@Michael C. Grant wrote that "the cost of FFTW isn't an easy formula based on log_2 anymore."
But the Wikipedia article says that the FFTW "can compute transforms of real and complex-valued arrays of arbitrary size and dimension in O(n log n) time."
So can it compute 1 forward FFT with the size 1920*1080? in kNlog(N)/log(2) =
5*1920*1080*log(1920*1080)/log(2)= 217559066 real operations?
Or should I perphaps take a different value for k?