When watching TV and the face of the person being interviewed is obscured by large pixels of moving colour – to conceal their Identity – I can make out the elements of their face more clearly If I squ

This technique is often used in real-life crime programmes to protect innocent individuals or to avoid suspects receiving unfair trials.

The process of pixilation works as follows: for each square pixel area, the computer measures the brightness and colour of each point and calculates the average. This average value is then assigned to the entire pixel. Thus the properties of the picture to be obscured, in this case a face, are largely retained. However, each square has sharp edges, and since the edges are a product of the pixilation process and not related to the elements of the face, they obscure the picture. The reason the face still seems to move is because although the position of each square will remain constant, the person “behind” the pixilated area will be moving, and therefore the average values for brightness and colour change. Squinting, screwing up your eyes or, even better, removing your spectacles blurs the image and prevents you from seeing the sharp edges of the squares. You are then left only with a face in which changes in brightness and colour have been smoothed by averaging, and which is then much easier to see and identify.

The same effect can be achieved by making the pixilated face smaller, or by moving it further away – the edges become too fine to observe their detail and disappear and the face is more easily seen. This effect was first described by Leon Harmon in 1973 (Scientific American, vol 229, p 70). 

The artist Salvador Dali made use of a sort of a “pixilation” technique in some of his paintings, one of which is pictured aside. These images were unreadable until one moved a certain distance from the picture or if one did some serious squinting. The pixilated painting below illustrates the point. When you view the picture from close quarters you see a woman walking through an opening. From further away, all you can see is the face of Abraham Lincoln, the former president of the U.S. With computers the effect is far more accessible. Simply take a bitmap image of anything, but preferably something small, and zoom in on it until you can see the pixels. The more you zoom in, the less clear the picture becomes, and the more you need to squint to see it.