Hi,

On 12-6-2013 10:49, Tomislav Maric wrote:

On 12-6-2013 10:49, Tomislav Maric wrote:

On 06/12/2013 01:51 AM, Adam Wulkiewicz wrote: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?Well, I am working on numerical methods for simulating fluid flow, and they need flow domain to be decomposed into polyhedra like MultiPolygon, so physics prevents the polygons of those polyhedra to have holes... What I am aiming at is working on the algorithms in 3D in boost.geometry, that I need in order to optimize my geometrical code for speed + efficiency, and then extend what I get later for a more general purpose. Triangles: that's a great question actually. Basically, the answer is yes and no. Initially the polyhedron is consisted of polygons, but then it is decomposed into tetrahedra to compute its volume and do subsequent intersections. This is done because some of its polygons will be *non planar* (search for voFoam on arXiv.org, page 12 I think). So, there are three concepts, I believe: polyhedron (multipolygon extended), tetrahedron and tetrahedral decomposition of a polyhedron (just triangle faces: e.g. optimized normal vector calculation).

The OGC model is our reference model for the concepts point, linestring, polygon, multi_point, multi_linestring, multi_polygon. Up to now we have followed this model.

See also this webpage, where it can be downloaded:

http://www.opengeospatial.org/standards/sfa

The OGC talks consequently about a "PolyhedralSurface".

I copy a part from the PDF here for convenience:

"

For any two polygons that share a common boundary, the “top” of the polygon shall be consistent. This means

Figure 14: Polyhedral Surface with consistent orientation

If each such LineString is the boundary of exactly 2 Polygon patches, then the PolyhedralSurface is a simple,

Regards, Barend