Rendering result on ImGui::Image is wrong during resizing ImGuiViewport

[shinji12345]

Version/Branch of Dear ImGui:

Version 1.90.5, Branch: docking

Back-ends:

imgui_impl_dx12.cpp + imgui_impl_win32.cpp

Compiler, OS:

Windows11 + msvc2022

Full config/build information:

Dear ImGui 1.90.5 (19050)
--------------------------------
sizeof(size_t): 8, sizeof(ImDrawIdx): 2, sizeof(ImDrawVert): 20
define: __cplusplus=199711
define: _WIN32
define: _WIN64
define: _MSC_VER=1938
define: _MSVC_LANG=201402
define: IMGUI_HAS_VIEWPORT
define: IMGUI_HAS_DOCK
--------------------------------
io.BackendPlatformName: imgui_impl_win32
io.BackendRendererName: imgui_impl_dx12
io.ConfigFlags: 0x00000443
 NavEnableKeyboard
 NavEnableGamepad
 DockingEnable
 ViewportsEnable
io.ConfigViewportsNoDecoration
io.ConfigInputTextCursorBlink
io.ConfigWindowsResizeFromEdges
io.ConfigMemoryCompactTimer = 60.0
io.BackendFlags: 0x00001C0E
 HasMouseCursors
 HasSetMousePos
 PlatformHasViewports
 HasMouseHoveredViewport
 RendererHasVtxOffset
 RendererHasViewports
--------------------------------
io.Fonts: 1 fonts, Flags: 0x00000000, TexSize: 512,64
io.DisplaySize: 983.00,744.00
io.DisplayFramebufferScale: 1.00,1.00
--------------------------------
style.WindowPadding: 8.00,8.00
style.WindowBorderSize: 1.00
style.FramePadding: 4.00,3.00
style.FrameRounding: 0.00
style.FrameBorderSize: 0.00
style.ItemSpacing: 8.00,4.00
style.ItemInnerSpacing: 4.00,4.00

Details:

My Issue/Question:

I would like to display the rendering result in ImGui::Image.

If the viewport is in the main window, it can be resized without problems.

If the viewport is outside the main window, the rendering result is wrong during resizing.

Could you please let me know if there is a solution to this problem.

Screenshots/Video:

Minimal, Complete and Verifiable Example code:

// Dear ImGui: standalone example application for DirectX 12

// Learn about Dear ImGui:
// - FAQ                  https://dearimgui.com/faq
// - Getting Started      https://dearimgui.com/getting-started
// - Documentation        https://dearimgui.com/docs (same as your local docs/ folder).
// - Introduction, links and more at the top of imgui.cpp

// Important: to compile on 32-bit systems, the DirectX12 backend requires code to be compiled with '#define ImTextureID ImU64'.
// This is because we need ImTextureID to carry a 64-bit value and by default ImTextureID is defined as void*.
// This define is set in the example .vcxproj file and need to be replicated in your app or by adding it to your imconfig.h file.

#include "imgui.h"
#include "imgui_impl_win32.h"
#include "imgui_impl_dx12.h"
#include <d3d12.h>
#include <dxgi1_4.h>
#include <d3dcompiler.h>
#include <tchar.h>

#ifdef _DEBUG
#define DX12_ENABLE_DEBUG_LAYER
#endif

#ifdef DX12_ENABLE_DEBUG_LAYER
#include <dxgidebug.h>
#pragma comment(lib, "dxguid.lib")
#endif

struct FrameContext
{
    ID3D12CommandAllocator* CommandAllocator;
    UINT64                  FenceValue;
};

// Data
static int const                    NUM_FRAMES_IN_FLIGHT = 3;
static FrameContext                 g_frameContext[NUM_FRAMES_IN_FLIGHT] = {};
static UINT                         g_frameIndex = 0;

static int const                    NUM_BACK_BUFFERS = 3;
static ID3D12Device*                g_pd3dDevice = nullptr;
static ID3D12DescriptorHeap*        g_pd3dRtvDescHeap = nullptr;
static ID3D12DescriptorHeap*        g_pd3dSrvDescHeap = nullptr;
static ID3D12CommandQueue*          g_pd3dCommandQueue = nullptr;
static ID3D12GraphicsCommandList*   g_pd3dCommandList = nullptr;
static ID3D12Fence*                 g_fence = nullptr;
static HANDLE                       g_fenceEvent = nullptr;
static UINT64                       g_fenceLastSignaledValue = 0;
static IDXGISwapChain3*             g_pSwapChain = nullptr;
static HANDLE                       g_hSwapChainWaitableObject = nullptr;
static ID3D12Resource*              g_mainRenderTargetResource[NUM_BACK_BUFFERS] = {};
static D3D12_CPU_DESCRIPTOR_HANDLE  g_mainRenderTargetDescriptor[NUM_BACK_BUFFERS] = {};

