Edge Detection Of A PPM Image In C?

So far this is what I've got:

What I'm trying to do is read in a PPM, store it in an array (done), then prompt the user to enter a threshold. Then detect the edges in an image and output to a new internal array image where pixels that are part of an edgeare white, other pixels are black.

I've been given this formula:
grad = abs(im(x+1, y) - im(x-1, y)) + abs(im(x, y+1) - im(x, y-1))
and am told to compare it to a threshold then sum grad over the colour channels. Any pointers would be welcome. Thanks.

Code:

---

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


#define maxheight 1080
#define maxwidth 1920
#define RGB_COMPONENT_COLOUR 255
#define pgmtype "P2"
#define ppmtype "P3"


typedef struct
{
    int red, green, blue;
} PPMPixel;


typedef struct
{
    int x, y;
    //PPMPixel *data;
} PPMImage;


typedef struct
{
    int rgb_comp_colour;
    char filetype[3];
    int height;
    int width;
} PPMHead;


PPMHead head[3];


PPMPixel r;


PPMPixel data; //Defines pointer to PPMPixel
PPMPixel **EdgeArray;

void edge_detection()
{
    int i, j;
    FILE *fed;
    char fnameed[100];
    printf("\tEnter first file's name: \n");
    scanf("%s", fnameed);
    fseek(stdin,0,SEEK_END);
    fed = fopen(fnameed, "r");
    if (fed == NULL)
    { 
        printf("\tError while opening the file\n");
    }
    else   
    {
        printf("\tWriting in %s\n", fnameed);
    }


    fscanf(fed, "%s %d %d %d", head[3].filetype, &head[3].width, &head[3].height, &head[3].rgb_comp_colour);
    printf("%s %d %d %d", head[3].filetype, head[3].width, head[3].height, head[3].rgb_comp_colour);


    EdgeArray = (PPMPixel **)malloc(head[3].height*sizeof(PPMPixel*)); //Points to malloc
    if((EdgeArray == NULL))
    {
        printf("\tError allocating memory to the array\n");
    }
    else
    {
        printf("\tMemory allocated to the PPM array sucessfully\n");
    }
    for (i=0;i<head[3].height;i++)
    {
        EdgeArray[i] = (PPMPixel *)malloc(head[3].width*sizeof(PPMPixel));
    }
   
    //Initialising each element
    for (j=0;j<head[3].height;j++)
    {
        for (i=0;i<head[3].width;i++)
        {
            fscanf(fed, "%d %d %d ", &r.red, &r.green, &r.blue);
            PPMPixel* data = &EdgeArray[j][i];
            data->red = r.red;
            data->green = r.green;
            data->blue = r.blue;
        }
    }
}

---

That doesn't look like edge detection. It looks like reading the image from a file.

Why not pull that out as a separate function, put in a main(), and go from there?

I compiled with warnings on - be mindful that C arrays start at element 0!

Code:

---

main.c:64:80: warning: array subscript 3 is above array bounds of ‘PPMHead[3]’ {aka ‘struct <anonymous>[3]’} [-Warray-bounds]
  64 |    printf("%s %d %d %d", head[3].filetype, head[3].width, head[3].height, head[3].rgb_comp_colour);
      |                                                                            ~~~~^~~
main.c:35:9: note: while referencing ‘head’
  35 | PPMHead head[3];
      |        ^~~~
main.c:64:64: warning: array subscript 3 is above array bounds of ‘PPMHead[3]’ {aka ‘struct <anonymous>[3]’} [-Warray-bounds]
  64 |    printf("%s %d %d %d", head[3].filetype, head[3].width, head[3].height, head[3].rgb_comp_colour);
      |                                                            ~~~~^~~
main.c:35:9: note: while referencing ‘head’
  35 | PPMHead head[3];
      |        ^~~~
main.c:64:49: warning: array subscript 3 is above array bounds of ‘PPMHead[3]’ {aka ‘struct <anonymous>[3]’} [-Warray-bounds]
  64 |    printf("%s %d %d %d", head[3].filetype, head[3].width, head[3].height, head[3].rgb_comp_colour);
      |                                            ~~~~^~~
main.c:35:9: note: while referencing ‘head’
  35 | PPMHead head[3];
      |        ^~~~
main.c:67:41: warning: array subscript 3 is above array bounds of ‘PPMHead[3]’ {aka ‘struct <anonymous>[3]’} [-Warray-bounds]
  67 |    EdgeArray = (PPMPixel **)malloc(head[3].height*sizeof(PPMPixel*)); //Points to malloc
      |                                    ~~~~^~~
main.c:35:9: note: while referencing ‘head’
  35 | PPMHead head[3];
      |        ^~~~
main.c:76:20: warning: array subscript 3 is above array bounds of ‘PPMHead[3]’ {aka ‘struct <anonymous>[3]’} [-Warray-bounds]
  76 |    for (i=0;i<head[3].height;i++)
      |                ~~~~^~~
main.c:35:9: note: while referencing ‘head’
  35 | PPMHead head[3];
      |        ^~~~
main.c:82:20: warning: array subscript 3 is above array bounds of ‘PPMHead[3]’ {aka ‘struct <anonymous>[3]’} [-Warray-bounds]
  82 |    for (j=0;j<head[3].height;j++)
      |                ~~~~^~~
main.c:35:9: note: while referencing ‘head’
  35 | PPMHead head[3];
      |        ^~~~
main.c:84:24: warning: array subscript 3 is above array bounds of ‘PPMHead[3]’ {aka ‘struct <anonymous>[3]’} [-Warray-bounds]
  84 |        for (i=0;i<head[3].width;i++)
      |                    ~~~~^~~
main.c:35:9: note: while referencing ‘head’
  35 | PPMHead head[3];
      |        ^~~~
main.c:78:47: warning: array subscript 3 is above array bounds of ‘PPMHead[3]’ {aka ‘struct <anonymous>[3]’} [-Warray-bounds]
  78 |        EdgeArray[i] = (PPMPixel *)malloc(head[3].width*sizeof(PPMPixel));
      |                                          ~~~~^~~
main.c:35:9: note: while referencing ‘head’
  35 | PPMHead head[3];

