GDI+ Compose bitmap with transparency

Question

I'd like to rewrite this method by using Graphics.DrawImage(). No matter the PixelFormat of bitmapToDrawOn and bitmapDrawn values (I tried all combinations) which should be typically PixelFormat.Format32bppRgb and PixelFormat.Format32bppArgb (or PixelFormat.Format32bppPArgb), I cannot obtain this result.

This compose operation seems pretty obvious to me, so it looks like I am missing something.

private static void ComposeBitmapWithTransparency(Bitmap bitmapToDrawOn, Bitmap bitmapDrawn) {
   Debug.Assert(bitmapToDrawOn != null);
   Debug.Assert(bitmapToDrawOn.PixelFormat == PixelFormat.Format32bppRgb);

   Debug.Assert(bitmapDrawn != null);
   Debug.Assert(bitmapDrawn.PixelFormat == PixelFormat.Format32bppArgb);

   var width = bitmapToDrawOn.Width;
   Debug.Assert(bitmapDrawn.Width == width);

   var height = bitmapToDrawOn.Height;
   Debug.Assert(bitmapDrawn.Height == height);

   var rect = new Rectangle(0, 0, width, height);
   var bitmapToDrawOnData = bitmapToDrawOn.LockBits(
         rect,
         ImageLockMode.ReadWrite,
         bitmapToDrawOn.PixelFormat);
   var bitmapDrawnData = bitmapDrawn.LockBits(
         rect,
         ImageLockMode.ReadOnly,
         bitmapDrawn.PixelFormat);

   unsafe {
      var pBitmapToDrawOn = (StructPixel*)bitmapToDrawOnData.Scan0;
      var pBitmapDrawn = (StructPixel*)bitmapDrawnData.Scan0;

      for (int y = 0; y < height; y++) {
         for (int x = 0; x < width; x++) {
            byte r = pBitmapToDrawOn->R;
            byte g = pBitmapToDrawOn->G;
            byte b = pBitmapToDrawOn->B;

            byte rX = pBitmapDrawn->R;
            byte gX = pBitmapDrawn->G;
            byte bX = pBitmapDrawn->B;

            // Only the transparency of bitmapDrawn is taken account.
            byte alphaX = pBitmapDrawn->Alpha;
            byte alphaXComplement = (byte)(255 - alphaX);

            pBitmapToDrawOn->R = (byte)((r * alphaXComplement + rX * alphaX) >> 8);
            pBitmapToDrawOn->G = (byte)((g * alphaXComplement + gX * alphaX) >> 8);
            pBitmapToDrawOn->B = (byte)((b * alphaXComplement + bX * alphaX) >> 8);

            pBitmapToDrawOn++;
            pBitmapDrawn++;
         }
      }
   }

   bitmapDrawn.UnlockBits(bitmapDrawnData);
   bitmapToDrawOn.UnlockBits(bitmapToDrawOnData);
}

The code for StructPixel:

   [StructLayout(LayoutKind.Explicit)]
   public struct StructPixel {
      [FieldOffset(0)]
      public byte B;
      [FieldOffset(1)]
      public byte G;
      [FieldOffset(2)]
      public byte R;
      [FieldOffset(3)]
      public byte Alpha;
   }

What is the actual problem here? You want to draw with Graphics.DrawBitmap()? That method doesn't exist. — Hans Passant, Mar 21 '14 at 14:29
Oups I meant DrawImage() and not DrawBitmap() sorry for the confusion. The problem is that I can't find a way to use DrawImage() to obtain the obvious *compose with transparency effect* coded by this method. I already read your answer http://stackoverflow.com/a/10661333/27194 but the compose effect I obtain is different than the obvious one and I don't known why. It looks like the composing algorithm is using [255] modulus on each RGB channel, so with an alpha of 50%, when composing the two channel values 100 and 200, instead of getting 150, DrawBitmap() ends up with 100+200=300 [255] = 45 — Patrick from NDepend team, Mar 21 '14 at 15:09

GDI+ Compose bitmap with transparency

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