This overlay of the painting shows our Necker cube in View A or the red-is-in-front view. We are looking up at the bottom of the cube and the foreground in the painting now shifts so the point B is closer to you than point A.The area in light blue follows a plane that comes out in space in front of the picture plane (the rectangle that defines the picture). The front figure now leans forward into the space and one feels she is no longer looking up at the other figure but in front and into a more forward space.
And this is why the right arm of the front figure was so difficult to judge - it depended on how I was reading it!
