= Epipolar line
An <epipolar line> is the image-plane trace of an <epipolar plane>. Given one point in the first <camera> image, the partner of that scene point must lie on the corresponding <epipolar line> in the other view. A <fundamental matrix (computer vision)> maps the first point to coefficients of that line.<Epipolar Geometry in the OpenCV-Python tutorials>[Basic Concepts, point-to-line mapping and computeCorrespondEpilines example]{ref}
For example, let $q_L=(100,50,1)^{\mathsf T}$ and
$$
F=\begin{pmatrix}0&0&0\\0&0&-1\\0&1&0\end{pmatrix}.
$$
Then $Fq_L=(0,-1,50)^{\mathsf T}$, so a candidate $q_R=(u,v,1)^{\mathsf T}$ satisfies $-v+50=0$. The partner can have different horizontal coordinate $u$, but must lie on row 50 in this ideal model. Intersecting candidate matches with this line reduces the search; it does not resolve repeated textures or guarantee that a visible partner exists.
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