156a7590f4
This plugin allows you to write data sources in the Python programming language. This is useful if you want to rapidly prototype a plugin, perform some custom manipulation on data or if you want to bind mapnik to a datasource which is most conveniently accessed through Python. The plugin may be used from the existing mapnik Python bindings or it can embed the Python interpreter directly allowing it to be used from C++, XML or even JavaScript. Mapnik already has excellent Python bindings but they only directly support calling *into* mapnik *from* Python. This forces mapnik and its input plugins to be the lowest layer of the stack. The role of this plugin is to allow mapnik to call *into* Python itself. This allows mapnik to sit as rendering middleware between a custom Python frontend and a custom Python datasource. This increases the utility of mapnik as a component in a larger system. There already exists MemoryDatasource which can be used to dynamically create geometry in Python. It suffers from the problem that it does not allow generating only the geometry which is seen by a particular query. Similarly the entire geometry must exist in memory before rendering can progress. By using a custom iterator object or by using generator expressions this plugin allows geometry to be created on demand and to be destroyed after use. This can have a great impact on memory efficiency. Since geometry is generated on-demand as rendering progresses there can be arbitrarily complex 'cleverness' optimising the geometry generated for a particular query. Obvious examples of this would be generating only geometry within the query bounding box and generating geometry with an appropriate level of detail for the output resolution.
83 lines
2.7 KiB
Python
83 lines
2.7 KiB
Python
"""A more complex example which renders an infinite series of concentric
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circles centred on a point.
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The circles are represented by a Python iterator which will yield only the
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circles which intersect the query's bounding box. The advantage of this
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approach over a MemoryDatasource is that a) only those circles which intersect
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the viewport are actually generated and b) only the memory for the largest
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circle need be available since each circle is created on demand and destroyed
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when finished with.
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"""
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import math
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import mapnik
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from shapely.geometry import *
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def box2d_to_shapely(box):
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import shapely.geometry
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return shapely.geometry.box(box.minx, box.miny, box.maxx, box.maxy)
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class ConcentricCircles(object):
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def __init__(self, centre, bounds, step=1):
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self.centre = centre
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self.bounds = bounds
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self.step = step
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class Iterator(object):
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def __init__(self, container):
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self.container = container
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centre = self.container.centre
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bounds = self.container.bounds
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step = self.container.step
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if centre.within(bounds):
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self.radius = 0
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else:
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self.radius = math.ceil(centre.distance(bounds) / float(step)) * step
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def next(self):
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circle = self.container.centre.buffer(self.radius)
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self.radius += self.container.step
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# has the circle grown so large that the boundary is entirely within it?
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if circle.contains(self.container.bounds):
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raise StopIteration()
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return ( circle.wkb, { } )
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def __iter__(self):
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return ConcentricCircles.Iterator(self)
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class TestDatasource(mapnik.PythonDatasource):
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def __init__(self):
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super(TestDatasource, self).__init__(
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geometry_type=mapnik.DataGeometryType.Polygon
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)
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def features(self, query):
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# Get the query bounding-box as a shapely bounding box
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bounding_box = box2d_to_shapely(query.bbox)
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centre = Point(-20, 0)
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return mapnik.PythonDatasource.wkb_features(
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keys = (),
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features = ConcentricCircles(centre, bounding_box, 0.5)
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)
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if __name__ == '__main__':
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m = mapnik.Map(640, 320)
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m.background = mapnik.Color('white')
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s = mapnik.Style()
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r = mapnik.Rule()
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r.symbols.append(mapnik.LineSymbolizer())
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s.rules.append(r)
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m.append_style('point_style',s)
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ds = mapnik.Python(factory='TestDatasource')
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layer = mapnik.Layer('python')
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layer.datasource = ds
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layer.styles.append('point_style')
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m.layers.append(layer)
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box = mapnik.Box2d(-60, -60, 0, -30)
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m.zoom_to_box(box)
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mapnik.render_to_file(m,'map.png', 'png')
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