Создание прозрачного окна на GDI+

HWND my_window = CreateWindowEx(NULL, WndClassName, "Transparent", WS_POPUP, X, Y, Width, Height, HWND_DESKTOP, NULL, hInstance, NULL);

SetWindowLong(my_window, GWL_EXSTYLE, GetWindowLong(my_window, GWL_EXSTYLE) ^ WS_EX_LAYERED);

Gdiplus::Bitmap *background_img = new Gdiplus::Bitmap(L"g:/test.png", false);

BITMAPINFO BMI;
BMI.bmiHeader.biSize = sizeof(BITMAPINFOHEADER);
BMI.bmiHeader.biBitCount = 32;
BMI.bmiHeader.biCompression = BI_RGB;
BMI.bmiHeader.biWidth = background_img->GetWidth();
BMI.bmiHeader.biHeight = background_img->GetHeight();
BMI.bmiHeader.biPlanes = 1;
 
HDC screendc;
 
HBITMAP hbmp = CreateDIBSection(screendc, &BMI, DIB_RGB_COLORS, NULL, NULL, 0);
 
//получаем контекст
screendc = GetDC(my_window);
 
//создаём контекст, совместимый с screendc
HDC backdc = CreateCompatibleDC(screendc);
 
//загружаем в него HBITMAP
HBITMAP h_old_bmp = (HBITMAP)SelectObject(backdc, hbmp);
 
//устанавливаем параметры смешивания
BLENDFUNCTION bf;
bf.BlendOp = AC_SRC_OVER;
bf.BlendFlags = 0;
bf.SourceConstantAlpha = 0xff;//100%
bf.AlphaFormat = AC_SRC_ALPHA;
 
 
POINT pt1 = {0, 0};//новая позиция многослойного окна
POINT pt2 = {0, 0};//позиция слоя в контексте устройства
 
SIZE WndSize;//новый размер многослойного окна
WndSize.cx = Width;
WndSize.cy = Height;

Graphics *graphics = new Graphics(backdc);
graphics->Clear(Color(0, 0, 0, 0));
graphics->DrawImage(background_img, 0, 0, Width, Height);

UpdateLayeredWindow(my_window, screendc, &pt1, &WndSize, backdc, &pt2, 0, &bf, ULW_ALPHA);

ShowWindow(my_window, SW_SHOW);

    // Get dimensions
    int iWidth = img.GetWidth();
    int iHeight = img.GetHeight();

    if (img.GetBPP() == 32)
    {
        int iPitch = img.GetPitch();
        for (int y = 0; y < iHeight; y++)
        {
            unsigned char* pBuffer = (unsigned char*)img.GetBits() + iPitch*y;
            for (int x = 0; x < iWidth; x++)
            {
                unsigned uAlpha = pBuffer[3];
                pBuffer[0] = (unsigned char)(uAlpha * pBuffer[0] / 255);
                pBuffer[1] = (unsigned char)(uAlpha * pBuffer[1] / 255);
                pBuffer[2] = (unsigned char)(uAlpha * pBuffer[2] / 255);
                pBuffer += 4;
            }
        }
    }

for (int i = 0; i < img.GetWidth(); i++)
{
    for (int j = 0; j < img.GetHeight(); j++)
    {
        BYTE* ptr = (BYTE*)img.GetPixelAddress(i, j);
        ptr[0] = ((ptr[0] * ptr[3]) + 127) / 255;
        ptr[1] = ((ptr[1] * ptr[3]) + 127) / 255;
        ptr[2] = ((ptr[2] * ptr[3]) + 127) / 255;
    }
}