Exporting band data
This tutorial shows how to convert ENVISAT raster information from dataset and generate flat binary rasters using PyEPR.
The program generates as many raster as the dataset specified in input.
The example code (examples/write_bands.py) is a direct
translation of the C sample program write_bands.c bundled with the
EPR API distribution.
The program is invoked as follows:
$ python write_bands.py <envisat-product> \
<output directory for the raster file> <dataset name 1> \
[<dataset name 2> ... <dataset name N>]
Import section
To use the Python EPR API one have to import epr module.
At first import time the underlaying C library is opportunely initialized.
#!/usr/bin/env python3
# This program is a direct translation of the sample program
# "write_bands.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_bands.c
import os
import sys
import epr
The main program
The main program in quite simple (this is just an example).
def main(*argv):
"""A program for converting producing ENVI raster information from
dataset.
It generates as many raster as there are dataset entrance parameters.
Call::
$ python3 write_bands.py <envisat-product>
<output directory for the raster file>
<dataset name 1>
[<dataset name 2> ... <dataset name N>]
Example::
$ python3 write_bands.py \
MER_RR__1PNPDK20020415_103725_000002702005_00094_00649_1059.N1 \
. latitude
"""
if not argv:
argv = sys.argv
if len(argv) <= 3:
print("Usage: write_bands.py <envisat-product> <output-dir> "
"<dataset-name-1>")
print(" [<dataset-name-2> ... <dataset-name-N>]")
print(" where envisat-product is the input filename")
print(" and output-dir is the output directory")
print(" and dataset-name-1 is the name of the first band to be "
"extracted (mandatory)")
print(" and dataset-name-2 ... dataset-name-N are the names of "
"further bands to be extracted (optional)")
print("Example:")
print(" write_bands MER_RR__2P_TEST.N1 . latitude")
print()
sys.exit(1)
product_file_path = argv[1]
output_dir_path = argv[2]
# Open the product; the argument is the path to product the data file
with epr.open(product_file_path) as product:
for band_name in argv[3:]:
write_raw_image(output_dir_path, product, band_name)
if __name__ == "__main__":
main()
It performs some basic command line arguments handling and then open the input product.
# Open the product; the argument is the path to product the data file
with epr.open(product_file_path) as product:
Finally the core function (write_raw_image()) is called on each band
specified on the command:
for band_name in argv[3:]:
write_raw_image(output_dir_path, product, band_name)
The write_raw_image() core function
The core function is write_raw_image().
def write_raw_image(output_dir, product, band_name):
"""Generate the ENVI binary pattern image file for an actual DS.
The first parameter is the output directory path.
"""
# Build ENVI file path, DS name specifically
image_file_path = os.path.join(output_dir, band_name + ".raw")
band = product.get_band(band_name)
source_w = product.get_scene_width()
source_h = product.get_scene_height()
source_step_x = 1
source_step_y = 1
raster = band.create_compatible_raster(source_w, source_h,
source_step_x, source_step_y)
print(f"Reading band {band_name!r}...")
raster = band.read_raster(0, 0, raster)
with open(image_file_path, "wb") as out_stream:
for line in raster.data:
out_stream.write(line.tostring())
# or better: raster.data.tofile(out_stream)
print(f"Raw image data successfully written to {image_file_path!r}.")
print("C data type is %r, element size %u byte(s), "
"raster size is %u x %u pixels." % (
epr.data_type_id_to_str(raster.data_type),
raster.get_elem_size(),
raster.get_width(),
raster.get_height()))
It generates a flat binary file with data of a single band whose name is specified as input parameter.
First the output file name is computed using the os module.
# Build ENVI file path, DS name specifically
image_file_path = os.path.join(output_dir, band_name + ".raw")
Then the desired band is retrieved using the epr.Product.get_band()
method and some of its parameters are loaded in to local variables:
band = product.get_band(band_name)
Band data are accessed by means of a epr.Raster object.
See also
The epr.Band.create_compatible_raster() is a facility method that
allows to instantiate a epr.Raster object with a data type compatible
with the band data:
raster = band.create_compatible_raster(source_w, source_h,
source_step_x, source_step_y)
Then data are read using the epr.Band.read_raster() method:
print(f"Reading band {band_name!r}...")
raster = band.read_raster(0, 0, raster)
Then the output file object is created (in binary mode of course) and data are copied to the output file one line at time
with open(image_file_path, "wb") as out_stream:
for line in raster.data:
out_stream.write(line.tostring())
Please note that it has been used epr.Raster.data attribute of the
epr.Raster objects that exposes epr.Raster data with the
powerful numpy.ndarray interface.
Note
copying one line at time is not the best way to perform the task in Python. It has been done just to mimic the original C code:
out_stream = fopen(image_file_path, "wb");
if (out_stream == NULL) {
printf("Error: can't open '%s'\n", image_file_path);
return 3;
}
for (y = 0; y < (uint)raster->raster_height; ++y) {
numwritten = fwrite(epr_get_raster_line_addr(raster, y),
raster->elem_size,
raster->raster_width,
out_stream);
if (numwritten != raster->raster_width) {
printf("Error: can't write to %s\n", image_file_path);
return 4;
}
}
fclose(out_stream);
A by far more “pythonic” solution would be:
raster.data.tofile(out_stream)