How do I clear a Direct2D render target to fully transparent

Question

I\'m trying to draw semi-transparent rectangles on an invisible HWND. However, clearing the window with ID2D1HwndRenderTarget::Clear just makes the entire w

Answer 1

When creating your RenderTarget, you'll have to tell D2D that you want to use alpha (in premultiplied mode) in the pixel format:

  HRESULT hr = mD2DFactory->CreateHwndRenderTarget(
    D2D1::RenderTargetProperties( D2D1_RENDER_TARGET_TYPE_DEFAULT, D2D1::PixelFormat( DXGI_FORMAT_UNKNOWN, D2D1_ALPHA_MODE_PREMULTIPLIED ) ),
    D2D1::HwndRenderTargetProperties( mWindow, size ),
    &mRenderTarget );

After this, calling Clear() with an alpha value of zero works just fine.