I am watching this video, in which the epipolar constraint is defined as
$x_l \cdot (x_l \times t)=0$
It means that the vector $x_l$ that passes through the observed point and the left camera origin is perpendicular to the normal of the epipolar plane, defined by that same vector, and the epipolar line.
What I fail to understand is, why is this a constraint at all?
Doesn't it hold for ANY two vectors $v$ and $w$ that $v \cdot (v \times w) = 0$?