Custom importance sampling question

[sabotage]

Hello guys I am trying to implement somekind of structured or wavelet importance sampling. For a start I tried using raydirections with gather function in renderman pro server 14.5 . Anyway the results are always grainy no matter how much samples I use. Can someone help or suggest a solution for an importance sampling via gather if it is possible.

Here is the function I tried first.

computeIsotropicWardDirs(float samples, alpha;
                         vector I; normal Ns; vector dPdu, dPdv;
                       output vector raydirs[1024]; ) // results
{

    vector ndir = vector(Ns), udir = normalize(dPdu), vdir;
    vector In = normalize(I), IinPlane, hdir;
    uniform float s;
    float delta, phi, r; // (delta is angle from the "up" axis)
    float u, v, n;

    // Create an orthonormal basis
    udir = udir - (Ns.udir) * Ns; // project udir onto plane perp. to normal
    normalize(udir);
    vdir = Ns ^ udir;

    // Compute directions
    for (s = 0; s < samples; s += 1) {
        // Compute (delta,phi) specifying the half vector h
        delta = atan(alpha * sqrt(-log(random())));
        phi = 2 * 3.141592 * random();

        // Compute half vector h
        n = cos(delta);
        r = sin(delta);
        u = r * cos(phi);
        v = r * sin(phi);
        hdir = u * udir + v * vdir + n * ndir;

    // Convert to a reflection direction
        raydirs[s] = reflect(In, hdir);
    }
}