Procedural terrain using 3D noise

Question

I'm coding a procedural terrain generation based on this article from GPU Gems 1.

But using CPU (not GPU). I'm stuck at generating the procedural terrain. I just can't figure out how make a 3D texture the way it looks in the article (fig. 1-10). Am I missing something big here? Here is the code:

float CTerrain::DensityFunction(float x, float y, float z) {
    CPerlin3DTexture perl_tex;
    float noise;
    uint32_t octaves = 3;
    float amplitude[10] = {0.25,0.5,1.0,1.9,4.1,8.1,16.1,9,10};
    float lacunarity = 1.5;
    float freq[10] = {4.03,1.96,1.01,0.51,0.2487,0.12563,0.8013,0.39913, 0, 0};
    uint32_t i;

noise = 0.0;
    for(i=0; i < octaves; ++i ) {
        noise += perl_tex.noise(x * freq[i],
                                y * freq[i],
                                z * freq[i]) * amplitude[i];
        x *= lacunarity;
        y *= lacunarity;
        z *= lacunarity;
    }

    return noise;
}

Values (nx,ny,nz) = (128,128,10)

float ***CTerrain::GenerateDensityTexture(uint32_t nx, uint32_t ny, uint32_t nz) {
float step[3];
int32_t i, j, k;

step[0] = 1.0 / (float)nx;
step[1] = 1.0 / (float)ny;
step[2] = 1.0 / (float)nz;
for(i = 1; i < nx-1; i++) {
    for(j = 1; j < ny-1; j++) {
        for(k = 1; k < nz-1; k++) {
            tex[i][j][k] = DensityFunction((float)i * step[0], 
                                           (float)j * step[1], 
                                           (float)k * step[2]);
        }
    }
 }

return tex;
}

Help, anybody? This is what I get from the above code:

Answer 1

In the article, the base density function is float density = -ws.y; This means that at high altitudes, the density will definitely >= 0 producing completely air, and at low altitudes density < 1 producing solid land.

From your screenshot, it looks like you just generated a solid block of uniformly 50% dense terrain instead of the gradient the article used.