A consistent CSG model is one which is made ! up of solid objects with no dangling surfaces. In , it is quite easy to construct inconsistent models, which will usually appear incorrect in the final images. ! In , CSG is implemented by maintaining the tree structure of the CSG operations. This tree is traversed, and the operators therein applied, on a per-ray basis. It is therefore difficult to verify the consistency of *************** *** 358,364 **** Such problems are often caused by polygons with incorrectly specified normals, or by surfaces that exactly coincide (which ! appear as partial ``swissh-cheese'' objects).
The following example illustrates an attempt to subtract a sphere from a pyramid defined using an incorrectly facing triangle. Note — 362,368 —- Such problems are often caused by polygons with incorrectly specified normals, or by surfaces that exactly coincide (which ! appear as partial ``Swiss cheese'' objects).
The following example illustrates an attempt to subtract a sphere from a pyramid defined using an incorrectly facing triangle. Note *************** *** 387,393 ****