Experiment
Find the Spot
Someone has painted a pit and a road barrier on a flat sidewalk. You are standing on the spot the artist marked for you. Take a look, then start walking.
You are standing on the spot.
You can also drag across the picture to walk.
From above: what is actually on the ground
What you just saw
From the marked spot, the painting and a real hole send the same pattern of light to your eye. That is the whole trick of anamorphosis, the same one Hans Holbein used for the stretched skull in The Ambassadors in 1533: the artist works out what a real object would look like from one point, and paints exactly that on a flat surface.
Step off the spot and the two come apart. With a real hole, the inside shifts against the rim as you move: you see more of one wall and less of another, and the floor slides behind the near edge. The painted pit cannot do that. It is glued to the sidewalk, so it tilts and smears along with the pavement, and the barrier, which seemed to stand up, stretches out flat across the ground. The view from above shows what was there all along.
The psychologist James J. Gibson argued that normal seeing happens on the move. The way surfaces shift against one another as you walk tells you which are near, which are far, and which are flat. A painting can match a real scene from one point. It cannot match it along a path, so a walker gets the full illusion for only a step or two.
The essay Stepping Around a Painted Hole asks why people still step around these paintings, when they know the ground is flat and the illusion breaks so quickly.
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