image processing

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C#

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

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

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

                int nOffset = stride - RImage.Width * 3;

                byte red, green, blue;

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

                        p[0] = p[1] = p[2] = (byte)(.299 * red + .587 * green + .114 * blue);

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

            RImage.UnlockBits(bmData);

lines of code in the example above we can see, that we see the pixels one by one from left to right.
or can be explained from x = 0 to x = max.
then down to y

for different issues I want to see and change one by one pixel from the top to the down.
or can be explained from the y = 0 to y = max.
then right onto x

Is this possible?
or perhaps this could
1. from x = max down to x = 0. then y.
2. of y = max down until y = 0. then x.