Re: [boost] Formal Review: Boost.Polygon starts today August 24, 2009

Subject: Re: [boost] Formal Review: Boost.Polygon starts today August 24, 2009
From: Andreas Fabri (andreas.fabri_at_[hidden])
Date: 2009-08-25 16:45:33

Hello,

When I compute union or intersection of two rectangles rotated by 45 degree
that form an X, I don't necessarily get a 45 degree polygon, namely when
the intersection point is not an integer.

For example when one polygon has an edge   0,0  -- 10,10
        and the other polygon has an edge  -1,10 --  9,9

then they intersect mathematically at  4.5,4.5
but as you snap to an integer, you get for example  5,5

Now -1,10 -- 5,5  is no longer a 45 degree edge.

Isn't the fact that the set of 45 degree polygons is not closed
under Boolean operations with snap rounding bothersome (even in practice).

Best regards,

andreas fabri