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Care must be used to ensure that the sense of the parameterization of curves and parameter space curves is correct. The parameter limits of the curve of an edge and the parameter space curves of any of its coedges are found from the edge as its start and end parameters that correspond to the start and end vertices of the edge.
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Recall a
pcurve is attached to each coedge of a face lying on a parametric surface. An edge common to two faces, each lying on a parametric surface, has an intersection curve (a 3D spline in model space) and two
pcurves, one in each of the parametric surfaces.
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Consider the edge first. If the edge has the opposite sense to its underlying curve, the edge parameter interval (a,b) recorded in the edge is taken to be the interval (-b,-a) when finding points along the curve. The stored form of the curve must be consistent with this convention.
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In the case of the model geometry class
INTCURVE, there is a further sense logical within the stored
intcurve so that the
intcurve may be reversed without altering the details of the stored
bs3_curve, and the parameter values are negated and interchanged if so.
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Now, trace out the points along a coedge, that is in the sense of the coedge as it bounds a face (metal on the left). It is possible to trace out points along the edge and then take the set of points in reverse order if the coedge sense bit records the coedge as reversed with respect to the edge.
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Alternatively, if the coedge bounds a face lying on a spline surface, it is possible to find the same set of points (strictly, a similar set of points) from the coedge and its parameter space curve. The procedure is to find the parameter limits (a,b) from the edge, reverse them to (-b,-a) if the coedge is reversed with respect to the edge, and then present these limits to the stored
pcurve to obtain the points ordered along it in the desired sense.
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As with an
intcurve, the stored form of a private
pcurvecontains a sense bit so that a private
pcurve is reversed without having to change the underlying details of the stored 2D parametric curve or 2D parametric surface. As described above, for
pcurves associated with an existing model geometry class
INTCURVE, sense reversal is denoted by a negative value of
n, where n refers to the
nth
pcurve of the referenced
intcurve.
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Because the interpretation of the curve stored is governed by all of the intervening sense bits, it is is essential to understand these details when setting
bs2_curves into
pcurves or
bs3_curves into
intcurves.
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