AForge.Math.FourierTransform.FFT C# (CSharp) Method

        public static void FFT(Complex[] data, Direction direction)
        {
            int n = data.Length;
            int m = Tools.Log2(n);

            // reorder data first
            ReorderData(data);

            // compute FFT
            int tn = 1, tm;

            for (int k = 1; k <= m; k++)
            {
                Complex[] rotation = GetComplexRotation(k, direction);

                tm = tn;
                tn <<= 1;

                for (int i = 0; i < tm; i++)
                {
                    Complex t = rotation[i];

                    for (int even = i; even < n; even += tn)
                    {
                        int odd = even + tm;
                        Complex ce = data[even];
                        Complex co = data[odd];

                        double tr = co.Re*t.Re - co.Im*t.Im;
                        double ti = co.Re*t.Im + co.Im*t.Re;

                        data[even].Re += tr;
                        data[even].Im += ti;

                        data[odd].Re = ce.Re - tr;
                        data[odd].Im = ce.Im - ti;
                    }
                }
            }

            if (direction == Direction.Forward)
            {
                for (int i = 0; i < n; i++)
                {
                    data[i].Re /= n;
                    data[i].Im /= n;
                }
            }
        }

        /// <summary>
        /// Two dimensional Fast Fourier Transform.
        /// </summary>
        ///
        /// <param name="data">Data to transform.</param>
        /// <param name="direction">Transformation direction.</param>
        ///
        /// <remarks><para><note>The method accepts <paramref name="data"/> array of 2<sup>n</sup> size
        /// only in each dimension, where <b>n</b> may vary in the [1, 14] range. For example, 16x16 array
        /// is valid, but 15x15 is not.</note></para></remarks>
        ///
        /// <exception cref="ArgumentException">Incorrect data length.</exception>
        ///
        public static void FFT2(Complex[,] data, Direction direction)
        {
            int k = data.GetLength(0);
            int n = data.GetLength(1);

            // check data size
            if (!Tools.IsPowerOf2(k) || !Tools.IsPowerOf2(n))
            {
                throw new ArgumentException("The matrix rows and columns must be a power of 2.");
            }

            if (k < minLength || k > maxLength || n < minLength || n > maxLength)
            {
                throw new ArgumentException("Incorrect data length.");
            }

            // process rows
            var row = new Complex[n];

            for (int i = 0; i < k; i++)
            {
                // copy row
                for (int j = 0; j < row.Length; j++)
                {
                    row[j] = data[i, j];
                }

                // transform it
                FourierTransform.FFT(row, direction);

                // copy back
                for (int j = 0; j < row.Length; j++)
                {
                    data[i, j] = row[j];
                }
            }

            // process columns
            var col = new Complex[k];

            for (int j = 0; j < n; j++)
            {
                // copy column
                for (int i = 0; i < k; i++)
                {
                    col[i] = data[i, j];
                }

                // transform it
                FourierTransform.FFT(col, direction);

                // copy back
                for (int i = 0; i < k; i++)
                {
                    data[i, j] = col[i];
                }
            }
        }