C#: Simulate gdi GetBitmapBits function and GdiFlush to fill a byte array with a bitmap

Question

I created a lib where I need to fill a byte array of 307200 elements (320x240x4(=32bit)) in the correct order for a bitmap, the display uses the format RGBA, and I would like to avoid using interop to use GetBitmapBits as I'm doing at the moment, while I prefer writing it in c# code to understand how bytes are printed on it.

Can someone help me?

Here is my actual code

    /// <summary>
    /// LONG GetBitmapBits(
    ///    __in   HBITMAP hbmp,
    ///    __in   LONG cbBuffer,
    ///    __out  LPVOID lpvBits
    ///  );
    /// </summary>
    /// <param name="hbmp"></param>
    /// <param name="cbBuffer"></param>
    /// <param name="lpvBits"></param>
    /// <returns></returns>
    [DllImport("Gdi32", EntryPoint = "GetBitmapBits")]
    private extern static long GetBitmapBits([In] IntPtr hbmp, [In] int cbBuffer, [Out] byte[] lpvBits);

    [DllImport("Gdi32", EntryPoint = "GdiFlush")]
    private extern static void GdiFlush();

    private void FillPixelArray(Bitmap bmp, ref byte[] array, bool bw = false)
    {
        Color tmp;
        if (!bw)
        {
            IntPtr hbmp = bmp.GetHbitmap();
            GdiFlush();
            GetBitmapBits(hbmp, array.Length * Marshal.SizeOf(typeof(byte)), array);
        }
        else
        {
            for (int x = 0; x < LgLcd.NativeConstants.LGLCD_BMP_WIDTH; ++x)
            {
                for (int y = 0; y < LgLcd.NativeConstants.LGLCD_BMP_HEIGHT; ++y)
                {
                    tmp = bmp.GetPixel(x, y);
                    array[y * 160 + x] = (byte)((tmp.R == 255 && tmp.G == 255 && tmp.B == 255) ? 0 : 255);
                }
            }
        }
    }

Another thing, is GetBitmapBits faster than any implementation I can do in C#?

max · Accepted Answer · 2011-01-05 07:39:31Z

2

var data = bmp.LockBits(new Rectangle(0, 0, bmp.Width, bmp.Height),
    System.Drawing.Imaging.ImageLockMode.ReadOnly,
    System.Drawing.Imaging.PixelFormat.Format32bppArgb);
Marshal.Copy(data.Scan0, array, 0, data.Stride * data.Height);
bmp.UnlockBits(data);

P.S. you don't need ref in ref byte[] array - array is a reference type already, and you are not modifying array variable in your function.

P.P.S. GetBitmapBits returns int, not long (don't confuse with LONG C macro), GdiFlush returns [return:MarshalAs(UnmanagedType.Bool)] bool, not void.

edited Jan 5, 2011 at 7:39

answered Jan 5, 2011 at 5:22

max

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2 Comments

Hans Passant Over a year ago

Technically you should use data.Stride instead of bmp.Width in the Copy() call. It now works by accident.

max Over a year ago

Yes, using Stride is correct, but in this particular case Stride will always be Width * 4 (because Stride rounds up to 4-byte boundary, and we are using ARGB - 4 bytes per pixel, and even more, our image width is 320 = 80 * 4). Changed code anyway.

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