The ideal backprojection filter in frequency domain (i.e. ||) is not integrable and thus, is not realizable. Thus an approximation to this ideal filter is implemented using windowing functions W(). Assume that a triangular window of width 0 is used. This windowing function is given by:

W() = 1 || /0 if || 0

0 otherwise

Derive the expression of this approximation filter in spatial domain (i.e. c(l))