Rotating landscapes

This is not some obscure Physics concept I am trying to introduce here. This blog post deals with the very familiar phenomenon of landscapes giving an illusion of rotation when we are travelling and the implications of it on 3d vision and future video systems.

Consider two poles to your right, one close to you and the other far away. Now, start walking in a line perpendicular to the line segment between the bases of the two poles. As you walk a certain distance, you will see that the angle with which you have to rotate the head to keep seeing the poles is different. The angle changed more for the closer pole than for the farther pole. This is a simple geometric concept.

Now consider poles which are placed close to each other between the two original poles. The closest pole travelled the greatest angle, the next pole a little less and the farthest the least. When you consider this kind of a motion where you see different distances travelling by different angles, our brain approximates it to a circle. This is what shows you rotating landscapes while travelling. This illusion becomes all the more apparent when the brain is clearly able to differentiate between the differences in angles made by different points. In other words, the effect is more noticeable when we have the farthest point at a very long distance and the speed of the closest point is high, as in the case of train travel. But, this circular motion never feels perfect because the farthest pole that we are focusing on is not really stationary, but moving. You can observe a perfect circular motion if you can focus on infinity and observe everything else out of the corner of your eye.

It now makes sense to move on to a useful related concept, the illusion of distant objects like the moon appearing to move with us when we are moving. This is just an extension of the rotating landscapes concept we just discussed. Consider the moon to the father pole or a point. If you look at the moon while moving, closer points like tress and building and people (if you are a kid) start making larger angles than the moon and hence move back. But, why did the moon not move back with a small angle? The answer is that, it did and the brain did not recognize it that way. We can observe the moon moving back only when we can compare it with a reference which is farther away from the moon. Our eyes have a problem guaging accurately such large distances and hence cannot locate reliably a reference point among the stars. The only reference points we are now left with are objects on the earth. These references make much larger angles than the moon, hence the objects move back with respect to the stationary moon or in other words, the moon moves forward with respect to those objects. This motion is same as the motion of our body, creating the illusion that the moon moves with us.

One aspect we touched upon while discussing about far away reference points is that the eyes cannot gauge very large distances accurately. To understand why, we need to understand how our eyes, more accurately the brain measures distances. As we all know, the brain makes use of images from two eyes which are slightly separated in space. The object which the two eyes perceive is different because the two eyes are spaced apart. The brain with its own algorithm makes use of the difference in these images to create a 3-dimensional view of the world. At a sufficiently large distance, the images in the two eyes are not different enough for the brain to calculate its distance. This is why, while a you can see the curvature of a marble which is close by, you see the moon as a flat surface. Seeing how we do not see far away objects in 3D, we would not know which star is a reliable farther reference to the moon because of our inability to gauge the distance of either. While, it is still possible to see that the moon is closer to us than the stars, and even see that some stars are closer to us than the others, the overwhelming impact of the close references cannot by overridden by these weak references.

The implication of it all is that we see only what our brain shows us. You see rotation where there is none and you see 2D where there is 3D all because of the brain’s algorithm in calculating 3D images and using angles to calculate distances. This could probably be an indication to the way of achieving 3d displays.