I'm implementing a system to search an image for occurrences of a smaller image inside it, using 2D FFT convolution, as described here : http://www.dspguide.com/ch24/7.htm and here : http://www.gamedev.net/reference/programming/features/imageproc/page3.asp (near the end)

My sketch : http://www.viz.nu/fft/

In an nutshell, the method is:
1. zero pad the search image and the "to-be-searched" image to a power of 2 (512)
2. In-place 2D DFT of both the images
3. Complex multiply each bin value in 2D.  (searchimage x image)
4. Reverse DFT the result
5. Linear scale and display this result
6. Maximum value of this 2D array should represent the match location

I feel like I have set up the system correctly and none of my code looks suspicious to me.  But the program only correctly finds the search images *some* of the time!  I'm even using the same test images as the second URL above.  But in my system it is not found in the right location.

Well anyway, if anyone happens to have an idea/resource/verbal abuse, your input is greatly appreciated.  I know this is pretty niche-y stuff..