Question

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?

Answer 1

I suggest optical flow. From Wikipedia:

Optical flow is the pattern of apparent motion of objects, surfaces, and edges in a visual scene caused by the relative motion between an observer (an eye or a camera) and the scene

Or, to quote a presentation from the Stanford Artificial Intelligence Lab:

Given a set of points in an image, find those same points in another image.

It means you can compute the displacement of a set of points belonging to the object you want to track from one image to another -> resulting in a set of vectors describing the direction of your object.

1. Find good image features to track using cvGoodFeaturesToTrack() — you should get good results as long as your sheet of paper is distinct from the table
2. Find its corners using cvFindCornerSubPix() and
3. Compute the optical flow using cvCalcOpticalFlowPyrLK() — “LK” means “Lucas-Kanade”, the name of the algorithm

See OpenCV’s Motion Analysis and Object Tracking documentation for details.