Accord.Imaging.ObjectiveFidelity.derivativeSNR C# (CSharp) Method

        private static unsafe double derivativeSNR(UnmanagedImage a, UnmanagedImage b)
        {
            int width = a.Width;
            int height = a.Height;

            int pixelSize = System.Drawing.Image.GetPixelFormatSize(a.PixelFormat) / 8;
            int stride = a.Stride;
            int offset = stride - a.Width * pixelSize;

            byte* ptrA = (byte*)a.ImageData.ToPointer();
            byte* ptrB = (byte*)b.ImageData.ToPointer();

            double sum1 = 0;
            double sum2 = 0;

            for (int y = 0; y < height - 1; y++)
            {
                for (int x = 0; x < width - 1; x++, ptrA++, ptrB++)
                {
                    // original image
                    int gradO = System.Math.Abs(*ptrA - ptrA[+stride]) + System.Math.Abs(*ptrA - ptrA[+1]);
                    sum1 += gradO * gradO;

                    // reconstructed image
                    int gradR = System.Math.Abs(*ptrB - ptrB[+stride]) + System.Math.Abs(*ptrB - ptrB[+1]);
                    sum2 += gradR * gradR;
                }

                ptrA += offset + 1;
                ptrB += offset + 1;
            }

            if (sum2 == 0)
                return 0;

            return 10 * System.Math.Log10(sum1 / sum2);
        }
    }