---

I think you need to do something like this:

Code:

---

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


typedef struct
{
    int red, green, blue;
} PPMPixel;


typedef struct
{
    int rgb_comp_colour;
    char filetype[3];
    int width,height;
    PPMPixel **data;
} PPMImage;


PPMImage *ppm_create(int height, int width) {
  PPMImage *image;
  // TODO: Allocate storage for the header (PPMImage)
  // TODO: Populate header
  // TODO: Allocate storage for the list of lines (image->height * sizeof(PPMPixel *));
  // TODO: Allocate storage for each line  (image->width * sizeof(PPMPixel))
  // TODO: Zero out the data
  return image; // Return NULL if unable to create image
}


PPMImage *ppm_read(char *fname) {
  PPMImage *image;
  // TODO: Allocate storage for the header
  // TODO: Read the header
  // TODO: Allocate storage for the list of lines
  // TODO: Allocate storage for each line
  // TODO: Read in each line
  return image; // Return NULL if unable to read in image
}


void ppm_free(PPMImage *img) {
  // Free lines
  for(int i = 0; i < img->height; i++)
      free(img->data[i]);


  // Free list of lines
  free(img->data);


  // Free header
  free(img);
}


int ppm_write(PPMImage *img, char *file_name) {
  // TODO: Open the file, return 0 if fails
  // TODO: Write out the header
  // TODO: Write out the pixel data
  return 1; // Success!
}


void ppm_edge_detect(PPMImage *src, PPMImage *dest)
{
  // TODO: process the data in 'src' to generate 'dest'
}


int main(int argc, char *argv[]) {
  PPMImage *src, *dest;


  src  = ppm_read("input.ppm");
  // TODO: need to check src is not NULL


  dest = ppm_create(src->width, src->height);
  // TODO: need to check dest is not NULL


  ppm_edge_detect(src,dest);
  ppm_write(dest, "output.ppm");
  // TODO: Need to check that the write succeeds


  ppm_free(src);
  ppm_free(dest);
  return 0;
}

---

Code:

---

FILE *f = fopen("test.ppm", "r+a+");
if (f == NULL)
{
perror("Unable to open file!");
exit(1);
}
char *line;
size_t len = 0;
int n = 0;
int a, b, c;
int aa, bb, cc;

while (getline(&line, &len, f) != -1) {
  if (n > 3) {
    if (n % 2 == 0) {
      sscanf(line, "%d %d %d", &a, &b, &c);
    } else {
      sscanf(line, "%d %d %d", &aa, &bb, &cc);
    }
    if (aa - a > threshold || bb - b > threshold || cc - c > threshold) {
      coordinates cord = returnarrayindex(n);
      printf("#%d::%d %d %d at x(%d),y(%d)\n", n, aa - a, bb - b, cc - c,
            cord.x, cord.y);
      buf[cord.x][cord.y] = "0 0 0";
    }
  } else if (n > 2) {
    sscanf(line, "%d %d %d", &aa, &bb, &cc);
  }
  n++;
}

fclose(f);
}

---