The reprojection error is a geometric error corresponding to the image distance
between a projected point and a measured one. It is used to quantify how closely an
estimate of a 3D point recreates the point's true projection
. More precisely, let be the
projection matrix

Camera matrix

In computer vision a camera matrix or projection matrix is a 3 \times 4 matrix which describes the mapping of a pinhole camera from 3D points in the world to 2D points in an image....

 of a camera

Pinhole camera

A pinhole camera is a simple camera without a lens and with a single small aperture – effectively a light-proof box with a small hole in one side. Light from a scene passes through this single point and projects an inverted image on the opposite side of the box...

and
be the image projection of , i.e.
.
The reprojection error of is given by
, where
denotes the Euclidean distance between the image points represented by vectors
and .