I41 Answer ― Image Smoothing

#include <stdlib.h>
#include <stdio.h>
#include <math.h>
#include "basicImage.c"

void smoothImage( image img, image smooth )
{
  int r, c, sum;
  unsigned char value ;

  /* Copy edge pixels from input image to output image */
  for ( r=0; r<img.nrows; r++ )
  {
    value = getPixel( img, r, 0 ) ;
    setPixel( smooth, r, 0, value );
    value = getPixel( img, r, img.ncols-1 ) ;
    setPixel( smooth, r, img.ncols-1 , value );
  }

  for ( c=0; c<img.ncols; c++ )
  {
    value = getPixel( img, 0, c ) ;
    setPixel( smooth, 0, c, value );
    value = getPixel( img, img.nrows-1, c ) ;
    setPixel( smooth, img.nrows-1, c, value );
  }

  /* Fill in the center with neighborhood averages */
  for ( r=1; r<img.nrows-1; r++ )
    for ( c=1; c<img.ncols-1; c++ )
    {
      sum  = getPixel( img, r-1, c-1 );
      sum += getPixel( img, r-1, c   );
      sum += getPixel( img, r-1, c+1 );
      sum += getPixel( img, r  , c-1 );
      sum += getPixel( img, r  , c   );
      sum += getPixel( img, r  , c+1 );
      sum += getPixel( img, r+1, c-1 );
      sum += getPixel( img, r+1, c   );
      sum += getPixel( img, r+1, c+1 );

      setPixel( smooth, r, c, sum/9 );
    }
}

int main ( int argc, char* argv[] )
{
  image img, smimg;
  
  if ( argc != 3 )
  {
    printf( "smooth oldImage smoothImage\n" );
    system( "pause" );
    exit( EXIT_FAILURE );
  }
  
  /* read in the image */
  readPGMimage( &img, argv[1] );

  /* create a blank image */
  newImage( &smimg, img.nrows, img.ncols );

  /* fill in values for the new image */
  smoothImage( img, smimg );
  
  /* write the image to disk and free memory */
  writePGMimage( smimg, argv[2] );
  freeImage( &img );
  freeImage( &smimg );
}

Comments: You may wish to replace the following

      sum  = getPixel( img, r-1, c-1 );
      sum += getPixel( img, r-1, c   );
      sum += getPixel( img, r-1, c+1 );
      sum += getPixel( img, r  , c-1 );
      sum += getPixel( img, r  , c   );
      sum += getPixel( img, r  , c+1 );
      sum += getPixel( img, r+1, c-1 );
      sum += getPixel( img, r+1, c   );
      sum += getPixel( img, r+1, c+1 );

with two for loops.