// Forward declarations of helper functions
void CreateFramebuffer(UINT64 w, UINT64 h, ID3D12Resource** framebuffer, ID3D12DescriptorHeap* ColorTargetDescHeap, ID3D12DescriptorHeap* TextureDescHeap);
bool CreateDeviceD3D(HWND hWnd);
void CleanupDeviceD3D();
void CreateRenderTarget();
void CleanupRenderTarget();
void WaitForLastSubmittedFrame();
FrameContext* WaitForNextFrameResources();
LRESULT WINAPI WndProc(HWND hWnd, UINT msg, WPARAM wParam, LPARAM lParam);

// Main code
int main(int, char**)
{
    // Create application window
    //ImGui_ImplWin32_EnableDpiAwareness();
    WNDCLASSEXW wc = { sizeof(wc), CS_CLASSDC, WndProc, 0L, 0L, GetModuleHandle(nullptr), nullptr, nullptr, nullptr, nullptr, L"ImGui Example", nullptr };
    ::RegisterClassExW(&wc);
    HWND hwnd = ::CreateWindowW(wc.lpszClassName, L"Dear ImGui DirectX12 Example", WS_OVERLAPPEDWINDOW, 100, 100, 1280, 800, nullptr, nullptr, wc.hInstance, nullptr);

    // Initialize Direct3D
    if (!CreateDeviceD3D(hwnd))
    {
        CleanupDeviceD3D();
        ::UnregisterClassW(wc.lpszClassName, wc.hInstance);
        return 1;
    }

    // Show the window
    ::ShowWindow(hwnd, SW_SHOWDEFAULT);
    ::UpdateWindow(hwnd);

    // Setup Dear ImGui context
    IMGUI_CHECKVERSION();
    ImGui::CreateContext();
    ImGuiIO& io = ImGui::GetIO(); (void)io;
    io.ConfigFlags |= ImGuiConfigFlags_NavEnableKeyboard;     // Enable Keyboard Controls
    io.ConfigFlags |= ImGuiConfigFlags_NavEnableGamepad;      // Enable Gamepad Controls
    io.ConfigFlags |= ImGuiConfigFlags_DockingEnable;         // Enable Docking
    io.ConfigFlags |= ImGuiConfigFlags_ViewportsEnable;       // Enable Multi-Viewport / Platform Windows
    //io.ConfigViewportsNoAutoMerge = true;
    //io.ConfigViewportsNoTaskBarIcon = true;

    // Setup Dear ImGui style
    ImGui::StyleColorsDark();
    //ImGui::StyleColorsLight();

    // When viewports are enabled we tweak WindowRounding/WindowBg so platform windows can look identical to regular ones.
    ImGuiStyle& style = ImGui::GetStyle();
    if (io.ConfigFlags & ImGuiConfigFlags_ViewportsEnable)
    {
        style.WindowRounding = 0.0f;
        style.Colors[ImGuiCol_WindowBg].w = 1.0f;
    }

    // Setup Platform/Renderer backends
    ImGui_ImplWin32_Init(hwnd);
    ImGui_ImplDX12_Init(g_pd3dDevice, NUM_FRAMES_IN_FLIGHT,
        DXGI_FORMAT_R8G8B8A8_UNORM, g_pd3dSrvDescHeap,
        g_pd3dSrvDescHeap->GetCPUDescriptorHandleForHeapStart(),
        g_pd3dSrvDescHeap->GetGPUDescriptorHandleForHeapStart());

