[evaal contest] computing accuracy

[Francesco Potortì]

As far as the paths with tracking are concerned, accuracy is computed
based on the error.  The error is the Euclidean distance from the
coordinates sent by the competing system and the real position.  The
competing systems sends a <t,x,y> datum.  The estimated position at time
t is (x,y).  The real position at time t is the linear interpolation
between the positions at integer numbers of seconds T just before and
just after t.  The real position at the integer number of seconds T is
the mean between the position of the two feet of the actor at time T.

As far as accuracy for areas of interest are concerned, accuracy is
based on a score.  For each sample sent by the competing system, the
score may be 0, 1/2 or 1, according to what is defined in the
refinement of criteria on the web site.

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