Geometry Boost List: Re: [geometry] 3D box -> 3D multi_polygon conversion

Subject: Re: [geometry] 3D box -> 3D multi_polygon conversion
From: Adam Wulkiewicz (adam.wulkiewicz_at_[hidden])
Date: 2013-06-11 19:51:45

Tomislav Maric wrote:
> On 06/11/2013 10:12 PM, Adam Wulkiewicz wrote:
>>
>> Correct me if I'm wrong but shouldn't ring, polygon, multipolygon,
>> etc. be always flat? It may be 3D, may have even some orientation and
>> position in 3D space, not only height, but should be flat. This way we
>> can perform some 2D operations on it by e.g. first projecting it into
>> the 2D plane. E.g. we can calculate convex hull (also flat) or
>> triangulate. I'm not so sure if using MultiPolygon concept to describe
>> 3D mesh is a good idea. I'd rather provide additional concept.
>
> IMHO this would be quite expensive. Coordinate transform is a Matrix
> Vector multiplication, and it costs for nothing, if its only done to
> enable 2D calculation on a 3d object. Another point, consider
> incremental convex hull in 3D: computing the visible face is not
> possible to do this way (mixed product makex only sense for non
> co-planar vectors) simply because the hull construction will lie in 3D
> and transforming it to 2D projection will not work. I'm sure the
> quickhull algorithm is similar.
>

Sure, this was only an example. My point was that maybe there should be 
introduced a new, as you've written, MultiPolygon-like concept. And then 
algorithms should be built for it. Maybe even you'd like to extend 
MultiPolygon somehow or change it to describe meshes in better way? E.g. 
should 3D mesh contain faces which are polygons with holes? Or could 
those containing only triangles be represented in some optimized way?

Regards,
Adam