title: See how far sheet of paper moves using webcam
body: <p>I have a webcam mounted ~12 inches off a table facing down.  I have a sheet of paper under it that can move in any direction, but only in a 2D plan on the table.  I want to use the webcam to figure out in which direction the sheet of paper is moving.  Is there an algorithm to do this?  What is it called?</p>