Lens shaders are not only used to change the direction of eye rays: they can also be used to manipulate colors and even the apparent shape of the rendered images (see mib_lens_stencil) .
This lens shader maps color components that are lying between a floor and a ceiling value to the unit interval. Values below and above those limits are clamped to 0 and 1, respectively.
If the luminance option is on, then the sample colors with a luminance below the floor value are replaced by the floor color, and sample colors with a luminance above the ceiling value are replaced by the ceiling color. Color values with luminance between those bounds are linearly scaled to luminance value between 0 and 1.
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