From: Ian Scott <ian.scott@st...>  20051202 10:35:20

Timothy Chan wrote: > Ian, > > Thanks for all your help and putting up with my stupid questions. I have > successfully used both fft fwd and bwd now on a number of images so that has > been a great help. > > There is one file in particular that I cannot seem to run the fft on, and > when I do I get the message "assertion failure '!'you probably gave a signal > size not of the form 2^p 3^q 5^r" Can you explain to me what this means and > how I can get around it? If there was somebody here that I could ask I > certain would, but unfortunately there isn't. We don't mind questions about VXL  its just nonVXL programming and toolchain management questions we would rather avoid. However, your questions would be better sent to vxlusers. I'm not an expert on the FFT code, but at a guess the size of your image needs to be (2^pi*3^qi*5^ri,2^pj*3^qj*5^rj), where all the variables are nonnegative integers. Most FFT algorithms require your image to be sized as a power of 2, in order to have a Fast Fourier Transform rather than just a Fourier Transform. VXL's FFT appears to be rather generous in allowing factors of 3 and 5 as well. I suspect the solution is to pad your image with some values so that it is the next appropriate size. Some of the links at http://www.google.com/search?q=padding+data+for+FFT appear to suggest that padding with 0 is generally acceptable. Ian. If your image is > > Tim Chan > 