How do pointers affect the speed of the method for reading color from a bitmap?

Question

C++ / C#

Please tell me, to understand: why this method for reading color from a bitmap

public unsafe static byte[, ,] BitmapToByteRgb(Bitmap bmp)
{
    int width = bmp.Width,
        height = bmp.Height;
    byte[, ,] res = new byte[3, height, width];
    BitmapData bd = bmp.LockBits(new Rectangle(0, 0, width, height), ImageLockMode.ReadOnly,
        PixelFormat.Format24bppRgb);
    try
    {
        byte* curpos;
        for (int h = 0; h < height; h++)
        {
            curpos = ((byte*)bd.Scan0) + h * bd.Stride;
            for (int w = 0; w < width; w++)
            {
                res[2, h, w] = *(curpos++);
                res[1, h, w] = *(curpos++);
                res[0, h, w] = *(curpos++);
            }
        }
    }
    finally
    {
        bmp.UnlockBits(bd);
    }
    return res;
}

works much faster than this

public static byte[, ,] BitmapToByteRgbNaive(Bitmap bmp)
{
    int width = bmp.Width,
        height = bmp.Height;
    byte[, ,] res = new byte[3, height, width];
    for (int y = 0; y < height; ++y)
    {
        for (int x = 0; x < width; ++x)
        {
            Color color = bmp.GetPixel(x, y);
            res[0, y, x] = color.R;
            res[1, y, x] = color.G;
            res[2, y, x] = color.B;
        }
    }
    return res;
}

On Habré it is said that the matter is in pointers. How exactly do they affect?

Answer 1

So they influence that you write / read directly to the memory, which will then be used by the Bitmap for drawing / saving and everything else. In particular, you don't have GetPixel/SetPixel calls, which are VERY SLOW for this task. You have a loop in two dimensions of the picture, i.e. calls to GetPixel will be width * height pieces. Believe me, it's a lot and hard. Address arithmetic, namely
much easier. Actually, the possibility of its use is provided by blocking the real data of the bitmap using the LockBits method. A lock here means a mark for the CLR code, in particular for the GC, that this area of ​​\u200b\u200bmemory cannot be touched (moved, for example) until you remove this mark.
Strictly speaking, more or less fast work with Bitmap is possible only through BitmapData, since using Get/SetPixel you will not wait until the end of your algorithm.