Oblique near clipping plane

void CameraMatrix::set_oblique_perspective(real_t p_fovy_degrees, real_t p_aspect, real_t p_z_near, real_t p_z_far, Quat oblique_near_plane, bool p_flip_fov) {
    if (p_flip_fov) {
        p_fovy_degrees = get_fovy(p_fovy_degrees, 1.0 / p_aspect);
    }

    real_t sine, cotangent, deltaZ;
    real_t radians = p_fovy_degrees / 2.0 * Math_PI / 180.0;

    deltaZ = p_z_far - p_z_near;
    sine = Math::sin(radians);

    if ((deltaZ == 0) || (sine == 0) || (p_aspect == 0)) {
        return;
    }
    cotangent = Math::cos(radians) / sine;

    set_identity();

    matrix[0][0] = cotangent / p_aspect;
    matrix[1][1] = cotangent;
    matrix[2][2] = -(p_z_far + p_z_near) / deltaZ;
    matrix[2][3] = -1;
    matrix[3][2] = -2 * p_z_near * p_z_far / deltaZ;
    matrix[3][3] = 0;

    // Here goes oblique magic!
    Quat q;
    q.x = (sign(oblique_near_plane.x) + matrix[2][0]) / matrix[0][0];
    q.y = (sign(oblique_near_plane.y) + matrix[2][1]) / matrix[1][1];
    q.z = -1.0F;
    q.w = (1.0F + matrix[2][2]) / matrix[3][2];
    Quat c = oblique_near_plane * (2.0F / oblique_near_plane.dot(q));
    matrix[0][2] = c.x;
    matrix[1][2] = c.y;
    matrix[2][2] = c.z + 1.0F;
    matrix[3][2] = c.w;
}