Light, Matter, Reflection Models. Light. The surface of an object may: emit light reflect light refract light.
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Light • The surface of an object may: • emit light • reflect light • refract light The color that we see when we look at a point on the surface of an object is the sum of all light which reaches that point on the surface and is reflected toward the viewer.
The color of each point on a surface can be described with a three component intensity or luminance function. Ir I = Ig Ib Light The value for each color component is a complex integral equation based on intensities, wavelengths, surface properties, distances, angular separations, orientations, attenuation by atmosphere, water, smoke ... and many other properties. Because the equation is not solvable in any practical sense, we use an approximation which yields a reasonably good visual result.
Light The approximation of the luminance function is based upon a lighting model. Two parts to problem .... 1 - modeling the light sources 2 - modeling the interaction between light and surface (reflection)
Light Sources • Light sources emit different • amounts of light • frequencies of light • and have different directional properties Examples of different types of Light Sources: Ambient light source Point Source Spotlight Distant Light Sources
Light Sources Ambient Light Source - - provides uniform illumination - large source with diffusers to scatter light in all directions - approximated by specifying an ambient intensity which is identical at every point in the scene Iar Ia = Iag Iab
1 0 0 0 2 0 Light Sources Point Source - - emits light equally in all directions - the intensity of illumination received from a point source is proportional to the inverse square of the distance between the source and the surface The intensity of light from source p received at point p on the surface is I(p, p ) = I (p) | p - p |
e Ps cos o, e is an exponent that determines how rapidly intensity dropsoff 0 o Is Light Sources Spotlight - - characterized by the narrow range of angles through which light is emitted (or can be seen) - intensity is a function of the angle between the direction of the source and a vector to a point on the surface - realistically modeled with concentration of light at center
Light Sources Distant Light Source - - direction with which light strikes any surface is approximately the same for each point on a surface - modeled by direction not location of light source
Lira Liga Liba Li = Lird Ligd Libd Lirs Ligs Libs Phong Reflection Model Supports three types of light-surface interactions - - ambient - diffuse - specular Each source has an ambient, a diffuse and a specular component for each of the three primary colors
Rira Riga Riba Ri = Rird Rigd Ribd Rirs Rigs Ribs Phong Reflection Model Supports three types of light-surface interactions - - ambient - diffuse - specular The amount of light reflected at each surface point has a has an ambient, a diffuse and a specular component for each of the three primary colors
We obtain the total intensity by adding the contributions of all sources and possibly a global ambient term Ir= ( Iira + Iird + Iirs ) + Iar i Phong Reflection Model We compute the contribution for each color source by adding the ambient, diffuse and specular components Lir= Rira Lira + Rird Lird + Rirs Lirs = Iira + Iird + Iirs