sobel.c

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/* sobel.c */
#include <float.h>
#include "mypgm.h"

int sobel(unsigned char *image1, unsigned char *image2, unsigned int x_size, unsigned int y_size)
     /* Spatial filtering of image data */
     /* Sobel filter (horizontal differentiation */
     /* Input: image1[y][x] ---- Outout: image2[y][x] */
{
  /* Definition of Sobel filter in horizontal direction */
  int weight[3][3] = {{ -1,  0,  1 },
                      { -2,  0,  2 },
                      { -1,  0,  1 }};
  double pixel_value;
  double min, max;
  int x, y, i, j;  /* Loop variable */
  
  /* Maximum values calculation after filtering*/
  min = DBL_MAX;
  max = -DBL_MAX;
  for (y = 1; y < y_size - 1; y++) {
    for (x = 1; x < x_size - 1; x++) {
      pixel_value = 0.0;
      for (j = -1; j <= 1; j++) {
            for (i = -1; i <= 1; i++) {
              pixel_value += weight[j + 1][i + 1] * (*(image1 + (y + j) + (x + i) * MAX_IMAGESIZE));
            }
      }
      if (pixel_value < min) min = pixel_value;
      if (pixel_value > max) max = pixel_value;
    }
  }
  if ((int)(max - min) == 0) {
    return -1;
  }

  /* Initialization of image2[y][x] */
  for (y = 0; y < y_size; y++) {
    for (x = 0; x < x_size; x++) {
      *(image2 + y + x * MAX_IMAGESIZE) = 0;
    }
  }
  /* Generation of image2 after linear transformtion */
  for (y = 1; y < y_size - 1; y++) {
    for (x = 1; x < x_size - 1; x++) {
      pixel_value = 0.0;
      for (j = -1; j <= 1; j++) {
            for (i = -1; i <= 1; i++) {
              pixel_value += weight[j + 1][i + 1] * (*(image1 + (y + j) + (x + i) * MAX_IMAGESIZE));
            }
      }
      pixel_value = MAX_BRIGHTNESS * (pixel_value - min) / (max - min);
      *(image2 + y + x * MAX_IMAGESIZE) = (unsigned char)pixel_value;
    }
  }

  return 0;
}