Python Swig Examples: Difference between revisions
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= More Complicated SWIG Python Interface Examples = | = More Complicated SWIG Python Interface Examples = | ||
== | == Overview == | ||
GRASS's libgis (C API) distance and area functions automatically switch to using geodetic calculations when in a Lat/Lon location. | GRASS's libgis (C API) distance and area functions automatically switch to using geodetic calculations when in a Lat/Lon location. | ||
As you might expect, they preform many common GIS functions in very well tested and mature code. Reimplementing them in python is a waste of time and potentially buggy. The idea of using the SWIG interface to GRASS's C libraries is to ''"Let the experts solve the problem for you."'' | As you might expect, they preform many common GIS functions in very well tested and mature code. Reimplementing them in python is a waste of time and potentially buggy. The idea of using the SWIG interface to GRASS's C libraries is to ''"Let the experts solve the problem for you."'' | ||
== Passing arrays and specific data types == | |||
Some of GRASS's functions want to be passed arrays of numbers or of a specific data type (e.g. integer, double). To do this from Python the 3rd party [http://geosci.uchicago.edu/csc/numptr/ NumPtr] module is used to access a memory pointer object which can be passed to the function. | Some of GRASS's functions want to be passed arrays of numbers or of a specific data type (e.g. integer, double). To do this from Python the 3rd party [http://geosci.uchicago.edu/csc/numptr/ NumPtr] module is used to access a memory pointer object which can be passed to the function. | ||
: (''any way to then free() the memory? "<tt>del SomePtr</tt>"?) | : (''any way to then free() the memory? "<tt>del SomePtr</tt>"?) | ||
=== Setting up the NumPtr module === | |||
NumPtr setup: | NumPtr setup: | ||
# NumPtr - Numeric Pointer Module for Python (GPL2) | # NumPtr - Numeric Pointer Module for Python (GPL2) | ||
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cp build/lib.linux-i686-2.4/*NumPtr.* /usr/src/grass/grass63/swig/python/ | cp build/lib.linux-i686-2.4/*NumPtr.* /usr/src/grass/grass63/swig/python/ | ||
== Examples == | |||
=== Distance and area calculations === | |||
Once NumPtr is installed with can run our script: | Once NumPtr is installed with can run our script: | ||
#!/usr/bin/python | #!/usr/bin/python | ||
# g.distance -- demo SWIG interface | # g.distance -- demo SWIG interface |
Revision as of 06:28, 3 March 2008
More Complicated SWIG Python Interface Examples
Overview
GRASS's libgis (C API) distance and area functions automatically switch to using geodetic calculations when in a Lat/Lon location. As you might expect, they preform many common GIS functions in very well tested and mature code. Reimplementing them in python is a waste of time and potentially buggy. The idea of using the SWIG interface to GRASS's C libraries is to "Let the experts solve the problem for you."
Passing arrays and specific data types
Some of GRASS's functions want to be passed arrays of numbers or of a specific data type (e.g. integer, double). To do this from Python the 3rd party NumPtr module is used to access a memory pointer object which can be passed to the function.
- (any way to then free() the memory? "del SomePtr"?)
Setting up the NumPtr module
NumPtr setup:
# NumPtr - Numeric Pointer Module for Python (GPL2) # http://geosci.uchicago.edu/csc/numptr/ # 23k .tgz ; 100k installed wget http://geosci.uchicago.edu/csc/numptr/NumPtr-1.1.tar.gz tar xzf NumPtr-1.1.tar.gz cd NumPtr-1.1 python setup.py build #python setup.py install --prefix=/home/user cp build/lib.linux-i686-2.4/*NumPtr.* /usr/src/grass/grass63/swig/python/
Examples
Distance and area calculations
Once NumPtr is installed with can run our script:
#!/usr/bin/python # g.distance -- demo SWIG interface # (c) 2008 Hamish Bowman, and the GRASS Development Team # Licensed as GPL >=2 # run this before starting python to append module search path: # export PYTHONPATH=/usr/src/grass/svn/grass63/swig/python # check with "import sys; sys.path" # or: import sys sys.path.append("/usr/src/grass/svn/grass63/swig/python") import python_grass6 as g6lib g6lib.G_gisinit('m.distance') # returns 0 on success ### calc distance ### g6lib.G_begin_distance_calculations() # returns 0 if projection has no metrix (ie. imagery) # returns 1 if projection is planimetric # returns 2 if projection is latitude-longitude # G63> g.region -d && g.region -g # G63> g.region -e # north-south extent: 14310.000000 # east-west extent: 19020.000000 # calc length of hypotenuse: # >>> pow( pow(14310,2) + pow(19020,2), 0.5 ) # 23802.027224587404 # calc area of current region # >>> 14310 * 19020 # 272176200 x1 = 609000 x2 = 589980 y1 = 4913700 y2 = 4928010 distance = g6lib.G_distance(x1, y1, x2, y2) print "distance is", distance # 23802.0272246 (ok, matches above calc.) ### calc area ### g6lib.G_begin_polygon_area_calculations() # returns 0 if the projection is not measurable (ie. imagery or xy) # returns 1 if the projection is planimetric (ie. UTM or SP) # returns 2 if the projection is non-planimetric (ie. latitude-longitude) # we dont' need this, but just to have a look g6lib.G_database_units_to_meters_factor() # 1.0 # passing an array of values import Numeric import NumPtr # do not need to close polygon x = [ x1, x2, x2, x1 ] y = [ y1, y1, y2, y2 ] npoints = len(x) # unset variables: #del [Xs, Xptr, Ys, Yptr] # or #Xs = Xptr = Ys = Yptr = None Xs = Numeric.array(x, Numeric.Float64) Xptr = NumPtr.getpointer(Xs) Ys = Numeric.array(y, Numeric.Float64) Yptr = NumPtr.getpointer(Ys) area = g6lib.G_area_of_polygon(Xptr, Yptr, npoints) print "area is", area # 272176200.0 (ok, matches above calc)