A pinhole camera projects a point 
from the 3-D projective space onto a point 
of the 2-D projective plane. This projection can be written as a 
homogeneous matrix 
of rank equal to 3 :
where 
is the equality up to a scale factor. If we restrict the 3-D projective space to the Euclidean space, then it is well known that 
can be written as :
and 
are the rotation and translation that link the camera frame to the 3-D Euclidean one. The most general form for the matrix of intrinsic parameters 
is :
where 
is the horizontal scale factor, k is the ratio between the vertical and horizontal scale factors, r is the image skew and 
and 
are the image coordinates of the center of projection.