When processing DHP (Digital Hemispherical Images), CAN-EYE determined the viewing angles of each pixels of the fish-eye lens+camera system knowing:

• The physical position of the optical center of the camera+fish-eye system.  The optical center within the camera represents the point in which all the light rays that form the image intersect.
• The projection function: in CAN-EYE, the distance along the radius of the circular image is a polynomial function (maximum order = 3) of the zenith angle. If the degree of the polynomial is 1, then the projection is assumed to be polar or equidistant.  

CAN-EYE proposes a method and a module to estimate the position of the optical center as well as to characterize the projection function (see the Documentation)

If the camera can not calibrated, it can be assumed perfect. In this case, the lens characteristics can be computed as the following:

• the optical sensor is at the center of the image: its coordinates are therefore half the number of lines and the number of rows
• the projection function is equidistant (polynomial order 1). The coefficient is then computed as the ratio of the maximum field of view of the fish-eye lens (FOV_max) divided by the length of the portion of the diagonal (L_D) of the image that does not include background fill color. The maximum field of view of the fish-eye lens is provided in the fish-eye technical specifications and corresponds to the field of view in the diagonal of the image.

We strongly recommend to not use these default parameters and to calibrate the system in order to get the most accurate results. Note that implementing the calibration experiment proposed with CAN-EYE should not take more than half a day.