Graphics Reference
In-Depth Information
Figure 14.22.
Cases when decomposing
horizontal trapezoid slices.
Figure 14.23.
Trapezoidal decomposition of a
trimmed Bezier patch.
reduced to drawing a sequence of subdivided triangles with a single bottom vertex,
subdivided triangles with a single top vertex, and/or subdivided parallelogram type
regions. See the document VattiTrim on the accompanying CD for more details.
Sample outputs using this approach are shown in Figures 14.20(b) and 14.21(b).
Finally, consider Figure 14.23. This shows the triangles and parallelograms that
are generated for a trimmed Bezier surface patch where the bounding curves are cubic
B-splines. Notice that parts of the domain were subdivided more than others. This
may not be desirable. The “feature” has two causes:
(1) All trapezoids are drawn no matter how thin they are.
(2) The vertical height of trapezoids is determined by scan beams and these are
usually thinner than the resolution of the bounding curves would suggest.
One can mitigate these causes by using a smaller resolution for the bounding curves,
expecting the trapezoids to be smaller anyway, or one could get fancier with the
algorithms described above and change the subdivision of the bounding curves in
