Great Illusion: Impossible 3D shape conceals six tricks

The spinning 3D shape in this video conceals six different illusions. To see it in 3D, you'll need to cross your eyes until the two images overlap and merge together. (If you're having trouble, try viewing the video in full screen or check out some tips here). Once the 3D rhombus appears, you should be able to perceive the following effects:

1. The spinning shape appears to switch directions
2. The striped band appears to spin in the opposite direction to the shape
3. The red cube seems to float in space
4. The red cube becomes transparent
5. The red cube rotates impossibly
6. The light falling on the shape appears to come from a source opposite its real source

The object in the video was created by Rex Young, an enthusiast with an illusions channel on YouTube, based on parts and instructions provided by artist Terry Pope. It was originally conceived to be viewed with a pseudoscope, an optical device that switches what the eyes are seeing using mirrors. But, by filming the structure with a stereoscopic camera and reversing the left and right frames, the same illusion can be seen just by crossing your eyes.

So why do we perceive this brain trick? When we view a scene, the image that appears on our retina is two-dimensional, so our visual system uses a variety of cues to add depth. One of these involves comparing the position of images on the left and right retinas to determine distance. Since the images in this video have been flipped, it reverses our distance cues, causing far away points to seem closer than nearer ones and altering our perception in a variety of ways.