The other day, after years of working with 3D images, I was surprised by a very basic phenomenon related to anaglyph images. I was walking through the ‘New Perspective on Mars’ exhibition with my red-and-blue glasses on, in search of a picture where adding the 3rd dimension to a flat image really makes a difference. What I found was the image of a slope on Olympus Mons that looks very boring in 2D, but a lot more interesting in 3D.
Most of the image is taken up by a gradually steepening slope, which is very hard to interpret in 2D. In most other images in the exhibition, shadows cast by the landscape provide some topographic clues even in 2D, but here we really need the stereoscopic depth to tell how steep the slope is at any one point. Or could you tell from the image above (without anaglyph glasses) which point is higher, A or B?
But here’s where I was struck by a phenomenon that I was familiar with in the horizontal, but not in the vertical direction: When I walked closer to the large scale image in the exhibition, the slope became steeper! If you are reading this on a desktop screen, you can simulate the effect by standing up, or stretching your neck and moving closer to the screen (which is a good loosening exercise, by the way…). On a laptop or tablet, simply tilt the screen up and down (with less exercise value). I was aware that 3D scenes move when I move in front of them, but since I usually sit in front of a screen image, I had never experienced a vertical movement.
I recommend you play around with this effect – if you have a pair of red-and-blue glasses, that is. The phenomenon is most striking at the “walk-in” scale, like in the exhibition at the Science Centre, but you can observe it on-screen too. Can you make the slope an overhang?
The effect is of course due to simple geometry (working that out is a nice mental loosening exercise…). It is a good demonstration of the limitations of stereoscopic 3D displays, from anaglyphic images and autostereograms to 3D movies: they only provide a true representation of a 3D scene when viewed from one position in front of the image. Move away from that point, and the scene gets distorted. This is different from true 3D displays, such as holograms, which allow the viewer to inspect a scene from different angles.