Handle OS window resizing when using Dear ImGui (DX11 backend) - c++

Question

I am trying to make a simple ImGui Win32 app with Visual C++.
I noticed a problem, that when I try to resize my host window,
the ImGui window gets stretched and the Widgets (also other stuff like lines) are deformed.
An example:
I am drawing a line from 0, 0 to 200, 200.
Theoretically, this line should always have a 45 degrees angle to the bottom of the window, but instead it stretches around when resizing the window.
I am pretty sure that this is not a D3D11 issue, because I have already worked with D3D11 a couple of times before and I know that it handles window resizing without any additional work/code.
I was looking through the imgui docs for a few days now, but I couldn't find any answer to my problem. This is really frustrating, as I thought this would be a trivial thing for such a well known framework.
Here's the rendering code:
std::optional<int> Window::beginRender() noexcept
{
if (const auto code = Window::processMessages())
return *code;
ImGui_ImplDX11_NewFrame();
ImGui_ImplWin32_NewFrame();
ImGui::NewFrame();
return {};
}
void Window::render(const char* windowTitle) noexcept
{
ImGui::SetNextWindowPos({ 0, 0 });
ImGui::SetNextWindowSize({ static_cast<float>(m_width), static_cast<float>(m_height) });
ImVec2 test{ ImGui::GetWindowSize() };
ImVec2 test2{ ImGui::GetContentRegionAvail() };
ImGui::Begin(windowTitle, nullptr, /*ImGuiWindowFlags_NoResize |*/ ImGuiWindowFlags_AlwaysAutoResize | ImGuiWindowFlags_NoSavedSettings | ImGuiWindowFlags_NoCollapse | ImGuiWindowFlags_NoMove | ImGuiWindowFlags_NoBackground | ImGuiWindowFlags_NoTitleBar);
ImGui::ShowDemoWindow();
ImGui::GetBackgroundDrawList()->AddLine(ImVec2{ 0.0f, 0.0f }, ImVec2{ 200.0f, 200.0f }, 0xFF0000FF);
ImGui::End();
}
void Window::endRender() noexcept
{
ImGui::EndFrame();
ImGui::Render();
constexpr float color[]{ 0.0f, 0.0f, 0.0f, 0.0f };
m_pContext->OMSetRenderTargets(1, &m_pRTV, nullptr);
m_pContext->ClearRenderTargetView(m_pRTV, color);
updateTargetData();
MoveWindow(m_hWnd, m_position.x, m_position.y, m_width, m_height, false);
//ImGuiIO& io{ ::ImGui::GetIO() };
//io.DisplaySize.x = m_width;
//io.DisplaySize.y = m_height;
ImGui_ImplDX11_RenderDrawData(ImGui::GetDrawData());
m_pSwapChain->Present(1, 0);
}
(About the App: It's tracking another window (notepad.exe), so I am actually resizing the target and then use MoveWindow() for resizing. But I guess this doesn't make any difference)
So how do I fix this issue? How do I correctly handle window resizing with ImGui?

Answer 1

In your Window Procedure Function, when you resize, you'll need to recreate the render target view, as such:
case WM_SIZE:
{
pMainRendertargetView->Release()
pSwapChain->ResizeBuffers(0, LOWORD(lParam), HIWORD(lParam), DXGI_FORMAT_UNKNOWN, 0);
ID3D11Texture2D* pBackBuffer;
pSwapChain->GetBuffer(0, IID_PPV_ARGS(&pBackBuffer));
pd3dDevice->CreateRenderTargetView(pBackBuffer, NULL, pMainRenderTargetView);
pBackBuffer->Release();
// Change Viewport Here
}