    // Load Fonts
    // - If no fonts are loaded, dear imgui will use the default font. You can also load multiple fonts and use ImGui::PushFont()/PopFont() to select them.
    // - AddFontFromFileTTF() will return the ImFont* so you can store it if you need to select the font among multiple.
    // - If the file cannot be loaded, the function will return a nullptr. Please handle those errors in your application (e.g. use an assertion, or display an error and quit).
    // - The fonts will be rasterized at a given size (w/ oversampling) and stored into a texture when calling ImFontAtlas::Build()/GetTexDataAsXXXX(), which ImGui_ImplXXXX_NewFrame below will call.
    // - Use '#define IMGUI_ENABLE_FREETYPE' in your imconfig file to use Freetype for higher quality font rendering.
    // - Read 'docs/FONTS.md' for more instructions and details.
    // - Remember that in C/C++ if you want to include a backslash \ in a string literal you need to write a double backslash \\ !
    //io.Fonts->AddFontDefault();
    //io.Fonts->AddFontFromFileTTF("c:\\Windows\\Fonts\\segoeui.ttf", 18.0f);
    //io.Fonts->AddFontFromFileTTF("../../misc/fonts/DroidSans.ttf", 16.0f);
    //io.Fonts->AddFontFromFileTTF("../../misc/fonts/Roboto-Medium.ttf", 16.0f);
    //io.Fonts->AddFontFromFileTTF("../../misc/fonts/Cousine-Regular.ttf", 15.0f);
    //ImFont* font = io.Fonts->AddFontFromFileTTF("c:\\Windows\\Fonts\\ArialUni.ttf", 18.0f, nullptr, io.Fonts->GetGlyphRangesJapanese());
    //IM_ASSERT(font != nullptr);

    // Our state
    bool show_demo_window = true;
    bool show_another_window = false;
    ImVec4 clear_color = ImVec4(0.45f, 0.55f, 0.60f, 1.00f);

    UINT64 ColorTargetWidth = 0, ColorTargetHeight = 0;
    ID3D12Resource* ColorTargetResource{ nullptr };
    ID3D12DescriptorHeap* ColorTargetDescHeap{ nullptr };
    ID3D12DescriptorHeap* TextureDescHeap{ nullptr };
    {
        D3D12_DESCRIPTOR_HEAP_DESC desc = {};
        desc.Type = D3D12_DESCRIPTOR_HEAP_TYPE_RTV;
        desc.NumDescriptors = 1;
        desc.Flags = D3D12_DESCRIPTOR_HEAP_FLAG_NONE;
        g_pd3dDevice->CreateDescriptorHeap(&desc, IID_PPV_ARGS(&ColorTargetDescHeap));

        desc.Type = D3D12_DESCRIPTOR_HEAP_TYPE_CBV_SRV_UAV;
        g_pd3dDevice->CreateDescriptorHeap(&desc, IID_PPV_ARGS(&TextureDescHeap));
    }
    const UINT64 GlobalHeapStart = 1;
    const UINT64 GlobalHeapLast = 1'000'000 - 1;
    UINT64 GlobalHeapOffset = GlobalHeapStart;
    const UINT HeapIncementSize = g_pd3dDevice->GetDescriptorHandleIncrementSize(D3D12_DESCRIPTOR_HEAP_TYPE_CBV_SRV_UAV);

    ID3D12RootSignature* signature{ nullptr };
    {
        D3D12_VERSIONED_ROOT_SIGNATURE_DESC desc{};
        desc.Version = D3D_ROOT_SIGNATURE_VERSION_1_0;
        desc.Desc_1_0.NumParameters = 0;
        desc.Desc_1_0.pParameters = nullptr;
        desc.Desc_1_0.NumStaticSamplers = 0;
        desc.Desc_1_0.pStaticSamplers = nullptr;
        desc.Desc_1_0.Flags = D3D12_ROOT_SIGNATURE_FLAG_NONE;

        ID3DBlob* blob{ nullptr }, * error{ nullptr };
        D3D12SerializeVersionedRootSignature(&desc, &blob, &error);
        g_pd3dDevice->CreateRootSignature(0, blob->GetBufferPointer(), blob->GetBufferSize(), IID_PPV_ARGS(&signature));
        if (blob) blob->Release();
        if (error) error->Release();
    }
    ID3D12PipelineState* PipelineState{ nullptr };
    {
        ID3DBlob* vs{ nullptr };
        ID3DBlob* ps{ nullptr };
        {
            static const char* vs_hlsl = R"(
            float4 main(uint VertexID : SV_VertexID) : SV_POSITION 
            {
                float4 Pos[3];
                Pos[0] = float4(-0.5, -0.5, 0.0, 1.0);
                Pos[1] = float4(+0.5, -0.5, 0.0, 1.0);
                Pos[2] = float4( 0.0, +0.5, 0.0, 1.0);

