| | The Precision package offers a set of functions defining precision criteria for use in conventional situations when comparing two numbers. Generalities It is not advisable to use floating number equality. Instead, the difference between numbers must be compared with a given precision, i.e. : Standard_Real x1, x2 ; x1 = ... x2 = ... If ( x1 == x2 ) ... should not be used and must be written as indicated below: Standard_Real x1, x2 ; Standard_Real Precision = ... x1 = ... x2 = ... If ( Abs ( x1 - x2 ) < Precision ) ... Likewise, when ordering floating numbers, you must take the following into account : Standard_Real x1, x2 ; Standard_Real Precision = ... x1 = ... ! a large number x2 = ... ! another large number If ( x1 < x2 - Precision ) ... is incorrect when x1 and x2 are large numbers ; it is better to write : Standard_Real x1, x2 ; Standard_Real Precision = ... x1 = ... ! a large number x2 = ... ! another large number If ( x2 - x1 > Precision ) ... Precision in Cas.Cade Generally speaking, the precision criterion is not implicit in Cas.Cade. Low-level geometric algorithms accept precision criteria as arguments. As a rule, they should not refer directly to the precision criteria provided by the Precision package. On the other hand, high-level modeling algorithms have to provide the low-level geometric algorithms that they call, with a precision criteria. One way of doing this is to use the above precision criteria. Alternatively, the high-level algorithms can have their own system for precision management. For example, the Topology Data Structure stores precision criteria for each elementary shape (as a vertex, an edge or a face). When a new topological object is constructed, the precision criteria are taken from those provided by the Precision package, and stored in the related data structure. Later, a topological algorithm which analyses these objects will work with the values stored in the data structure. Also, if this algorithm is to build a new topological object, from these precision criteria, it will compute a new precision criterion for the new topological object, and write it into the data structure of the new topological object. The different precision criteria offered by the Precision package, cover the most common requirements of geometric algorithms, such as intersections, approximations, and so on. The choice of precision depends on the algorithm and on the geometric space. The geometric space may be : More...
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