BLENDFUNCTION bf;
   bf.BlendFlags = 0;
   bf.AlphaFormat = AC_SRC_ALPHA;
   bf.BlendOp = AC_SRC_OVER;
   bf.SourceConstantAlpha= 255;

				
   AlphaBlend(myDestination->getBackBufferDC(), static_cast<int>(X + myShadowOffsetX), static_cast<int>(Y + myShadowOffsetY), myShadow->width, myShadow->height, myShadow->ImageDC, 0, 0, myShadow->width, myShadow->height, bf);

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Original post by Colin Jeanne AlphaBlend() does not work in the way you are trying to use it. It applies an alpha value over the entire image. Bitmaps do not contain per-pixel alpha values and so you cannot define a set of pixels to be transparent.

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You have to premultiply each pixel of the source bitmap with its alpha value. The reason you're seeing a white background is because those pixels should be black: 255 times 0 yields 0 for each channel red, green and blue. Note that although AlphaBlend requires you to premultiply, it does need the alpha channel, so keep your bitmaps intact.

	  int bufferSize = target->widthBytes * target->height;
	  BYTE* buffer = (BYTE*)malloc(bufferSize);

	  // retrieve the pixel values
	  GetBitmapBits(target->NewBitmap, bufferSize, buffer);

	      // calculate the new alpha values
		  int i = 0;

		  while (i < bufferSize) {
		  	  buffer[i + 0] = buffer[i + 0] * buffer[i + 3];
			  buffer[i + 1] = buffer[i + 1] * buffer[i + 3];
			  buffer[i + 2] = buffer[i + 2] * buffer[i + 3];
			  
			  i = i + 4;
		  }

	  // set the pixel values
	  SetBitmapBits(target->NewBitmap, bufferSize, buffer);
	  delete buffer;

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Also, take notice that AlphaBlend is slooow. I'm pretty sure there are better alternatives to the native GDI, even if they are implemented entirely in software.