                return Pos[VertexID];
            })";
            static const char* ps_hlsl = R"(
            float4 main() : SV_TARGET0
            {
                return float4(0, 1, 0, 1); 
            })";
            D3DCompile(vs_hlsl, strlen(vs_hlsl), nullptr, nullptr, nullptr, "main", "vs_5_0", 0, 0, &vs, nullptr);
            D3DCompile(ps_hlsl, strlen(ps_hlsl), nullptr, nullptr, nullptr, "main", "ps_5_0", 0, 0, &ps, nullptr);
        }
        D3D12_GRAPHICS_PIPELINE_STATE_DESC desc{};
        desc.pRootSignature = signature;
        desc.VS = { vs->GetBufferPointer(), vs->GetBufferSize() };
        desc.PS = { ps->GetBufferPointer(), ps->GetBufferSize() };
        desc.BlendState.AlphaToCoverageEnable = FALSE;
        desc.BlendState.IndependentBlendEnable = FALSE;
        desc.BlendState.RenderTarget[0].BlendEnable = FALSE;
        desc.BlendState.RenderTarget[0].LogicOpEnable = FALSE;
        desc.BlendState.RenderTarget[0].RenderTargetWriteMask = D3D12_COLOR_WRITE_ENABLE_ALL;
        desc.SampleMask = D3D12_DEFAULT_SAMPLE_MASK;
        desc.RasterizerState.MultisampleEnable = false;
        desc.RasterizerState.CullMode = D3D12_CULL_MODE_NONE;
        desc.RasterizerState.FillMode = D3D12_FILL_MODE_SOLID;
        desc.RasterizerState.DepthClipEnable = true;
        desc.DepthStencilState.DepthEnable = FALSE;
        desc.InputLayout.pInputElementDescs = nullptr;
        desc.InputLayout.NumElements = 0;
        desc.IBStripCutValue = D3D12_INDEX_BUFFER_STRIP_CUT_VALUE_DISABLED;
        desc.PrimitiveTopologyType = D3D12_PRIMITIVE_TOPOLOGY_TYPE_TRIANGLE;
        desc.NumRenderTargets = 1;
        desc.RTVFormats[0] = DXGI_FORMAT_R8G8B8A8_UNORM;
        desc.SampleDesc.Count = 1;
        desc.NodeMask = 0;
        desc.Flags = D3D12_PIPELINE_STATE_FLAG_NONE;
        g_pd3dDevice->CreateGraphicsPipelineState(&desc, IID_PPV_ARGS(&PipelineState));
        assert(PipelineState);
        vs->Release();
        ps->Release();
    }

    // Main loop
    bool done = false;
    while (!done)
    {
        // Poll and handle messages (inputs, window resize, etc.)
        // See the WndProc() function below for our to dispatch events to the Win32 backend.
        MSG msg;
        while (::PeekMessage(&msg, nullptr, 0U, 0U, PM_REMOVE))
        {
            ::TranslateMessage(&msg);
            ::DispatchMessage(&msg);
            if (msg.message == WM_QUIT)
                done = true;
        }
        if (done)
            break;

        // Start the Dear ImGui frame
        ImGui_ImplDX12_NewFrame();
        ImGui_ImplWin32_NewFrame();
        ImGui::NewFrame();

        if (ImGui::Begin("Viewport"))
        {
            if (ImGui::BeginChild("ViewportChild"))
            {
                // Resize ColorTarget
                const ImVec2 window_size = ImGui::GetContentRegionAvail();

                if (ColorTargetWidth != window_size.x || ColorTargetHeight != window_size.y)
                {
                    static int count = 0;
                    ColorTargetWidth = window_size.x;
                    ColorTargetHeight = window_size.y;
                    CreateFramebuffer(ColorTargetWidth, ColorTargetHeight, &ColorTargetResource, ColorTargetDescHeap, TextureDescHeap);
                    ColorTargetResource->SetName(L"ColorTarget");
                }

                // Copy to shader visible descriptor heap
                auto CpuHandleShaderVisibleHeap = g_pd3dSrvDescHeap->GetCPUDescriptorHandleForHeapStart();
                CpuHandleShaderVisibleHeap.ptr += GlobalHeapOffset * HeapIncementSize;
                g_pd3dDevice->CopyDescriptorsSimple(1, CpuHandleShaderVisibleHeap,
                    TextureDescHeap->GetCPUDescriptorHandleForHeapStart(), D3D12_DESCRIPTOR_HEAP_TYPE_CBV_SRV_UAV);

