Project the velocity field to be approximately mass-conserving,
       using a few iterations of Gauss-Seidel.

def project(vx, vy, occlusion):
    """Project the velocity field to be approximately mass-conserving,
       using a few iterations of Gauss-Seidel."""
    p = np.zeros(vx.shape)
    div = -0.5 * (np.roll(vx, -1, axis=1) - np.roll(vx, 1, axis=1)
                + np.roll(vy, -1, axis=0) - np.roll(vy, 1, axis=0))
    div = make_continuous(div, occlusion)

    for k in range(50):
        p = (div + np.roll(p, 1, axis=1) + np.roll(p, -1, axis=1)
                 + np.roll(p, 1, axis=0) + np.roll(p, -1, axis=0))/4.0
        p = make_continuous(p, occlusion)

    vx = vx - 0.5*(np.roll(p, -1, axis=1) - np.roll(p, 1, axis=1))
    vy = vy - 0.5*(np.roll(p, -1, axis=0) - np.roll(p, 1, axis=0))

    vx = occlude(vx, occlusion)
    vy = occlude(vy, occlusion)
    return vx, vy