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"""A more complex example which renders an infinite series of concentric
circles centred on a point.
The circles are represented by a Python iterator which will yield only the
circles which intersect the query's bounding box. The advantage of this
approach over a MemoryDatasource is that a) only those circles which intersect
the viewport are actually generated and b) only the memory for the largest
circle need be available since each circle is created on demand and destroyed
when finished with.
"""
import math
import mapnik
from shapely.geometry import *
def box2d_to_shapely(box):
import shapely.geometry
return shapely.geometry.box(box.minx, box.miny, box.maxx, box.maxy)
class ConcentricCircles(object):
def __init__(self, centre, bounds, step=1):
self.centre = centre
self.bounds = bounds
self.step = step
class Iterator(object):
def __init__(self, container):
self.container = container
centre = self.container.centre
bounds = self.container.bounds
step = self.container.step
if centre.within(bounds):
self.radius = 0
else:
self.radius = math.ceil(centre.distance(bounds) / float(step)) * step
def next(self):
circle = self.container.centre.buffer(self.radius)
self.radius += self.container.step
# has the circle grown so large that the boundary is entirely within it?
if circle.contains(self.container.bounds):
raise StopIteration()
return ( circle.wkb, { } )
def __iter__(self):
return ConcentricCircles.Iterator(self)
class TestDatasource(mapnik.PythonDatasource):
def __init__(self):
super(TestDatasource, self).__init__(
geometry_type=mapnik.DataGeometryType.Polygon
)
def features(self, query):
# Get the query bounding-box as a shapely bounding box
bounding_box = box2d_to_shapely(query.bbox)
centre = Point(-20, 0)
return mapnik.PythonDatasource.wkb_features(
keys = (),
features = ConcentricCircles(centre, bounding_box, 0.5)
)
if __name__ == '__main__':
m = mapnik.Map(640, 320)
m.background = mapnik.Color('white')
s = mapnik.Style()
r = mapnik.Rule()
r.symbols.append(mapnik.LineSymbolizer())
s.rules.append(r)
m.append_style('point_style',s)
ds = mapnik.Python(factory='TestDatasource')
layer = mapnik.Layer('python')
layer.datasource = ds
layer.styles.append('point_style')
m.layers.append(layer)
box = mapnik.Box2d(-60, -60, 0, -30)
m.zoom_to_box(box)
mapnik.render_to_file(m,'map.png', 'png')
|