                // Image
                auto GpuHandleShaderVisibleHeap = g_pd3dSrvDescHeap->GetGPUDescriptorHandleForHeapStart();
                GpuHandleShaderVisibleHeap.ptr += GlobalHeapOffset * HeapIncementSize;
                ImGui::Image((ImTextureID)(GpuHandleShaderVisibleHeap.ptr), window_size);

                GlobalHeapOffset = GlobalHeapOffset == GlobalHeapLast ? GlobalHeapStart : GlobalHeapOffset + 1;
            }
            ImGui::EndChild();
        }

        ImGui::ShowDemoWindow();
        ImGui::End();

        // Rendering
        ImGui::Render();

        FrameContext* frameCtx = WaitForNextFrameResources();
        UINT backBufferIdx = g_pSwapChain->GetCurrentBackBufferIndex();
        frameCtx->CommandAllocator->Reset();

        D3D12_RESOURCE_BARRIER barrier = {};
        barrier.Type                   = D3D12_RESOURCE_BARRIER_TYPE_TRANSITION;
        barrier.Flags                  = D3D12_RESOURCE_BARRIER_FLAG_NONE;
        barrier.Transition.pResource   = g_mainRenderTargetResource[backBufferIdx];
        barrier.Transition.Subresource = D3D12_RESOURCE_BARRIER_ALL_SUBRESOURCES;
        barrier.Transition.StateBefore = D3D12_RESOURCE_STATE_PRESENT;
        barrier.Transition.StateAfter  = D3D12_RESOURCE_STATE_RENDER_TARGET;
        g_pd3dCommandList->Reset(frameCtx->CommandAllocator, nullptr);
        g_pd3dCommandList->ResourceBarrier(1, &barrier);

        g_pd3dCommandList->SetGraphicsRootSignature(signature);
        g_pd3dCommandList->SetDescriptorHeaps(1, &g_pd3dSrvDescHeap);
        {
            D3D12_RESOURCE_BARRIER desc[1]{};
            desc[0].Type = D3D12_RESOURCE_BARRIER_TYPE_TRANSITION;
            desc[0].Flags = D3D12_RESOURCE_BARRIER_FLAG_NONE;
            desc[0].Transition.pResource = ColorTargetResource;
            desc[0].Transition.StateBefore = D3D12_RESOURCE_STATE_PIXEL_SHADER_RESOURCE;
            desc[0].Transition.StateAfter = D3D12_RESOURCE_STATE_RENDER_TARGET;
            desc[0].Transition.Subresource = D3D12_RESOURCE_BARRIER_ALL_SUBRESOURCES;
            g_pd3dCommandList->ResourceBarrier(_countof(desc), desc);

            D3D12_VIEWPORT viewport{};
            viewport.Width = ColorTargetWidth;
            viewport.Height = ColorTargetHeight;
            viewport.MinDepth = 0.0;
            viewport.MaxDepth = 1.0f;
            viewport.TopLeftX = 0.0f;
            viewport.TopLeftY = 0.0f;
            g_pd3dCommandList->RSSetViewports(1, &viewport);

            D3D12_RECT rect{};
            rect.left = 0;
            rect.top = 0;
            rect.right = LONG(ColorTargetWidth);
            rect.bottom = LONG(ColorTargetHeight);
            g_pd3dCommandList->RSSetScissorRects(1, &rect);

            const D3D12_CPU_DESCRIPTOR_HANDLE rtvs[]{ ColorTargetDescHeap->GetCPUDescriptorHandleForHeapStart() };
            const FLOAT ClearColor[4]{ 1.0f, 0.0f, 0.0f, 1.0f };
            g_pd3dCommandList->ClearRenderTargetView(rtvs[0], ClearColor, 0, nullptr);
            g_pd3dCommandList->OMSetRenderTargets(_countof(rtvs), rtvs, false, nullptr);

            g_pd3dCommandList->IASetPrimitiveTopology(D3D_PRIMITIVE_TOPOLOGY_TRIANGLESTRIP);
            g_pd3dCommandList->SetPipelineState(PipelineState);
            g_pd3dCommandList->DrawInstanced(3, 1, 0, 0);

            desc[0].Transition.StateBefore = D3D12_RESOURCE_STATE_RENDER_TARGET;
            desc[0].Transition.StateAfter = D3D12_RESOURCE_STATE_PIXEL_SHADER_RESOURCE;
            g_pd3dCommandList->ResourceBarrier(_countof(desc), desc);
        }

