Tolerance Variables
In order to maintain model integrity, the modeler must work to specified tolerances. Applying tolerances insures proper interpretation of positions, such as maintaining that the points of vertices lie on the curves of the edges they bound or correctly determining if a position is inside or outside a volume.
ACIS uses the tolerance variables SPAresabs, SPAresnor, and SPAresfit to control modeling operations. These are global variables defined in the system that affect modeling functionality. All modeling operations in ACIS use these tolerance variables to maintain consistency of mathematical operations. Although SPAresnor should not be changed by the application, SPAresabs and SPAresfit may be changed, with great care, as explained in this section.
SPAresabs
SPAresabs is named for resolution absolute. It is the smallest meaningful quantity representable in ACIS. This can be interpreted as the distance below which ACIS considers two points to be coincident. If two points, A and B, are separated by less than SPAresabs, they are considered to be the same point.
SPAresabs also represents the smallest feature being modeled, since it is the smallest distance between two points. The default value is 10-6. The default value was chosen assuming that at least an order of magnitude guard band around SPAresabs is required. Refer to the section Dynamic Range below for more information.
SPAresnor
SPAresnor is named for resolution normalized. This is the ratio of the smallest meaningful quantity representable in ACIS (SPAresabs) to the largest. This reflects the precision to which numerical values are calculated and stored. The default value is 10-10.
From the definition SPAresnor = SPAresabs/largest, the largest quantity representable in ACIS is:
SPAresfit
SPAresfit is named for resolution fit. This is used as a guide to the fitting algorithms for the fit tolerance of an approximate curve or surface. The default value is 10-3.
Polynomial approximations are computed for some curves and surfaces in ACIS. The approximations are stored in the model together with their corresponding curve or surface definitions. The approximations are used:
- Alone when approximate geometry is sufficient (for example, for drawing).
- Together with the curve or surface definitions when more precise geometry is required; using the two together can make algorithms faster than using the curve or surface definitions alone.
Some fitting algorithms are adaptive, and therefore may produce tighter fits than SPAresfit in certain circumstances; for example, in regions of high curvature.
SPAresmch
SPAresmch is named for resolution machine. When comparing numbers that are within SPAresmch of one another, we recognize that for some computations, these two numbers can be considered equal. In other words, SPAresmch is the precision at which the machine is operating.
Given that our model space is 104 and the smallest geometric feature that can be modeled is 10-6, we need the machine to ensure a resolution of 10-6/104 = 10-10. To provide algorithms which deal with exceedingly small numbers a bit more computational freedom, we actually set SPAresmch to 10-11.
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