I saw a documentary a few years ago (about 5) demonstrating a photographic manipulation formula, which can reduce the width of an image, without stretching, or cropping. It would do this by calculating the paths of least resistance (low contrast) from the top to the bottom of the image, and removing those lines one at a time until the image had shrunk enough. The effect is that objects with well defined boundaries are left untouched, and larger less well defined objects have strips removed from them.
Does anyone know what this technique is called? Are there any implementations of it?