The other day I overheard a discussion between me and myself about pigeons. It was more about what their eyes and brains see when they walk back and forth picking up breadcrumbs that little kids and their granddads throw on the ground in front of a park bench. We have all seen it, haven´t we?
It´s like a discussion in stereo, you and yourself take a slightly different angle on a subject and discuss it vigorously in your head. The result is a confusing, but a slightly more complete picture of the matter. After a while you might get to agree with yourself that both of you are actually seeing the same thing and the two points of view improved both´s understanding.
The discussion went something like this.
– Why do pigeons move their head back and forth?
– Obvious, it helps them to balance their walk.
– Well, ducks don´t do that and they seem to move even more unevenly. I think they might improve their vision by doing that. Maybe they can see in stereo using the parallax from the motion. Similar to the 3D impression one gets from clever camera motion. If the field of view from your eyes does not overlap significantly, as is the case for pigeons, that might be the way to go to get depth information from your environment.
– Could be, but well, it´s pure speculation.
– But one could easily imagine that the brain puts together an image from before and after the fast head movement to generate depth information in a similar way we see depth from the simultaneous images that come from our eyes.
– We will never know for sure.
– There you could be right. Too bad really.
– Conclusion: From just looking at the pigeons we actually don´t know much more about their vision, but learnt that there are several possibilities to consider.
Once hooked on the topic, with a bit of literature search it was easy to find out that there has been quite a bit of scientific research on the topic – turns out that the idea of pigeon stereo vision wasn´t that silly after all (see for example Necker 2007). Pigeons, and a few other bird species, actually keep their head stationary with respect to the ground for a moment and then quickly move it forward. Just like from my own little discussion, scientific research has not reached a consensus on the subject. Both possibilities raised above, balance and stereo vision, are the most discussed reasons for the behavior.
Whatever is the case for the birds, the idea of using delayed images from a single camera is, of course, not new. The following link takes you to a stereo movie that we obtained by rotating the camera horizontally around a model nebula and delaying the frames on the right by several frames. If you know how to look at such double images to see a single stereo image in the middle, you will be able to appreciate the 3D structure directly and not only from the changing perspective of the camera with time. Unfortunately rotating the camera around the object is not the best way to obtain stereo images, since at the edges the parallax effect is not as strong as it is in “proper” stereo viewing with parallel cameras that are at a certain distance from one another.