Shading
Virtually all imaging systems produce shading. By this we mean that if the
physical input image a(x,y) = constant, then the
digital version of the image will not be constant. The source of the shading
might be outside the camera such as in the scene illumination or the result of
the camera itself where a gain and offset might vary from pixel
to pixel. The model for shading is given by:
where a[m,n] is the digital image that would have been
recorded if there were no shading in the image, that is,
a[m,n] = constant. Techniques for reducing or
removing the effects of shading are discussed in Section 10.1.