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<title>Sweep face splitting - Sketch</title>
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<h5 class="subsubsection">3.2.6.5 Sweep face splitting</h5>

<p>Before sending each four-sided body polygon of a <code>sweep</code>
to the output, <code>sketch</code> tests to see if it is roughly planar. 
<a name="index-polygon_002c-planar-318"></a><a name="index-planarity-of-polygons-319"></a>Since planarity is necessary for proper functioning of the hidden
surface algorithm, &ldquo;warped&rdquo; polygons are automatically split into
two triangles.

   <p>Hole-filling polygons produced by closure-tagged
<a name="index-_003c_003e_0040r_007b_002c-closure-tag_007d-320"></a><a name="index-closure-tag_002c-_0040code_007b_003c_003e_007d-321"></a>line sweeps are not
split.  Nor are original polygons in polygon sweeps.  It is the user's
responsibility to ensure these are planar.

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