bool TestPointVisiblity(vect3d v)

It translate this point in world space to screen point, and it check the depth buffer on translated point. After it you need reset the AlphaMode, or NoAlphaMode.

                Result:

                True: If the point is visiblity from screen (The depth was larger than the depth of point)

                False: If the point is not visiblity from screen (The depth of point was cover some other points, anyway it is under the screen)