As per optical-flow, why is there a heavy motion at image margins?

Question

See some examples:

(1)

(2)

I am curious about, why is there a high magnitude area at the margins of the image? (Like the blue area for the first frame, and red area for the second)

Also, I am seeing a similar trend around a stationary objects in the frame. (As for example, see the heading "RACE 9" in the first image...and look at the high magnitude area around it)

Even though they are not moving very fast, why is this trend coming up? Is it because the pixels are considered to have moved out of the frame, and thus Opt-Flow assumes high motion, corresponding to them? Any possible explanation will be highly appreciated.

Answer 1

What you see in the second picture is a scissor problem. Look at the white triangle at the bottom, which is just a part of the white strip. Imagine that on the next frame the camera will move up just a bit. Because the white stip is almost parallel to the edge of the frame, the point of intersection of the white strip with the bottom edge of the frame will move a long distance to the right. It's similar to scissors, where the point of intersection of blades moves very fast if the angle between the blades is small. So if you just try to follow the motion of the bottom white triangle on your frame you'll see that it's sliding to the right at high speed as the camera moves up.

This type of problems appear near the edge of the screen and along straight edges in a picture in general. For example, you can shift stripes of the american flag to the right or left without significant change of a picture. It means that insignificant changes of the flag picture can be as well interpreted as large shifts to the left or right.