contrast of image

Question

can somebody explain me this code for contrast of image.
public Bitmap contrast(Bitmap b, sbyte nContrast)
        {
            if (nContrast < -100) return b;
            if (nContrast > 100) return b;

            double pixel = 0, contrast = (100.0 + nContrast) / 100.0;

            contrast *= contrast;

            int red, green, blue;

            // GDI+ still lies to us - the return format is BGR, NOT RGB.
            BitmapData bmData = b.LockBits(new Rectangle(0, 0, b.Width, b.Height), ImageLockMode.ReadWrite, PixelFormat.Format24bppRgb);

            int stride = bmData.Stride;
            System.IntPtr Scan0 = bmData.Scan0;

            unsafe
            {
                byte* p = (byte*)(void*)Scan0;

                int nOffset = stride - b.Width * 3;

                for (int y = 0; y < b.Height; ++y)
                {
                    for (int x = 0; x < b.Width; ++x)
                    {
                        blue = p[0];
                        green = p[1];
                        red = p[2];

                        pixel = red / 255.0;
                        pixel -= 0.5;
                        pixel *= contrast;
                        pixel += 0.5;
                        pixel *= 255;
                        if (pixel < 0) pixel = 0;
                        if (pixel > 255) pixel = 255;
                        p[2] = (byte)pixel;

                        pixel = green / 255.0;
                        pixel -= 0.5;
                        pixel *= contrast;
                        pixel += 0.5;
                        pixel *= 255;
                        if (pixel < 0) pixel = 0;
                        if (pixel > 255) pixel = 255;
                        p[1] = (byte)pixel;

                        pixel = blue / 255.0;
                        pixel -= 0.5;
                        pixel *= contrast;
                        pixel += 0.5;
                        pixel *= 255;
                        if (pixel < 0) pixel = 0;
                        if (pixel > 255) pixel = 255;
                        p[0] = (byte)pixel;

                        p += 3;
                    }
                    p += nOffset;
                }
            }

            b.UnlockBits(bmData);

            return b; ;
        }