Direct2D renders in main window but not child window

Question

I'm learning how to use Direct2D and DirectWrite. I wrote a sample application that uses these libraries to render content directly in the main (top-level) window. It works fine.

Now I'm trying to rewrite the painting code for a child window in another application. The original code uses GDI and works fine. But when I convert it to use Direct2D, nothing appears at all (the child window appears black).

I've simplified the painting code down to a simple fill of the client area.

LRESULT PlotWnd::OnPaint(UINT, WPARAM, LPARAM, BOOL &) {
    ATL::CComPtr<ID2D1Factory> pD2DFactory;
    HRESULT hr = ::D2D1CreateFactory(D2D1_FACTORY_TYPE_SINGLE_THREADED, &pD2DFactory);
    assert(SUCCEEDED(hr));
    ATL::CComPtr<ID2D1HwndRenderTarget> pRT;
    RECT rc;
    GetClientRect(&rc);
    D2D1_SIZE_U size = D2D1::SizeU(rc.right - rc.left, rc.bottom - rc.top);
    hr = pD2DFactory->CreateHwndRenderTarget(
        D2D1::RenderTargetProperties(/* D2D1_RENDER_TARGET_TYPE_SOFTWARE */),
        D2D1::HwndRenderTargetProperties(m_hWnd, size), &pRT);
    assert(SUCCEEDED(hr));

    pRT->BeginDraw();
    pRT->Clear(D2D1::ColorF(D2D1::ColorF::Beige));
    hr = pRT->EndDraw();
    assert(SUCCEEDED(hr));
    ValidateRect(nullptr);

    return 0;
}

All of the Direct2D calls succeed with S_OK, so I'm kind of at a loss as to where to look for the problem. At this point, the only significant difference I can see between my working experiment and this program is that in this one, I'm only using Direct2D in one child window.

So my questions are: Is there an issue that I'm missing when using Direct2D in a child window? Any tips for debugging this further?

Update: I've removed statics to avoid confusion. I'm completely setting up and tearing down Direct2D with each paint. There is exactly one paint call, with a real size.

If I switch to software rendering (by using D2D1_RENDER_TARGET_TYPE_SOFTWARE in the render target properties), the problem goes away--I get the beige fill as expected.

This led me to suspect my graphics driver, which was out of date. But I've now updated my graphics driver, and it still fails in the same manner. What else could cause hardware rendering to fail when software rendering works as expected?

Update 2: I'm still trying to reproduce this is a small, self-contained example. In the mean time, I noticed that simply starting Spy++ will sometimes cause the window to actually render to the screen (one time).

Have you overridden the WM_ERASEBKGND handler so it doesn't keep blanking your window? — Roger Rowland
@Roger Rowland: Yes, my WM_ERASKBKGND handler does nothing. If I have it fill with an arbitrary color, then I see that color instead of the beige fill I expect from the Direct2D Clear command. — Adrian McCarthy
Ok, just running through some "obvious" things - does the parent window have the WS_CLIPCHILDREN style set? — Roger Rowland
Yes, both ancestors have WS_CLIPCHILDREN (confirmed with Spy++). — Adrian McCarthy
Have you verified that OnPaint first gets called with a non-zero size? Just thinking that your GetClientRect may be called too soon (e.g. if you create with a dummy size and then move/size the child window). — Roger Rowland

Rick Brewster Rick Brewster · Accepted Answer · 2014-01-19T02:36:56

Well, for starters, you're supposed to create the factory at application start, and the render target with the window (HWND). Those are not supposed to be "temporary" objects that only live as long as your OnPaint (WM_PAINT) is executing.

Direct2D renders in main window but not child window

3 Answers