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SciPy Reference Guide, Release 0.8.dev
The value of the shift is obtained and then assigned to a dynamically allocated memory location. Both this data pointer
and the function pointer are then wrapped in a PyCObject, which is returned. Additionally, a pointer to a destructor
function is given, that will free the memory we allocated for the shift value when the PyCObject is destroyed. This
destructor is very simple:
static void
_destructor(void* cobject, void *cdata)
{
if (cdata)
free(cdata);
}
To use these functions, an extension module is built:
static PyMethodDef methods[] = {
{"shift_function", (PyCFunction)py_shift_function, METH_VARARGS, ""},
{NULL, NULL, 0, NULL}
};
void
initexample(void)
{
Py_InitModule("example", methods);
}
This extension can then be used in Python, for example:
>>> import example
>>> array = arange(12, shape=(4,3), type = Float64)
>>> fnc = example.shift_function(0.5)
>>> print geometric_transform(array, fnc)
[[ 0. 0. 0. ]
[ 0. 1.3625 2.7375]
[ 0. 4.8125 6.1875]
[ 0. 8.2625 9.6375]]
C callback functions for use with ndimage functions must all be written according to this scheme. The next section
lists the ndimage functions that acccept a C callback function and gives the prototype of the callback function.
1.10.10 Functions that support C callback functions
The ndimage functions that support C callback functions are described here. Obviously, the prototype of the function
that is provided to these functions must match exactly that what they expect. Therefore we give here the prototypes
of the callback functions. All these callback functions accept a void callback_data pointer that must be
wrapped in a PyCObject using the Python PyCObject_FromVoidPtrAndDesc function, which can also accept
a pointer to a destructor function to free any memory allocated for callback_data. If callback_data is not needed,
PyCObject_FromVoidPtr may be used instead. The callback functions must return an integer error status that is
equal to zero if something went wrong, or 1 otherwise. If an error occurs, you should normally set the python error
status with an informative message before returning, otherwise, a default error message is set by the calling function.
The function generic_filter (see Generic filter functions) accepts a callback function with the following prototype:
The calling function iterates over the elements of the input and output arrays, calling the callback function
at each element. The elements within the footprint of the filter at the current element are passed through
1.10. Multi-dimensional image processing (ndimage) 79