Principle of associating each pixel p in an image with a form factor f f_p. In green: each p pixel in the picture corresponds to a rectangle in the focal plane of the camera. In purple: from the observer point of view (i.e., the camera optical center), this rectangle corresponds to a solid angle ω_p making a θ_p angle (in blue) with the viewing direction (in red). Then the form factor associated with pixel p is $f{f}_p=\frac{1}{\pi }{\omega }_p\cos \left({\theta }_p\right)$.