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1.2 Free-Form Surfaces

mental ray supports  free-form curves and surfaces in non-uniform rational  B-spline (NURBS),  Bézier,  Taylor (monomial),  cardinal or  basis matrix form. Any of these forms may be rational and may be of degree up to twenty-one.1.2 Surfaces may be trimmed.

Internally,  free-form surfaces are triangulated (approximated) before rendering. A variety of  approximation techniques is available, including uniform and regular parametric, edge length, distance, angle, and combined methods.

The length, distance, and angle criteria can be combined; in this case refinement continues until all criteria are met. The any keyword changes this so that refinement stops when any criterion is met; this is useful to stop the angle criterion from adding more triangles when the triangle edges become smaller than the limit specified by the length criterion, for example.

View-dependent subdivision, enabled by the view keyword, changes the length and distance criteria to work in raster space, interpreting the parameters in pixels instead of 3D units. Effectively, closer objects are tessellated more finely. This means that objects that are instanced more than once must be triangulated in multiple ways. A tradeoff between the additional memory required to store multiple objects, and the reduced total number of instanced triangles must be evaluated to achieve optimal speed.

The length, distance, and angle  approximation techniques use a recursive subdivision process that can also be controlled by two additional parameters, specifying the minimum and maximum number of recursion levels. The subdivision can be forced to proceed at least as far as the given minimum level, and refinement can be halted at the maximum level.

All subdivisions of a  free-form surface apart from the regular parametric technique and the Delaunay technique begin at the patch level. (A patch is the smallest unit of a surface.) If, for example, a plane is modeled with ten by ten patches it will be approximated by at least two hundred triangles, although two triangles might be adequate. If mental ray seems to be producing a large number of triangles in spite of a low  approximation accuracy, this is often due to the selected patch subdivision algorithm.

The combined angle and distance  approximation technique is usually the most useful technique for high quality rendering. For a quick rendering to examine materials or  object positions, the uniform parametric technique may be used with a factor of zero.

Free-form curves ( trimming curves) may also be approximated by any of the above described methods using a technique and tolerances which are distinct from those of the surface which the curve trims. The definitions are essentially the same if one considers a curve segment to correspond to a surface patch. An important difference is that the edge length, distance, and angle  approximation techniques will coalesce curve segments if possible. A straight line consisting of one hundred co-linear segments may be approximated by a single line segment.



Footnotes

... twenty-one.1.2
Although the user-settable degree is currently limited to 21, mental ray has no inherent limit.

next up previous contents
Next: 1.3 Edge Merging and Up: 1. Functionality Previous: 1.1 Parallelism
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