Library demo - reading a image (GNU Astronomy Utilities)

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12.4.1 Library demo - reading a FITS image ¶

The following simple program demonstrates how to read a FITS image into memory and use the void *array pointer in of Generic data container (gal_data_t). For easy linking/compilation of this program along with a first run see BuildProgram (in short: Compile, link and run ‘myprogram.c’ with this command: ‘astbuildprog myprogram.c). Before running, also change the filename and hdu variable values to specify an existing FITS file and/or extension/HDU.

This is just intended to demonstrate how to use the array pointer of gal_data_t. Hence it does not do important sanity checks, for example in real datasets you may also have blank pixels. In such cases, this program will return a NaN value (see Blank pixels). So for general statistical information of a dataset, it is much better to use Gnuastro’s Statistics program which can deal with blank pixels and many other issues in a generic dataset.

To encourage good coding practices, this script contains a copyright notice with a place holder for your name and your email (as you customize it for your own purpose). Always keep a one-line description and copyright notice like this in all your scripts, such “metadata” is very important to accompany every source file you write. Of course, when you write the source file from scratch and just learn how to use a single function from this manual, only your name/year should appear. The existing name of the original author of this example program is only for cases where you copy-paste this whole file.

/* Reading a FITS image into memory.
 *
 * The following simple program demonstrates how to read a FITS image
 * into memory  and use the 'void *array' pointer. This is just intended
 * to demonstrate how to use the array pointer of 'gal_data_t'.
 *
 * Copyright (C) 2024      Your Name <your@email.address>
 * Copyright (C) 2020-2024 Mohammad Akhlaghi <mohammad@akhlaghi.org>
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful, but
 * WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
 * General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program. If not, see <http://www.gnu.org/licenses/>.
 */

#include <stdio.h>
#include <stdlib.h>

#include <gnuastro/fits.h> /* includes gnuastro's data.h and type.h */
#include <gnuastro/statistics.h>

int
main(void)
{
  size_t i;
  float *farray;
  double sum=0.0f;
  gal_data_t *image;
  char *filename="img.fits", *hdu="1";


  /* Read `img.fits' (HDU: 1) as a float32 array. */
  image=gal_fits_img_read_to_type(filename, hdu, GAL_TYPE_FLOAT32,
                                  -1, 1, NULL);


  /* Use the allocated space as a single precision floating
   * point array (recall that `image->array' has `void *'
   * type, so it is not directly usable). */
  farray=image->array;


  /* Calculate the sum of all the values. */
  for(i=0; i<image->size; ++i)
    sum += farray[i];


  /* Report the sum. */
  printf("Sum of values in %s (hdu %s) is: %f\n",
         filename, hdu, sum);


  /* Clean up and return. */
  gal_data_free(image);
  return EXIT_SUCCESS;
}