This is a necker cube, the image was first published in 1832 by Swiss scientist Louis Albert Necker. Which plane is in front A (red) or B (blue)? If A is in front, (closer to you) then we are looking at the underside of the cube if B is in front then we see the top plane of our cube. Since your perception of 3D is constructed in your brain you can see both possibilities interchangeably.
Eventually the Necker cube helped me make sense of the strange experiences I had while copying the Boucher at the museum. My perception of the length of the front figure's arm kept changing, it would feel right one day and wrong the next. I didn't solve that mystery at the time but I did start to see that there were strong horizontal and vertical connections in the picture. I explored these ideas some years later working from a B&W reproduction. The drawing, (#3) and the extended Necker cube (#4) tell that story.(-> #3)
