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Focal length and size of the image are referred to as intrinsic pa-
rameters of the camera, since they depend only on the specific camera
and on nothing else. In the general case, coordinates of scene points
are defined in respect to a world coordinate system, which is different
from the camera system. In this case, it is necessary to convert the
coordinates to their expression in the camera system before calculating
the projection on the image plane. Therefore, a set of extrinsic param-
eters (position and orientation of the camera in respect to the world
system) are needed to obtain image coordinates. Given the extrinsic
parameters, it is possible to determine the transformation matrix be-
tween world and camera systems, thus the function to map points of
the world system to the camera system.
The pinhole model does not consider camera distortions, thus it
accurately correspond to reality only in those cases where distortions
are neglectable (typically, in good quality cameras or in the central zone
of the image). Several camera models exist which include distortion
factors in the projection function. For example, the model used in
Heikkila and Silven [41] differs from the pinhole model in two aspects:
• the position of the principal point can be different from the center
of the image;
• radial and tangential distortion (as defined in [42]) are present,
and apply a non-linear transformation to final projection coordi-
nates.
Other models [43] consider the possibility for the camera axes to be non-
orthogonal; others [44, 45] introduce a prism distortion due to camera
imperfect manufacturing.
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