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When light does not diffuse and it does reflect at the same angle as it hits an object, it is called specular reflection. Such mirror-like reflection is controlled by the reflection keyword in a finish statement. For example:
finish { reflection 1.0 ambient 0 diffuse 0 }
This gives the object a mirrored finish. It will reflect all other elements in the scene. Usually a single float value is specified after the keyword even though the syntax calls for a color. For example a float value of 0.3 gets promoted to the full color vector < 0.3,0.3,0.3,0.3,0.3> which is acceptable because only the red, green and blue parts are used.
The value can range from 0.0 to 1.0. By default there is no reflection.
Adding reflection to a texture makes it take longer to render because an additional ray must be traced. The reflected light may be tinted by specifying a color rather than a float. For example
finish { reflection rgb <1,0,0> }
gives a red mirror that only reflects red light.
POV-Ray uses a limited light model that cannot distinguish between objects which are simply brightly colored and objects which are extremely bright. A white piece of paper, a light bulb, the sun, and a supernova, all would be modeled as rgb<1,1,1> and slightly off-white objects would be only slightly darker. It is especially difficult to model partially reflective surfaces in a realistic way. Middle and lower brightness objects typically look too bright when reflected. If you reduce the reflection value, it tends to darken the bright objects too much. Therefore the reflection_exponent keyword has been added. It produces non-linear reflection intensities. The default value of 1.0 produces a linear curve. Lower values darken middle and low intensities and keeps high intensity reflections bright. This is a somewhat experimental feature designed for artistic use. It does not directly correspond to any real world reflective properties. We are researching ways to deal with this issue in a more scientific model. The reflection_exponent has no effect unless reflection is used.
Note that although such reflection is called specular it is not controlled by the specular keyword. That keyword controls a specular highlight.
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