mental ray supports several types of curves for different purposes, like to define outer bounds or holes with trim curves on free-form surfaces, specify curves using the hair geometry for hair and fur rendering, or provide true 3D space curves for modeling operations or custom applications.
The geometric representation of hair and fur typically requires a lot of data to be able to define and render all the details expected from human hair or animal fur. The support for a specialized hair geometry description allows to provide this detail with the help of a simplified curve model and additional data like radii and custom values assigned to these curves or their vertices. This implicit definition of cylindrical shapes can be handled much more efficiently than a true surface definition would, and allows mental ray to render scenes with large hair counts. Hair and fur is normally shaded like a flat ribbon which is sufficient to achieve pleasing visual results for renderings of objects from a distance. More advanced shading models can be implemented easily, for example to fake a round shape in close-ups.
Certain applications want to generate surfaces or other geometric shapes at render time only, like extruding a curve along a path. The input curves can be given as a space curve, which is a true free-form curve description in three-dimensional space.
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