Figuring out what is popping up at you and what is popping down and away from you in an image can be very difficult. Our brain just doesn’t have enough information to do this right a lot of the time. So, it guesses. And it uses shadows to help make those guesses. But sometimes, it still guesses wrong. Very wrong.

Take a look at these two images. They are the same image, but one has been altered so that the light and shadows have been inverted. What do they look like to you? Do you see hills in one and craters in the other? Which one is which for you? Now, think about which direction the sun is shining from in each image. Is it the same in both or different?

Image credit: NASA/GSFC/Arizona State University (I. Antonenko additional editing)

What’s happening here is an optical illusion. Your brain interprets the relief of the features based on the way the sun is shining on them, that is the illumination. In the left image, the sun is shining from the top left of the image. For most people, the features here will look like craters, depressions in the surface. In the right image, the illumination is from the bottom right of the image.  Knowing that, you should be able to reason out that these are craters too. The light is shining over the holes of the craters, illuminating their insides on the far left and casting shadows on the right. But, for most people, this just doesn’t happen. No matter how hard they try, the right image looks like a bunch of pimples and hills to them.

What is going on? Why is our brain not using the information we give it, when we tell it the sun is coming from the bottom right? It appears that our brains have been trained to prefer illumination from a top direction, and more specifically, upper left for many people. Even if we tell our brain the illumination is coming from another direction, it sometimes continues to interpret things using its preference.  It’s quite strange, because in real life, the sun doesn’t usually come at you from a particular direction, unless you specifically orient your body so that it does. And even if you did, you would have to keep adjusting your position during the day, as the sun moved across the sky, to keep it that way. I can’t say I know too many people who do that. Ever.

So, why would our brain have gotten used to objects being illuminated from the top of an image? We think it is a product of ergonomics and culture. Most people are right handed, so when we write or draw we tend to place desk lighting in front of us, so as not to cast shadows over our work. In addition, most writing in western culture tends to move from left to right, and top to bottom, so as to minimize the possibility of smearing (for a right handed person, at least). Again, placing our desk lighting to the left helps to eliminate shadows on our work. So, most people have trained their brains to prefer illumination from the upper left by years of desk work!

Recognizing that illumination affects whether we see hills popping up at us or craters popping away, Moon Mappers has added a little sun symbol to the task interface to tell you which direction the illumination is coming from.

Image credit: CosmoQuest Moon Mappers

Since this direction will not always be optimal for every person, you may find yourself looking at an image where the illumination direction does not work for you. In preparation for these occasions, Stuart has written instructions on how to adjust your computer monitor so that the images will pop correctly for you.

For more information on the effects of lighting, check out our Moon Mappers: Lighting Effects Guide