[CSC 315] Strange Tesselation Error

[Andrew J. Pounds]

Some of you have noted that when you use the ear-clipping algorithm to 
tesselate something like

unfilled

you actually end up with...

filled


This is happening because I may not have been clear on the check related 
to "can I draw the line back to the initial vertex".   I think in class 
I may have made it too simple -- I stated that as long as the line did 
not intersect any other lines in the polygon. In addition the line 
connecting the point back to the vertex must lie in the INTERIOR of the 
polygon.

There are multiple ways to check for this, but the easiest in my 
opinion  is to check the angle back to the original vertex and the angle 
of the next vertex in the list.  If the angle to the next vertex in the 
list is less than the angle to the initial point, then you can't 
tesselate that triangle and you have to move to the next one in the list.

Sit down with a piece of paper and play with it for a few minutes and it 
should make sense.  If not, see me.


-- 
Andrew J. Pounds, Ph.D.  (pounds_aj at mercer.edu)
Professor of Chemistry and Computer Science
Mercer University,  Macon, GA 31207   (478) 301-5627
http://faculty.mercer.edu/pounds_aj

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