static float a = 0, da = 0.01;
    matrix3D_t tilting = rotationXMTX(0.5) * rotationYMTX(a);

    point3D_t bg1[4] = {
        {0,0,0}, {100,0,0}, {100,100,0},{0,100,0}
    };
    point2D_t pbg1[4];
    vector3D_t vec;
    for(int i = 0 ; i < 4 ; i++) {
        vec = Point2Vector(bg1[i]);
        vec = tilting * vec;
        pbg1[i] = Vector2Point2D(vec);
    }
    drawPolygon(pbg1,4);
    a+=da;