Marc Gregoire’s Blog

A while ago I wrote an article: “Introduction to WIC: How to use WIC to load an image, and draw it with GDI?”

The code in that article didn’t handle transparency.
It’s actually trivial to implement transparency in that sample code.
First thing you have to do is modify CNuonImg::Open() and specify GUID_WICPixelFormat32bppPBGRA, which is a pre-multiplied alpha format, instead of GUID_WICPixelFormat32bppBGR.
Second, modify CNuonImg::Render() as follows (changes are highlighted):

void CNuonImg::Render(HDC hDC, UINT x, UINT y, UINT cx, UINT cy)
{
	// Make sure an image has been loaded
	if (!IsLoaded())
		throw WINCODEC_ERR_WRONGSTATE;

	// Get the WIC factory from the singleton wrapper class
	IWICImagingFactory* pFactory = CWICImagingFactory::GetInstance().GetFactory();
	if (!pFactory)
		throw WINCODEC_ERR_NOTINITIALIZED;

	// Create a WIC image scaler to scale the image to the requested size
	CComPtr<IWICBitmapScaler> pScaler = nullptr;
	IfFailedThrowHR(pFactory->CreateBitmapScaler(&pScaler));
	IfFailedThrowHR(pScaler->Initialize(m_pConvertedFrame, cx, cy, WICBitmapInterpolationModeFant));

	// Render the image to a GDI device context
	HBITMAP hDIBBitmap = NULL;
	HDC memDC = NULL;
	try
	{
		// Get a DC for the full screen
		HDC hdcScreen = GetDC(NULL);
		if (!hdcScreen)
			throw 1;

		HDC memDC = CreateCompatibleDC(hdcScreen);
		if (!memDC)
			throw 2;

		BITMAPINFO bminfo;
		ZeroMemory(&bminfo, sizeof(bminfo));
		bminfo.bmiHeader.biSize         = sizeof(BITMAPINFOHEADER);
		bminfo.bmiHeader.biWidth        = cx;
		bminfo.bmiHeader.biHeight       = -(LONG)cy;
		bminfo.bmiHeader.biPlanes       = 1;
		bminfo.bmiHeader.biBitCount     = 32;
		bminfo.bmiHeader.biCompression  = BI_RGB;		

		void* pvImageBits = nullptr;	// Freed with DeleteObject(hDIBBitmap)
		hDIBBitmap = CreateDIBSection(hdcScreen, &bminfo, DIB_RGB_COLORS, &pvImageBits, NULL, 0);
		if (!hDIBBitmap)
			throw 3;

		ReleaseDC(NULL, hdcScreen);

		HBITMAP hOldBitmap = (HBITMAP)::SelectObject(memDC, hDIBBitmap);

		// Calculate the number of bytes in 1 scanline
		UINT nStride = DIB_WIDTHBYTES(cx * 32);
		// Calculate the total size of the image
		UINT nImage = nStride * cy;
		// Copy the pixels to the DIB section
		IfFailedThrowHR(pScaler->CopyPixels(nullptr, nStride, nImage, reinterpret_cast<BYTE*>(pvImageBits)));

		// Copy the bitmap to the target device context
		BLENDFUNCTION bf;
		bf.AlphaFormat = AC_SRC_ALPHA;
		bf.BlendFlags = 0;
		bf.BlendOp = AC_SRC_OVER;
		bf.SourceConstantAlpha = 255;
		::AlphaBlend(hDC, x, y, cx, cy, memDC, 0, 0, cx, cy, bf);

		::SelectObject(memDC, hOldBitmap);
		DeleteDC(memDC);
		DeleteObject(hDIBBitmap);
	}
	catch (...)
	{
		if (hDIBBitmap)
			DeleteObject(hDIBBitmap);
		if (memDC)
			DeleteDC(memDC);
		// Rethrow the exception, so the client code can handle it
		throw;
	}
}