Convert RGB to Gray Scale without using Pointers

YvesDaoust

0/5 (0 vote)

Jan 17, 2012

CPOL

15219

You shouldn't rely on methods from the .NET classes (Bitmap::SetPixel, Bitmap::GetPixel, Color::FromArgb) in the inner loop as these are called intensively, million times per image, and their implementation are opaque to you.If their implementation is opaque to the compiler too, no code...

You shouldn't rely on methods from the .NET classes (Bitmap::SetPixel, Bitmap::GetPixel, Color::FromArgb) in the inner loop as these are called intensively, million times per image, and their implementation are opaque to you.

If their implementation is opaque to the compiler too, no code optimization can be done via inlining.

Doing the linear combinations using floating-point arithmetic is overkill. Fixed-point is good enough. You will be penalized by the overhead of data type conversion and floating-point arithmetic.
What's so scary with pointers ? Is the code below less readable?

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); // 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
            byte* Bmp = (byte*)BMPData.Scan0 + i * BMPData.Stride;
            byte* Gray = (byte*)GRAYData.Scan0 + i * GRAYData.Stride;

            // Loop on the pixels
            for (int j = 0; j < Width; j++, Bmp += 3, Gray += 3)
            {
                // Extract the luminance of a source pixel using 14 bits fixed-point
                byte Y = (byte)((4897 * Bmp[2] + 9617 * Bmp[1] + 1868 * Bmp[0]) >> 14);

                // Assign it to the corresponding destination pixel
                Gray[2] = Gray[1] = Gray[0] = Y;
            }
        }
    }

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

    picGray.Image = GRAY;
}