Python Ctypes Examples

• See GRASS and Python for more information.

• See also raster, vector sample scripts. (GRASS 7)

Raster

Working with (geographic) coordinates for each raster cell and the computational region

The ctypes interfaces are thin wrappers around the corresponding C functions. Any script which uses them should have essentially the same structure as a module written in C. Structures can be allocated by using the structure name as a function, and passed by reference using ctypes' byref() function.

The second argument to G_col_to_easting() and G_row_to_northing() is a "const struct Cell_head *". The information can be obtained from G_get_window(), e.g.:

       import sys
       import grass.lib.gis as gis
       from ctypes import *
 
       gis.G_gisinit(sys.argv[0])
 
       region = gis.Cell_head()
       gis.G_get_window(byref(region))
       coor_col = gis.G_col_to_easting((thecolumn + 0.5), byref(region))
       coor_row = gis.G_row_to_northing((therow + 0.5), byref(region))

Basic raster metadata access methods

This is a simple demonstration script to show the ctypes style access to some of the raster metadata.

(GRASS 7)

from ctypes import *
import grass.lib.gis as libgis
import grass.lib.raster as libraster
 
import grass.script as grass
import grass.temporal as tgis
 
def print_raster_info(name):
 
  grass.run_command("r.mapcalc", expr=name + " = 5.0", overwrite=True)
  grass.run_command("r.timestamp", map=name, date="13 Jan 2003 / 15 Jan 2003")
 
  # Create a region struct
  region = libgis.Cell_head()
 
  # Read the raster region
  libraster.Rast_get_cellhd("test", "", byref(region))
  print region.cols
  print region.rows
  print region.north
  print region.south
  print region.east
  print region.west
  print region.ns_res
  print region.ew_res
  print region.top
  print region.bottom
  print region.tb_res
  print region.depths
  print region.proj
 
  # Get Map type
  maptype = libraster.Rast_map_type(name, "")
 
  if maptype == libraster.DCELL_TYPE:
    print "DCELL"
  elif maptype == libraster.FCELL_TYPE:
    print "FCELL"
  elif maptype == libraster.CELL_TYPE:
    print "CELL"
  else:
    print "Unknown type"
 
  # Read range
  if libraster.Rast_map_is_fp(name, ""):
    print "Floating point map"
    range = libraster.FPRange()
    libraster.Rast_read_fp_range(name, "", byref(range))
    min = libgis.DCELL()
    max = libgis.DCELL()
    libraster.Rast_get_fp_range_min_max(byref(range), byref(min), byref(max))
  else:
    print "Cell map"
    range = libraster.Range()
    libraster.Rast_read_range(name, "", byref(range))
    min = libgis.CELL()
    max = libgis.CELL()
    libraster.Rast_get_fp_range_min_max(byref(range), byref(min), byref(max))
 
  minimum = min.value
  maximum = max.value
  print minimum
  print maximum
 
  # Read time stamp
  tstmp = libgis.TimeStamp()
  libgis.G_read_raster_timestamp(name, "", byref(tstmp))
 
  print tstmp.count
 
  dt1 = tstmp.dt[0]
  dt2 = tstmp.dt[1]
 
  if tstmp.count >= 1:
    print dt1.mode
    print dt1.year
    print dt1.month
    print dt1.day
    print dt1.hour
    print dt1.minute
    print dt1.second
  if tstmp.count > 1:
    print dt2.mode
    print dt2.year
    print dt2.month
    print dt2.day
    print dt2.hour
    print dt2.minute
    print dt2.second
 
 
if __name__ == "__main__":
    print_raster_info("test")

Vector

Accessing feature geometry

This script switches X, Y coordinates and multiple them by -1.

#!/usr/bin/python
 
import sys
 
from grass.lib.vector import *
 
if len(sys.argv) < 2:
    sys.exit("Usage: %s: vector" % sys.argv[0])
 
name = sys.argv[1]
 
map_info = pointer(Map_info())
points = Vect_new_line_struct()
cats = Vect_new_cats_struct()
c_points = points.contents
 
level = Vect_open_update(map_info, name, "")
if level < 2:
    sys.exit("Topology not available")
 
nlines = Vect_get_num_lines(map_info)
 
for line in range(1, nlines + 1):
    ltype = Vect_read_line(map_info, points, cats, line)
    if line % 100 == 0:
        sys.stderr.write("\r%d" % line)
 
    for i in range(c_points.n_points):
        x = c_points.x[i]
        c_points.x[i] = -1 * c_points.y[i]
        c_points.y[i] = -1 * x
 
    Vect_rewrite_line(map_info, line, ltype,
                      points, cats)
    line += 1
 
sys.stderr.write("\r")
 
Vect_destroy_line_struct(points)
Vect_destroy_cats_struct(cats)
 
Vect_build_partial(map_info, GV_BUILD_NONE)
Vect_build(map_info)
Vect_close(map_info)

See also

• http://docs.python.org/library/ctypes.html