How to draw a PostGIS Polygon with holes into Blender using Python

Question

i would like to draw polygons in Blender. The vertices positions are given as a text in PostGIS format. Example :

POLYGON((0 0, 1 0, 1 1, 0 1, 0 0))

I already came up with a script working pretty well for any polygon without holes. However i also need to draw some polygons with holes. Example (the second polygon defines the hole) :

POLYGON((0 0, 10 0, 10 10, 0 10, 0 0),(1 1, 1 2, 2 2, 2 1, 1 1))

Here is the python script so far to draw any sequence of polygons without holes :

import io
import csv
import bmesh
data="""
0;2;POLYGON((0 0,30 0,30 30,0 30,0 0))
0;2;POLYGON((4 4,26 4,26 26,4 26,4 4))
"""

f = io.StringIO(data)
csv_f = csv.reader(f, delimiter=';')

i = 0
for row in csv_f:
    i+=1
    if (len(row)>2):
        verts = eval('['+row[2].replace(',','),(').replace(' ',',')[8:-1]+']')
        bm = bmesh.new()
        for v in verts:
            bm.verts.new((v[0], v[1], float(row[0])))
        bm.faces.new(bm.verts)
        bm.normal_update()
        me = bpy.data.meshes.new(row[0])
        bm.to_mesh(me)
        ob = bpy.data.objects.new(row[0], me)
        bpy.context.scene.objects.link(ob)
bpy.context.scene.update()

Executing this script into Blender will create two squares, one inside the other :

My question is : how to remove the inner square from the big square (and resulting into a hole)

I tried to Add a Boolean modifier on the big square, applied to the inner square object, but i get a "Cannot execute boolean operation".

Is there any other way doing this from Blender and that can be scripted into Python ?

Answer 1

Polygons can't have holes without a pathway between the hole and the perimeter. ( see: BMesh polygons with holes )

It is perhaps more convenient to use a 2D Curve. They can represent a face with holes easily using a Spline type called 'POLY'. For Example:

Then later Convert to a Polygon based Mesh and let Blender take care of how it arranges the tessellation.

here a script you could use:

import bpy  

# weight  
w = 1 

def MakeFilledPolyLine(objname, curvename, cLists):
    curvedata = bpy.data.curves.new(name=curvename, type='CURVE')  
    curvedata.dimensions = '2D'  

    odata = bpy.data.objects.new(objname, curvedata)  
    odata.location = (0,0,0) # object origin  
    bpy.context.scene.objects.link(odata)  

    for cList in cLists:
        polyline = curvedata.splines.new('POLY')  
        polyline.points.add(len(cList)-1)  
        for num in range(len(cList)):  
            polyline.points[num].co = cList[num][0], cList[num][1], 0, w

        polyline.order_u = len(polyline.points)-1
        polyline.use_endpoint_u = True
        polyline.use_cyclic_u = True


# using the cyclic switch auto closes the loops, notice i've   
# dropped the last coordinates.
vectors = [
    [[0,0], [10,0], [10,10], [0,10]], 
    [[1,1], [1,2], [2,2], [2,1]]
]
MakeFilledPolyLine("NameOfMyCurveObject", "NameOfMyCurve", vectors)

The upside of this is it would handle polygons with no holes, or with any number of holes using the same code.

If you've come this far, I expect it will be a trivial matter to convert the strings of that PostGIS file into the appropriately formatted nested lists for the MakeFilledPolyLine function.

Answer 2

Simply use bmesh.ops.triangle_fill to accomplish your task:

bmesh.ops.triangle_fill(bm, use_beauty=True, use_dissolve=False, edges=outer_and_inner_edges)

Here outer_and_inner_edges is the list of BMesh edges (both outer and inner) that constitute your polygon with holes.

The operator will create a bunch of triangles that cover the polygon in question.