Convert RGB to Gray Scale without using Pointers

A piece of warning, though: the LockBits method is not a transparent one, it does image format conversion before returning a pointer to the bitmap (this is why it has a PixelFormat argument). And conversely, the UnlockBits method converts back to the original format. My best guess is that when the requested format is the native one (PixelFormat property of the Bitmap), no conversion takes place.

struct BGR { public byte B, G, R; }

void GrayConversion()
{
    Bitmap BMP= (Bitmap)pictureBox1.Image; // BMP is the source image
    int Width= BMP.Width, Height= BMP.Height;
    Bitmap GRAY= new Bitmap(Width, Height, PixelFormat.Format24bppRgb); // GRAY is the resultant image

    // Access the bitmap data
    BitmapData BMPData= BMP.LockBits(new Rectangle(0, 0, Width, Height), 
               ImageLockMode.ReadOnly, PixelFormat.Format24bppRgb);
    BitmapData GRAYData= GRAY.LockBits(new Rectangle(0, 0, Width, Height), 
               ImageLockMode.WriteOnly, PixelFormat.Format24bppRgb);

    // Loop on the rows
    for (int i= 0; i < Height; i++)
    {
        unsafe
        {
            // Start-of-row addresses
            BGR* Bmp= (BGR*)(BMPData.Scan0 + i * BMPData.Stride);
            BGR* Gray= (BGR*)(GRAYData.Scan0 + i * GRAYData.Stride);

            // Loop on the pixels
            for (int j= 0; j < Width; j++, Bmp++, Gray++)
            {
                // Extract the luminance of a source pixel using 14 bits fixed-point
                byte Y= (byte)((4897 * Bmp->R + 9617 * Bmp->G + 1868 * Bmp->B) >> 14);

                // Assign it to the corresponding destination pixel
                Gray->R= Gray->G= Gray->B= Y;
            }
        }
    }

    // Release
    BMP.UnlockBits(BMPData);
    GRAY.UnlockBits(GRAYData);

    picGray.Image= GRAY;
}

It is no big deal to adapt this function to the Format32bppArgb format.