Matrix dot product: is the multiplication of the corresponding
elements of the matrix
Matrix cross product: Matrix multiplication is the Mth row of matrix
A multiplied by the Nth column of matrix B. Each element is multiplied
and summed as the values of the Mth row and column.
Why is it common to see the image plane in front of the camera's
optical center: Generally, in order to simplify the model, we uniformly
symmetric the imaging plane to the front of the optical center of the
camera, so that the imaging plane and three-dimensional space points are
placed on the same side of the camera coordinate system, which is called
virtual image.