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@scardine
Created August 23, 2013 14:39
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Experiments trying to implement topojson in Python using shapely.
from shapely.geometry import shape, Point, MultiPoint
import math
import json
def get_bounds(geometries):
"""Returns bounding box of geometries. Implementation creates a MultiPoint
from the boxes of all polygons in order to get the result"""
points = []
for g in geometries:
bounds = g.bounds
points.extend([Point(bounds[:2]), Point(bounds[2:])])
return MultiPoint(points).bounds
def lines2arcs(geometries, kx, ky, x0, y0):
"""Naive approach to create arcs from lines. TODO: implement detection of
shared points/arcs"""
for g in geometries:
try:
boundary = g.boundary
except AttributeError:
continue
if boundary.type == 'LineString':
boundary = [boundary]
arcs = []
x = y = 0
for line in boundary:
arc = []
for a, b in line.coords:
a = int(round((a - x0) / kx - x))
b = int(round((b - y0) / ky - y))
x += a
y += b
arc.append([a, b])
arcs.append(arc)
return arcs
def geo2topo(geojson, quant_factor=1e4):
"""Given a geojson dict, returns topojson"""
assert geojson['type'] == "FeatureCollection"
geometries = []
for feature in geojson['features']:
geometries.append(shape(feature['geometry']))
x0, y0, x1, y1 = get_bounds(geometries)
kx, ky = 1 / ((quant_factor - 1) / (x1 - x0)), 1/ ((quant_factor - 1) / (y1 - y0))
arcs = lines2arcs(geometries, kx, ky, x0, y0)
return {
"type" : "Topology",
"transform" : {
"scale" : [kx, ky],
"translate" : [x0, y0]
},
"objects" : {},
"arcs" : arcs
}
@prologic
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Have you done anything more with this? Used it anywhere in anything?

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