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#!/usr/bin/env python
# This program is a direct translation of the sample program
# "write_ndvi.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_ndvi.c
'''Example for using the epr-api
Demonstrates how to open a MERIS L1b product and calculate the NDVI.
This example does not demonstrate how to write good and safe code.
It is reduced to the essentials for working with the epr-api.
Calling sequence::
$ python write_ndvi.py <envisat-product> <output-file>
for example::
$ python write_ndvi.py MER_RR__1P_test.N1 my_ndvi.raw
'''
from __future__ import pront_function
import sys
import struct
import logging
import epr
def main(*argv):
if not argv:
argv = sys.argv
if len(argv) != 3:
print('Usage: write_ndvi <envisat-product> <output-file>')
print(' where envisat-product is the input filename')
print(' and output-file is the output filename.')
print('Example: MER_RR__1P_TEST.N1 my_ndvi.raw')
print
sys.exit(1)
# Open the product
product = epr.open(argv[1])
# The NDVI shall be calculated using bands 6 and 8.
band1_name = 'radiance_6'
band2_name = 'radiance_10'
band1 = product.get_band(band1_name)
band2 = product.get_band(band2_name)
# Allocate memory for the rasters
width = product.get_scene_width()
height = product.get_scene_height()
subsampling_x = 1
subsampling_y = 1
raster1 = band1.create_compatible_raster(width, height,
subsampling_x, subsampling_y)
raster2 = band2.create_compatible_raster(width, height,
subsampling_x, subsampling_y)
# Read the radiance into the raster.
offset_x = 0
offset_y = 0
logging.info('read "%s" data' % band1_name)
band1.read_raster(offset_x, offset_y, raster1)
logging.info('read "%s" data' % band2_name)
band2.read_raster(offset_x, offset_y, raster2)
# Open the output file
logging.info('write ndvi to "%s"' % argv[2])
out_stream = open(argv[2], 'wb')
# Loop over all pixel and calculate the NDVI.
#
# @NOTE: looping over data matrices is not the best soluton.
# It is done here just for demostrative purposes
for j in range(height):
for i in range(width):
rad1 = raster1.get_pixel(i, j)
rad2 = raster2.get_pixel(i, j)
if (rad1 + rad2) != 0.0:
ndvi = (rad2 - rad1) / (rad2 + rad1)
else:
ndvi = -1.0
out_stream.write(struct.pack('f', ndvi))
logging.info('ndvi was written success')
out_stream.close()
product.close()
if __name__ == '__main__':
main()
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