如何画透明位图

{//必须是全局的，这样使位图的混合过程alpha设置相同static BLENDFUNCTION bf; CPaintDC ldc(this);CDC MemDC;CBitmap Bitmap1, Bitmap2;MemDC.CreateCompatibleDC( &ldc ); Bitmap1.LoadBitmap(IDB_BITMAP1); //纯红Bitmap2.LoadBitmap(IDB_BITMAP2); //纯黑BITMAP bmp1, bmp2;Bitmap1.GetBitmap( &bmp1 );Bitmap2.GetBitmap( &bmp2);MemDC.SelectObject( &Bitmap2 );ldc.BitBlt(0,0,bmp2.bmWidth, bmp2.bmHeight, &MemDC, 0,0, SRCCOPY);MemDC.SelectObject( &Bitmap1 );bf.BlendOp = AC_SRC_OVER; //必须bf.BlendFlags = 0;// 必须bf.AlphaFormat = 0; bf.SourceConstantAlpha = 150; //不透明度// 直接与窗口混合ldc.AlphaBlend(0,0,bmp1.bmWidth,bmp1.bmHeight, &MemDC,0,0,bmp1.bmWidth,bmp1.bmHeight, bf );}