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.
Although the mapnik::query class is exposed to the Python bindings, the
resolution attribute is a raw boost::tuple. If you attempt to access
this tuple from Python, boost complains strongly.
This patch adds the required magic to marshal the raw boost::tuple which
is query::resolution_type into an honest-to-goodness Python tuple.
The png library uses setjmp/longjmp to throw exceptions when reading. If this
is not set up, the png library calls abort(). This change handles the errors
and throws a C++ exception instead.
This issue was found by testing images from pngsuite at
http://www.schaik.com/pngsuite/. These images are included and a unit test was
added to test both images that should be successful and images that should
throw an exception.
