In this section a geometric interpretation of a camera projection matrix is given. It is seen that constraints on the internal camera parameters can easily be given a geometric interpretation in space.
A camera projection plane defines a set of three planes. The first one is parallel to the image and goes through the center of projection. This plane can be obtained by back-projecting the line at infinity of the image (i.e. ). The two others respectively correspond to the back-projection of the image - and -axis (i.e. and resp.). A line can be back-projected through equation (3.9):
The AC is also possible to give a geometric interpretation to a known focal length or aspect ratio. Put a plane parallel with the image at distance from the center of projection (i.e. in camera centered coordinates). In this case a horizontal motion in the image of pixels will move the intersection point of the line of sight over a distance . In other words a known focal length is equivalent to knowing that the length of two (typically orthogonal) vectors are equal. If the aspect ratio is defined as the ratio between the horizontal and vertical sides of a pixel (which makes it independent of ), a similar interpretation is possible.