What do you see: convex “eggs” that “stick out” of the screen or concave “egg crates”? Click on them to flip them around!

Why do I see 3D shapes on my 2D screen?

We live in a 3D world but your visual system has access only to the 2D images on your retina. Yes, you have two eyes and because images that fall on them are slightly different you can see in “stereo”. However, stereoscopic depth alone cannot help you if an object is further than a few meters away or if you can see it clearly only with one eye. Which is why your visual system uses a variety of tricks to recover the missing 3D information. (Close one of your eyes to see that the world does not suddenly become flat in the absence of the stereoscopic depth.) For example, it may look at how the object moves. Or it can look at a characteristic pattern of bright and shaded regions to infer its 3D structure and the location of the light source.

Why do I see either an “egg” or an “egg crate”?

The shading pattern defines the shape of an object but cannot tell you whether it is convex or concave. For this, you need to make assumptions about the location of the light source. Typically, your visual system assumes that the light comes from above (sun). If you examine the picture, you will notice that “eggs” have bright parts on top, whereas “egg crates” - at the bottom.