This tutorial shows how to generate bit masks from ENVISAT flags information as “raw” image using PyEPR.
The example code (examples/write_bitmask.py) is a direct translation of the C sample program write_bitmask.c bundled with the EPR API distribution.
The program is invoked as follows:
$ python write_bitmask.py <envisat-product> <bitmask-expression> \
<output-file>
The examples/write_bitmask.py code consists in a single function that also includes command line arguments handling:
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 | #!/usr/bin/env python
# This program is a direct translation of the sample program
# "write_bitmask.c" bundled with the EPR-API distribution.
#
# Source code of the C program is available at:
# https://github.com/bcdev/epr-api/blob/master/src/examples/write_bitmask.c
'''Generates bit mask from ENVISAT flags information as "raw" image
for (e.g.) Photoshop
Call::
$ python write_bitmask.py <envisat-product> <bitmask-expression>
<output-file>
Example to call the main function::
$ python write_bitmask.py MER_RR__2P_TEST.N1 \
'l2_flags.LAND and !l2_flags.BRIGHT' my_flags.raw
'''
import sys
import epr
def main(*argv):
if not argv:
argv = sys.argv
if len(argv) != 4:
print('Usage: write_bitmask <envisat-product> <bitmask-expression> '
'<output-file>')
print(' where envisat-product is the input filename')
print(' and bitmask-expression is a string containing the bitmask '
'logic')
print(' and output-file is the output filename.')
print('Example:')
print(" MER_RR__2P_TEST.N1 'l2_flags.LAND and !l2_flags.BRIGHT' "
"my_flags.raw")
print
sys.exit(1)
product_file_path = argv[1]
bm_expr = argv[2]
image_file_path = argv[3]
# Open the product; an argument is a path to product data file
product = epr.open(product_file_path)
offset_x = 0
offset_y = 0
source_width = product.get_scene_width()
source_height = product.get_scene_height()
source_step_x = 1
source_step_y = 1
bm_raster = epr.create_bitmask_raster(source_width, source_height,
source_step_x, source_step_y)
product.read_bitmask_raster(bm_expr, offset_x, offset_y, bm_raster)
with open(image_file_path, 'wb') as out_stream:
bm_raster.data.tofile(out_stream)
print 'Raw image data successfully written to "%s".' % image_file_path
print 'Data type is "byte", size is %d x %d pixels.' % (source_width,
source_height)
if __name__ == '__main__':
main()
|
In order to use the Python EPR API the epr module is imported:
import epr
As usual the ENVISAT product is opened using the epr.open() function that returns an epr.Product instance.
# Open the product; an argument is a path to product data file
product = epr.open(product_file_path)
Scene size parameters are retrieved form the epr.Product object using the epr.Product.get_scene_width() and epr.Product.get_scene_height() methods:
source_width = product.get_scene_width()
source_height = product.get_scene_height()
The EPR API allows to manage data by means of epr.Raster objects, so the function epr.create_bitmask_raster(), specific for bitmasks, is used to create a epr.Raster instance.
See also
Data are actually read using the epr.Product.read_bitmask_raster() method of the epr.Product class:
product.read_bitmask_raster(bm_expr, offset_x, offset_y, bm_raster)
The epr.Product.read_bitmask_raster() method receives in input the bm_expr parameter that is set via command line:
bm_expr = argv[2]
bm_expr is a string that define the logical expression for the definition of the bit-mask. In a bit-mask expression, any number of the flag-names (found in the DDDB) can be composed with “(”, ”)”, “NOT”, “AND”, “OR”.
Valid bit-mask expression are for example:
flags.LAND OR flags.CLOUD
or:
NOT flags.WATER AND flags.TURBID_S
Finally data are written to disk as a flat binary file using the numpy.ndarray.tofile() method of the epr.Raster.data attribute of the epr.Raster objects that exposes data via the numpy.ndarray interface:
with open(image_file_path, 'wb') as out_stream:
bm_raster.data.tofile(out_stream)