I am trying to apply a simple relief (emboss-like) effect to an image in a WinForms application using Bitmap.GetPixel and SetPixel.

The idea is to compare the brightness of the current pixel with a shifted pixel (offset by 5 pixels) and compute a new grayscale value.

Here is my code:

public static void ApplyReliefEffect(Form form)
{
    if (ImageStorage.bitmap != null)
    {
        int brightness;
        int brightness1;
        int brightness2;

        Bitmap sourceBitmap = new Bitmap(ImageStorage.bitmap);
        Bitmap offsetBitmap = new Bitmap(ImageStorage.bitmap);

        Color currentPixel;
        Color offsetPixel;

        Graphics graphics = form.CreateGraphics();

        for (int x = 5; x < sourceBitmap.Width; x++)
        {
            for (int y = 5; y < sourceBitmap.Height; y++)
            {
                currentPixel = sourceBitmap.GetPixel(x, y);
                offsetPixel = offsetBitmap.GetPixel(x - 5, y - 5);

                brightness1 = CalculateBrightness(currentPixel);
                brightness2 = CalculateBrightness(offsetPixel);

                brightness = 150 + (brightness2 - brightness1);

                if (brightness < 0)
                {
                    brightness = 0;
                }
                else if (brightness > 255)
                {
                    brightness = 255;
                }

                sourceBitmap.SetPixel(x, y, Color.FromArgb(brightness, brightness, brightness));
            }
        }

        graphics.DrawImage(sourceBitmap, 25, 50);
        graphics.Dispose();
    }
}

AI tried to tell me like a million times, that this is crazily unoptimal way to do it, and it suggests to use LockBits instead. What I dont understand though is, what makes this so slow? I dont understand the difference between the two (and boy did I try to understand :D). Could you maybe help out please? Thanks in advance!