MeshBuilder causes self intersections

[MughtyWinky]

I'm trying to convert roof geometries, based on a 3d city model, into DXF. These roofs usually consist of >4 3d-points and should be exported as "areas". Thus they are converted using the MeshBuilder. During conversion, the MeshBuilder seems to call subdivide_ngons, which adds a point. Depending on the geometry, this leads to self intersections. Is there a better way to solve this?

import ezdxf

#create a new document
doc = ezdxf.new(dxfversion="R2013")
msp = doc.modelspace()
layout = doc.layouts.get("Model")

# create a easy 3d shape
# z = y for planarity
coords = [[0,0,0], [3,0,0], [3,1,1], [8,1,1], [8,0,0], [10,0,0], [10,10,10], [0,10,10], [0,0,0]]

# add coords as mesh
mb = ezdxf.render.MeshBuilder()
mb.add_face(coords)
# render calls subdivide_ngons,
# which adds a point at 4.67, 2.44
# and causes some self-intersections
mb.render_3dfaces(layout)

# how it should look
# but only as line
pl = msp.add_polyline3d(coords)
pl.rgb=[255,0,0]

doc.saveas("c:/temp/example.dxf")

[mozman]

Ezdxf is not a sophisticated 3d library so try to keep things simple. In this case you add an n-gon to a MeshBuilder which is basically ok because the main target entity of MeshBuilder is the MESH entity, which support n-gons. The problem here is the rendering of the mesh as 3DFACE objects, which supports only triangles and quadrilaterals but not n-gons. The subdiv algorithm which divides n-gons into triagles is very simple and can not handle concave shapes as you see. The only solution is to subdivide the n-gons into convex shapes by yourself or using the MESH entity. A third party library like CGAL may help to subdivide the polygon.

[MughtyWinky]

Thank's for the answer, that's what I assumed. Luckily the database backend PostGIS provides ST_Tesselate(), which uses CGAL via SFCGAL. Something like

SELECT 
	CASE 
		WHEN ST_NPoints(geom) >4 THEN ST_Tesselate(geom)
		ELSE geom
	END as geom
FROM
	test

will do the trick. But that's kind of offtopic.

I appreciate your quick answer and work at ezdxf. Thank you!

[mozman]

Added MeshBuilder.tessellation() method which uses the "ear clipping" algorithm to process ngons for rendering POLYFACE and 3DFACE entities. This should solve this issue.