Kenny’s 2nd Law of Vision
7 February 2009
My 2nd Law of Vision is called Kenny’s Rule of Fist. It’s a pun on a rule of thumb, as each of my Laws of Vision really are. The idea behind the Rule of Fist is that vision systems can get difficult or complicated when the object being analyzed is larger than your fist. Again, as with the 3rd law, it means you may have to make special choices, play tricks, purchase expensive hardware, etc…
Why a fist?
Well you don’t want to cut off your thumb!
It’s because your fist is pretty big and the resolution in the standard camera is not that great, so as objects get bigger you can not evaluate things that are significant relative to human vision. When you want to look at things larger than your fist you will be looking at courser details or measurements.. People, or worse customers, ask “Why can’t it see that, it’s obviously wrong!”.
Objects being big also bring into play and can create parallax problems of z-order measurements as you have to move the camera toward and away from objects. In addition, they can also create distortion problems of the lens if you try to use wide angle lenses. One tool to help solve z-order parallax is a telecentric lens which is a really cool thing, but you have to get a really big and really expensive lens if the object is as big or bigger than your fist.
Objects larger than your fist can be problematic for machine vision system design. So always consider my 2nd law or the Rule of Fist.