21 October 2005

Relief reversal effect

The picture shows paw prints of possibly a dog and a racoon. When the picture is rotated around its horizontal axis, the impressions in the original now appear as if they were reliefs on the surface of the sand ("negative" relief becomes "positive") and vice versa.

The reason for the apparent reversal of relief is explained at this site: "If a feature has positive relief the shadow will fall on the far side (away from the sun). If the feature has negative relief the shadow will fall on the near side (toward the sun)."

This is a well-known effect with practical uses. For example, Blaker1 explained, long before digital photography, how one can create a photograph of the inscription (positive relief) on a seal (when the seal itself is missing) by photographing its impression (negative relief) and then by reversing the film during printing.

1. Blaker, A.A. 1977. Handbook for Scientific Photography (pp. 223-224). W.H. Freeman & Co.


T. Beth said...

Very strange! The second photo does look like a positive relief after you look at the first photo.

Pamela Martin said...

I've often wondered about this phenomenon--aggravating when I'm playing around with a track image. Good to know what causes it--Thanks!