thank you for your question. The meaning "Foward" as value of Parameter "Transformation Mode" of the operator "FFT" means transformation to frequency space. With value "Inverse" the FFT operator transforms the frequency space to "normal"image space. The functionality of FFT operator in this mode corresponds to Inverse Fast Fourier Transform. To transform an image with no imaginary part to frequency space set the lower (imaginary ) input node of operator "FFT" to constant value "Zero". In your VisualApplets installation directory under Examples/processing/Advanced/FastFourierTransform you can find an example implementation.