I have a 2D image and I convolve it with a 2D kernel image using FFT. So far I was always using symmetric kernels (e.g., Gaussian with stddev_x = stddev_y). As a result, I never bothered thinking whether I have to flip my kernel image or not because it wouldn't have made any difference. However, now I want to convolve my image using an elliptical Gaussian kernel with stddev_x != stddev_y and an arbitrary angle. Now, do I have to flip my kernel image prior the FFT convolution? Or the flipping is required only when using the usual convolution algorithm and not the FFT-based one?