image mapping for zernike moments

To compute the Zernike moments of a given image, the center of the image is taken as the origin and pixel coordinates are mapped to the range of unit circle, i.e. x^2+y^2≤1.

In a paper I have found that they use these formula:

x= (2i+1-N)/D,y=(2j+1-N)/D D={N for inscribed circular disc contained in the square image N√2 for outer circular disc containing the whole square image)┤

But I don't understand this formula