Re: [Algorithms] more plane-poly clipping woes

[Angel P. <ju...@bi...>]

If I understood you correctly - the newly created split vertex
does not lie within +/-EPSILON distance from the plane
that generated it. There are tree reasonable ways to handle
this:
1. Increase EPSILON. It MUST be big enough to handle such
precision errors.
2. Increase your FP precision by using doubles.
3. Split polygons with very long edges to smaller polygons.
I am quite certain that you get such inconsistencies only
when splitting long edges.

> Well, I'm now using "thick" BSP planes as suggested
> to me by Angel Popov.  This basically means treating
> all your BSP planes as having some thickness; then
> any vert which lies in that thickness is treated as
> being already on the plane.  When you clip a poly
> against a plane, the verts which are "on" the plane
> are not modified.  This prevents you from clipping
> segments where one point has a distance of 999
> and the other has a distance of -0.001
> 
> The problem is that the clipper is still not consistent.
> That is, if I take a polygon and clip it against a
> plane to produce two new polygons, and then test those
> against the plane, they should tell me they're on the front
> and back sides; instead they can occasionally report that
> they intersect the plane, because a vertex produced by
> clipping was not inside the thickness of the plane.
> 
> The only solution I can think of for this is to have a
> larger epsilon for plane-testing than for plane-clipping
> (double seems to work), but that concerns me a bit (is
> there a way that could lead to inconsistency?).
> 
> 
> 
> --------------------------------------
> Charles Bloom           www.cbloom.com
> 
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