ImGui overlay with UpdateLayeredWindow function - overlay

Question

As title, I'm trying to create a partially transparent in-game overlay using ImGui that's clickable on the UI but click-through otherwise, i.e. you can click on the ImGUI elements but outside the elements you can interact with the game.
I was able to do it using
https://github.com/ocornut/imgui/blob/master/examples/example_win32_directx11/main.cpp
by making the window style as WS_EX_TOPMOST | WS_EX_LAYERED, and using
SetLayeredWindowAttributes(hwnd, RGB(0, 0, 0), 0, ULW_COLORKEY);
However this would impact the rendering performance of the underlying game.
So, I decided to try the UpdateLayeredWindow function.
Here is one of the templates I referenced :
https://github.com/riley-x/TransparentWindow
This does not impact the performance and worked perfectly, and now I have to integrate ImGUI rendering to replace the green ellipse.
However, ImGUI uses the following code :
g_pd3dDeviceContext->OMSetRenderTargets(1, &g_mainRenderTargetView, NULL);
g_pd3dDeviceContext->ClearRenderTargetView(g_mainRenderTargetView, clear_color_with_alpha);
But in order to use the
BOOL UpdateLayeredWindow(
HWND hWnd,
HDC hdcDst,
POINT *pptDst,
SIZE *psize,
HDC hdcSrc,
POINT *pptSrc,
COLORREF crKey,
BLENDFUNCTION *pblend,
DWORD dwFlags
);
function, I have to tell ImGUI to render to an HDC.
I've thought of some possible solutions,
I'm not sure how g_mainRenderTargetView can possibly bind to an HDC, unlike ID2D1Factory::CreateDCRenderTarget which can actually bind to an HDC with BindDC.
Or I could let the ImGUI render as usual but retrieve the back buffer as a bitmap, and then do the following
HDC hdcWnd = GetDC(hwnd);
HDC hdcMem = CreateCompatibleDC(hdcWnd);
HBITMAP memBitmap = CreateCompatibleBitmap(hdcWnd, rect.right - rect.left, rect.bottom - rect.top);
SelectObject(hdcMem, memBitmap);
m_pRenderTarget->BindDC(hdcMem, &rect);
m_pRenderTarget->BeginDraw();
m_pRenderTarget->Clear({ 0 });
m_pRenderTarget->DrawBitmap(bitmap.Get()); // ???
m_pRenderTarget->EndDraw();
POINT pt0 = { 0 };
SIZE sz = { rect.right - rect.left, rect.bottom - rect.top };
BLENDFUNCTION bfunc = { 0 };
bfunc.AlphaFormat = AC_SRC_ALPHA;
bfunc.BlendFlags = 0;
bfunc.BlendOp = AC_SRC_OVER;
bfunc.SourceConstantAlpha = 255;
UpdateLayeredWindow(hwnd, hdcWnd, &pt0, &sz, hdcMem, &pt0, 0, &bfunc, ULW_COLORKEY);
DeleteObject(memBitmap);
ReleaseDC(hwnd, hdcWnd);
ReleaseDC(0, hdcMem);
where m_pRenderTarget is made from :
D2D1_RENDER_TARGET_PROPERTIES props = D2D1::RenderTargetProperties(
D2D1_RENDER_TARGET_TYPE_DEFAULT,
D2D1::PixelFormat(
DXGI_FORMAT_B8G8R8A8_UNORM,
D2D1_ALPHA_MODE_PREMULTIPLIED),
0,
0,
D2D1_RENDER_TARGET_USAGE_NONE,
D2D1_FEATURE_LEVEL_10
);
d2Factory->CreateDCRenderTarget(&props, m_pRenderTarget.GetAddressOf());
The problem is when I tried to retrieve the backbuffer bitmap, it always failed.
Or maybe there is some other way to make the overlay work without using UpdateLayeredWindow?