Calculate the difference (or error if you have ground truth optical flow
at hand) between two optical flow estimates:

	floerr left-file right-file output-file

The input files may either be in the Middleburry .FLO format or the UUlm
.FLOW format (a simple extension to .FLO with a third band of
information per vector, namely the confidence).

The results are written to .ERR files, which are of the format:
	FERRwhmza0a1...an
where FERR is a header string tag, w and h are the width and height,
respectively. m is the mean difference/error, z is the number of values
used during computation of m and a1 to an are the per-position errors.
Storage of a1...an is in row-order. Thus, to get the error at pixel
position (x, y) you need get the (x + y * w)-th a.

Note that the calculation of m contains a possible pitfall:

If the left-file contains flow at a position x,y, corresponding to the
i-then entry of the a vector (->ai) but the right-file does not, the
mean error is increase by PI as this is the maximally possible error. In
addition ai is set to 1e9.

If the left-file contains no flow, but the right-file does, ai is marked
with -1e9. However, the mean error and z are not increased!

This behavious might change in the future.


The file readErr.m contains a matlab function to load .ERR files.