for(int y = 0; y < HEIGHT; y++) {
    for(int x = 0; x < WIDTH; x++) {
        float fx = (1.0f / (float)WIDTH); // multiply fx, fy
        float fy = (1.0f / (float)HEIGHT); // to scale perlin sampling.
        float height = perlin_2d(fx, fy); // maybe add seed somewhere.

        int offset = x + (y * WIDTH);
        verts[offset].x = (float)x;
        verts[offset].y = height;
        verts[offset].z = (float)y;
    }
}