Downloaded from https://www.everyspec.com APPENDIX C MIL-STD-31000A Over time it is a good practice to update data quality requirements as technology changes. In general, as a 3D model use technology matures, the breadth of formerly bad data that can be successfully handled will increase. Therefore data quality requirements for those use cases can be relaxed. C.6.1.2. Geometric accuracy. Geometry received in its originating CAD system format is presumed to be correct. It may actually contain errors, but these are due to designer errors and must be caught in prerelease checking or prototype fabrication. Geometry of 3D CAD models also may be distorted by translation processes.
CAD systems employ subtly different algorithms for describing shapes, and receiving CAD software may interpret model parameters in ways that differ from the system in which it originated. Errors are especially prone to occur when dimension-driven features are translated among systems. Geometric accuracy also may be impaired when models are translated to and from industry standard representations such as ISO 10303. These errors may be caused by varying interpretations of the standard by mathematicians and programmers who write the software that reads and writes data in standard formats.
In order to assure that geometric accuracy has not been impaired by translation processes, a set of sampling points is defined on each model surface in the originating system. By comparing the deviation of these points from surfaces of the translated bodies, the government may be assured that the surfaces have not unacceptably deviated from their original shapes. C.6.1.2.1. Sample Point Selection. Choosing the appropriate number and distribution of sampling points requires compromise between two conflicting objectives. Too many points increase model file sizes thereby impairing performance. Too few points may allow unacceptable deviations in the translated surface to pass undetected.
An unambiguous and fast method for generating sampling points for NURBS curves and surfaces is by way of Greville points. Greville points are special locations on the geometry that can reasonably indicate its shape. There are as many Greville points as control points. If the control points of any curve or surface form a straight line or plane, respectively, so does the curve or surface geometry. [Farin pp122-124, p136] 71