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RE290 - Where the same surface is illuminated by several...

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Unformatted text preview: Where the same surface is illuminated by several point sources, the diffusely reflected light must be summed from all of these to find the resulting intensity 21.1. The intensity of illumination received from any given point source will fall off (according to an inverse square law) with the distance from that source. This fact might need to be taken account of in views of interiors lit by electric light for example; but for outdoor scenes in sunshine the distance from the sun to all surfaces in the scene can be taken as equal, and the value of [p as the same throughout. If the surface is polished and shiny — it has a high reflectance —- then light from a point source is reflected off it in a particular direction; the angle 6 between the reflected ray and the surface normal 15 equal or nearly equal to the angle between the surface normal and the direction to the light source (Figure 14.25). This is specular reflection, of which reflection in a mirror is the extreme case. It is specular reflection which produces the bright highlights on polished objects. Figure 14.25 shows the angle or between the direction in which light is reflected off a shiny surface, and the direction from the surface to the viewpoint. Highlights are only seen when this angle on is small. The effect can be observed with any shiny spheroid such as an apple or orange. Notice that the highlight takes its colour from the light source (and is usually therefore white or yellow) rather than from the surface itself. This is in contrast with diffuse reflection, where the colour seen depends both on the colour of the incident light and on the properties of the surface. Fig. 14.25 Specular _ Light reflecnon Surface normal Reflected ray The phenomenon of specular reflection has been modelled for computer graphics by Phong Bui-Tuong [1975] , using an empirically-derived formula for the fall-off of intensity with the angle a. An alternative model based more closely on the theoretical optics has been implemented by Blinn [1977]. This gives rather different results especially for light falling at shallow angles of incidence (that is, where 0 in Figure 14.25 is large). For objects made ‘of transparent materials, there may be light transmitted through their surfaces from sources beyond them. We will consider this possibility shortly. Meanwhile for opaque materials the intensity of the reflected light from any surface can be calculated from the sum of diffuse and specular reflections from all point sources, together with a further contribution of diffuse reflection from ambient light. If the image is to be coloured then these calculations must be made separately for different colours. For example in the red, green, blue (RGB) system used in colour television and in most colour computer displays, three calculations would be made with appropriate values of ID and reflectance k in each case. The calculations would be made after the elimination of hidden surfaces, display for each remaining visible surface in order to decide pixel values to ...
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