gistfile1.cs

private static void PreMultiplyImage(string infile, string outfile)
        {
            using (System.Drawing.Image image = System.Drawing.Image.FromFile(infile))
            {
                int w = image.Width;
                int h = image.Height;
                System.Drawing.Bitmap bitmap = new System.Drawing.Bitmap(image);
                for (int i = 0; i != bitmap.Width; ++i)
                {
                    for (int j = 0; j != bitmap.Height; ++j)
                    {
                        System.Drawing.Color pixel = bitmap.GetPixel(i, j);
                        System.Drawing.Color prepxiel = System.Drawing.Color.FromArgb(
                             pixel.A,
                             (pixel.R * pixel.A / 255),
                             (pixel.G * pixel.A / 255),
                             (pixel.B * pixel.A / 255)                            
                        );
                        bitmap.SetPixel(i,j,prepxiel);
                    }
                }

                bitmap.Save(outfile);
               
            }
        }