Setting Alpha values for a Format32bppArgb

I have been trying to set the alpha values manually for a Bitmap with the format of Format32bppArgb . In this code example, I am setting them all to 0.5f, however, they will be different values in the future and not all 0.5f/128 (as this is my test case to just get this working). How can I properly set the alpha values for a bitmap quickly? I could use SetPixel(), however, SetPixel() is horrifically slow for large images compared to just locking/unlocking the bitmap.

        Rectangle rect = new Rectangle(0, 0, bmp.Width, bmp.Height);
        System.Drawing.Imaging.BitmapData bmpData =
            bmp.LockBits(rect, System.Drawing.Imaging.ImageLockMode.ReadWrite,
            b开发者_StackOverflowmp.PixelFormat);

        // Get the address of the first line.
        IntPtr ptr = bmpData.Scan0;

        // Declare an array to hold the bytes of the bitmap.
        int bytes = Math.Abs(bmpData.Stride) * bmp.Height;
        byte[] rgbValues = new byte[bytes];

        // Copy the RGB values into the array.
        System.Runtime.InteropServices.Marshal.Copy(ptr, rgbValues, 0, bytes);

        for (int counter = 0; counter < rgbValues.Length; counter += 4)
        {
            rgbValues[counter] = 255;
            rgbValues[counter + 1] = 255;
            rgbValues[counter + 2] = 255;
            rgbValues[counter + 3] = 128;
        }

        // Copy the RGB values back to the bitmap
        System.Runtime.InteropServices.Marshal.Copy(rgbValues, 0, ptr, bytes);

        // Unlock the bits.
        bmp.UnlockBits(bmpData);

The best way to do this, providing you want to have the same alpha value on your entire bitmap, is to use a ColorMatrix. Check out this example by Microsoft:

http://msdn.microsoft.com/en-us/library/w177ax15(v=vs.71).aspx