void Frustum::genProjectionMatrix(double near, double far, double aspect,
        double fov)
{
    projectionMatrix->zero();
    float v1, v2, v3, v4;
    float fovR = fov * M_PI / 180;

    v1 = 1 / (aspect*tan(fovR/2));
    v2 = 1 / tan(fovR/2);
    v3 = (near+far) / (near-far);
    v4 = (2*near*far) / (near-far);

    projectionMatrix->set(0, 0, v1);
    projectionMatrix->set(1, 1, v2);
    projectionMatrix->set(2, 2, v3);
    projectionMatrix->set(2, 3, v4);
    projectionMatrix->set(3, 2, -1);
}