Calculating slope of the power spectrum

I would like to calculate the slope of the power spectrum of an image and was wondering if any code exists.

the basic algorithm is:
first compute the fft and power spectrum (S),
S(u,v)=FFT(I)/N^2

then, represent them in polar coordinate and sum them over all directions:
S(f)=sum_theta{S(f,theta)}.=Af^(-alpha)
the *alpha* is called the slope of power spectrum and it is what i want to estimate

Thanks!