling a shiny surface
Consider an inverted shiny metallic pot (spherical surface) and a bright point light source. Depending on the position of the light source you can see a bright spot on the metal surface. Around the spot, brightness is gradually reduced and ultimately it fades out completely as you move away from the brightest point. As you move your head, the viewpoint, the bright spot tends to move. The illumination model which is responsible for this phenomenon is known as Phong illumination model. This model is an expression for spherical illumination and considers the following two aspects:
Shading
Implementation of illumination model at the pixel points or polygonal surfaces of the graphics object. The calculation of intensity profile involves determination of surface normal at every point.
If we assume that point light source and the viewer are at infinity besides assuming geometric approximation of the bounding surface as satisfying the object shape. This becomes a world of constant shading with uniform light source and the viewpoint does not influence the perceptual reflectivity of the surface. Even though such shading may not always be realistic, a good tradeoff between computational simplicity and the desired shading quality can be achieved.