Sobel Operator C - Edge detection gone wrong - c

Question

I ve been trying hard to calculate the gradient (and dips) of a binary image in C using the Sobel operator. I ve already checked the operators several times and went through tons of internet sites. Nevertheless, I have to admit that I have no experience with image processing and I am quite a rookie in C coding. I dont get any error messages, but the result does not show the desired gradients on the edges.
Somehow the gradient in the x-direction is not calculated - but why?
Thanks for the help!
#include <stdio.h>
#include <math.h>
#include <stdlib.h>
FILE *infile, *outfile;
int main(int argc, char *argv[])
{
int nx,nz,k,i,nu;
int j,m,l;
int km, kp, im, ip, kpp, ipp;
float Gx, Gz, G, Gmax, Gmin;
float T;
float **dip;
float pi;
float *tmp;
float *bufz;
float **temp1;
char *velfile_in = "vel";//"vel_315_273";
char *velfile_out = "dip";
void *alloc1 (size_t n1, size_t size);
void **alloc2(size_t n1, size_t n2, size_t size);
void ***alloc3(size_t n1, size_t n2, size_t n3, size_t size);
pi = 4. * atan(1.);
// Initiate constants
T = atof(argv[1]);
nx = atoi(argv[2]);
nz = atoi(argv[3]);
Gmax = 0.;
Gmin = 10e8;
// border handling (cyclic)
km = (k+nz-1) % nz;
kp = (k+1) % nz;
kpp = (k+2) % nz;
im = (i+nx-1) % nx;
ip = (i+1) % nx;
ipp = (i+2) % nx;
// allocate 1D, 2D and 3D arrays
tmp = (float *)alloc1(nz,sizeof(float));
bufz = (float *)alloc1(nz,sizeof(float));
temp1 = (float **)alloc2(nx,nz,sizeof(float));
dip = (float **)alloc2(nx,nz,sizeof(float));
//READ FILE
//***********************************************************
infile = fopen(velfile_in, "r");
if (infile == NULL) err("Error: could not open file.");
for (i=0; i<nx; i++) {
nu = fread(tmp,sizeof(float),nz,infile);
for (k=0; k<nz; k++) {
temp1[k][i] = tmp[k];
}
}
fclose(infile);
// APPLY SOBEL****************************************
for (i = 0; i < nx; i++)
{
for (k = 0; k < nz; k++)
{
Gx = (temp1[km][im] - temp1[km][ip] + 2 * temp1[k][im] - 2 * temp1[k][ip] + temp1[kp][im] - temp1[kp][ip]);
Gz = (temp1[km][im] - temp1[kp][im] + 2 * temp1[km][i] - 2 * temp1[kp][i] + temp1[km][ip] - temp1[kp][ip]);
G = sqrtf(Gx * Gx + Gz * Gz);
Gmax = (Gmax > G ? Gmax : G);
Gmin = (Gmin < G ? Gmin : G);
dip[k][i] = abs(atan(Gz/Gx) * 180. / pi);
printf("(%d,%d)\tGx:%5.3f\tGz%5.3f\tG%5.3f\n",i,k,Gx,Gz,G);
}
}
printf("Gmax:%5.3f\tGmin:%5.3f\n",Gmax,Gmin);
// write file ********************************************************
outfile = fopen(velfile_out,"w");
for (i=0; i<nx; i++) {
for (k=0; k<nz; k++) bufz[k] = dip[k][i];
fwrite(bufz,sizeof(float),nz,outfile);
}
fclose(outfile);
return 0;
}

Answer 1

The obvious problem is that your convolution kernel isn't moving - the array indices are all separate variables and need updating to stay in sync with i and j. Moving the assignments inside the loops should fix that:
...
for (i = 0; i < nx; i++)
{
im = (i+nx-1) % nx;
ip = (i+1) % nx;
ipp = (i+2) % nx;
for (k = 0; k < nz; k++)
{
km = (k+nz-1) % nz;
kp = (k+1) % nz;
kpp = (k+2) % nz;
...