Perlin noise - What am I doing wrong?

Question

I just started with world generation and I've been looking for tutorials on perlin noise everywhere but sadly there are not alot to be found on Google. The last days I followed a tutorial but I can't get my code to work.

Here is my Java method.

private static double[][] createNoise(int xn, int yn, int sps) {
    int m = yn * sps;
    int n = xn * sps;
    double[][] u = new double[yn + 1][];
    double[][] v = new double[yn + 1][];

    double[][] x = new double[m][];
    double[][] y = new double[m][];
    double[][] z = new double[m][];

    for (int i = 0; i < m; i++) {
        x[i] = new double[n];
        y[i] = new double[n];
        z[i] = new double[n];
    }
    for (int i = 0; i < yn + 1; i++) {
        u[i] = new double[xn + 1];
        v[i] = new double[xn + 1];
    }

    for (int i = 0; i < xn + 1; i++) {
        for (int j = 0; j < yn + 1; j++) {
            u[i][j] = nextRandom();
            v[i][j] = nextRandom();
        }
    }
    double hx = xn / (n - 1);
    double hy = yn / (m - 1);
    for (int i = 0; i < m; i++) {
        for (int j = 0; j < n; j++) {
            x[i][j] = hx * j;
            y[i][j] = hy * i;
        }
    }

    for (int i = 0; i < m; i++) {
        for (int j = 0; j < n; j++) {
            int xc = (int)x[i][j];
            int yc = (int)y[i][j];

            if (x[i][j] % 1 == 0 && x[i][j] != 0 ) xc = xc - 1;
            if (y[i][j] % 1 == 0 && y[i][j] != 0 ) yc = yc - 1;

            double xr = x[i][j] - xc;
            double yr = y[i][j] - yc;
            double s11[] = {-xr, -yr};
            double s21[] = {-xr, 1 - yr};
            double s22[] = {1 - xr, 1 - yr};
            double s12[] = {1 - xr, -yr};

            double q11 = s11[0] * u[yc][xc] + s11[1] * v[yc][xc];
            double q21 = s21[0] * u[yc + 1][xc] + s21[1] * v[yc + 1][xc];
            double q22 = s22[0] * u[yc + 1][xc + 1] + s22[1] * v[yc + 1][xc + 1];
            double q12 = s12[0] * u[yc][xc + 1] + s12[1] * v[yc][xc + 1];

            z[i][j] = lerp(x[i][j], y[i][j], xc, xc + 1, yc, yc + 1, q11, q12, q21, q22);
        }
    }

    return z;
}

The heightmap that the method returns sadly enough looks like this

As you can see, the first row/column is working, but after that the algorithm seems to fail. I made sure that the method

nextRandom();

returns a float value between -1 & 1.

Thanks alot!

Answer 1

Thanks to user @gawi for pointing this out.

hx = xn / (n - 1);

will divide 2 integers, and most likely return 0. You can fix this by casting a double to it:

double hx = (double)xn / (n - 1);

Then the map just works!