Distance between points on a face Given a 2D picture of a face, how is it possible to measure the distance between two different points on the surface of the face?
Thanks
Joel
 A: First you want to build a 3d-mesh out of your face. To get a general idea of this, the following paper might be interesting for you: http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.58.8435&rep=rep1&type=pdf . Of course you can improve this algorithm with knowledge, that you already have about the appearance of human faces.
Then you select your points in the mesh and run a path-finding algorithm to get a good approximation for the distance. When you did everything properly you will get something like this (note that the red line should mark shortest path):

I hope this is what you wanted
A: Use a ruler. If your needs are more specific, perhaps you could explain them?
A: 1) Determine the longitude and lattitude of the points. (Using polar co-ordinates)
2) http://en.wikipedia.org/wiki/Great-circle_distance
A: A 2D image of a face is a certain projection of a three-dimensional object into the plane. The pre-image of any point is a line, whence a lot of information gets lost in the process. It follows that it is ${\it impossible}$ to tell the true distance between two original points by measurements in the 2D image, even if you are able to identify the individual image points with their corresponding points on the face.
