UpdateLayeredWindow的透明度

Question

我有分层窗口（WS_EX_LAYERED），我正在尝试使用UpdateLayeredWindow在窗口上显示位图。

位图是不规则形状，背景色为RGB(30, 30, 30)。我想使用ULW_COLORKEY选项使背景透明。

当我致电UpdateLayeredWindow时，图像会在屏幕上呈现，但背景不透明。

    // Draw background rectangle
    COLORREF transparent_colour = RGB(30, 30, 30);
    HBRUSH hTranparentBrush = CreateSolidBrush(transparent_colour);
    HBRUSH hOldBrush = SelectObject(hSrcDC, hTransparentBrush);

    // the rectangle is the size of the splash screen / layered window
    hresult =  Rectangle(hSrcDC, 0, 0, szSplash.cx, szSplash.cy);
    //error checking has been removed

    // Paint rotated bitmap over background
    hresult = SetGraphicsMode(hSrcDC, GM_ADVANCED);

    float s = math::sin(degree * pi / 180);
    float c = math::cos(degree * pi / 180);
    XFORM x;
    x.em11 = c;
    x.em12 = s;
    x.em21 = -s;
    x.em22 = c;
    x.eDx = (1-c)*szImg.cx + s*szImg.cy;
    x.eDy = (1-c)*szImg.cy - s*szImg.cx;

    hresult = SetWorldTransform(hSrcDC, x);

    // the BitBlt below succeeds. hSrcDC contains a bitmap with a solid
    // background and the rotated image over it.
    hresult = BitBlt(hSrcDC, 100, 100, szImg.cx, szImg.cy, hImgDC, 0, 0, SRCCOPY);

    // Print to screen
    POINT ptZero = {0, 0};

    SIZE szBmp = szSplash; //size of the splash screen

    BLENDFUNCTION blend;
    blend.BlendOp = ARC_SRC_OVER;
    blend.BlendFlags = 0;
    blend.SourceConstantAlpha = 255;
    blend.AlphaFormat = AC_SRC_ALPHA;

    //this call succeeds - the bitmap is printed to the screen, but the
    //background is still visible?
    hresult = UpdateLayeredWindow(hwndSplash, 
                                  hSplashDC, 
                                  &ptZero, 
                                  &szBmp, 
                                  hSrcDC, 
                                  &ptZero, 
                                  transparent_colour, 
                                  &blend, 
                                  ULW_COLORKEY);

如果我将ULW_COLORKEY替换为ULW_ALPHA，则图像的背景和其他部分都会变得透明，看起来很糟糕。我希望背景透明 - 有什么我想念的吗？