        // Render Dear ImGui graphics
        const float clear_color_with_alpha[4] = { clear_color.x * clear_color.w, clear_color.y * clear_color.w, clear_color.z * clear_color.w, clear_color.w };
        g_pd3dCommandList->ClearRenderTargetView(g_mainRenderTargetDescriptor[backBufferIdx], clear_color_with_alpha, 0, nullptr);
        g_pd3dCommandList->OMSetRenderTargets(1, &g_mainRenderTargetDescriptor[backBufferIdx], FALSE, nullptr);
        g_pd3dCommandList->SetDescriptorHeaps(1, &g_pd3dSrvDescHeap);
        ImGui_ImplDX12_RenderDrawData(ImGui::GetDrawData(), g_pd3dCommandList);
        barrier.Transition.StateBefore = D3D12_RESOURCE_STATE_RENDER_TARGET;
        barrier.Transition.StateAfter  = D3D12_RESOURCE_STATE_PRESENT;
        g_pd3dCommandList->ResourceBarrier(1, &barrier);
        g_pd3dCommandList->Close();

        g_pd3dCommandQueue->ExecuteCommandLists(1, (ID3D12CommandList* const*)&g_pd3dCommandList);

        // Update and Render additional Platform Windows
        if (io.ConfigFlags & ImGuiConfigFlags_ViewportsEnable)
        {
            ImGui::UpdatePlatformWindows();
            ImGui::RenderPlatformWindowsDefault(nullptr, (void*)g_pd3dCommandList);
        }

        g_pSwapChain->Present(1, 0); // Present with vsync
        //g_pSwapChain->Present(0, 0); // Present without vsync

        UINT64 fenceValue = g_fenceLastSignaledValue + 1;
        g_pd3dCommandQueue->Signal(g_fence, fenceValue);
        g_fenceLastSignaledValue = fenceValue;
        frameCtx->FenceValue = fenceValue;
    }

    WaitForLastSubmittedFrame();

    // Cleanup
    ImGui_ImplDX12_Shutdown();
    ImGui_ImplWin32_Shutdown();
    ImGui::DestroyContext();

    CleanupDeviceD3D();
    ::DestroyWindow(hwnd);
    ::UnregisterClassW(wc.lpszClassName, wc.hInstance);

    return 0;
}

void CreateFramebuffer(UINT64 w, UINT64 h, ID3D12Resource** framebuffer,
    ID3D12DescriptorHeap* ColorTargetDescHeap, ID3D12DescriptorHeap* TextureDescHeap)
{
    HRESULT hr{};
    {
        D3D12_HEAP_PROPERTIES heap{};
        heap.Type = D3D12_HEAP_TYPE_DEFAULT;

        D3D12_RESOURCE_DESC desc{};
        desc.Dimension = D3D12_RESOURCE_DIMENSION_TEXTURE2D;
        desc.Alignment = 0;
        desc.Width = w;
        desc.Height = h;
        desc.DepthOrArraySize = 1;
        desc.MipLevels = 1;
        desc.Format = DXGI_FORMAT_R8G8B8A8_UNORM;
        desc.SampleDesc.Count = 1;
        desc.SampleDesc.Quality = 0;
        desc.Flags = D3D12_RESOURCE_FLAG_ALLOW_RENDER_TARGET;
        desc.Layout = D3D12_TEXTURE_LAYOUT_UNKNOWN;

        D3D12_CLEAR_VALUE clear_color{};
        clear_color.Format = DXGI_FORMAT_R8G8B8A8_UNORM;
        clear_color.Color[0] = 1.0f;
        clear_color.Color[1] = 0.0f;
        clear_color.Color[2] = 0.0f;
        clear_color.Color[3] = 1.0f;

        g_pd3dDevice->CreateCommittedResource(
            &heap, D3D12_HEAP_FLAG_NONE, &desc,
            D3D12_RESOURCE_STATE_PIXEL_SHADER_RESOURCE,
            &clear_color, IID_PPV_ARGS(framebuffer));
    }

    {
        D3D12_RENDER_TARGET_VIEW_DESC desc{};
        desc.Format = DXGI_FORMAT_R8G8B8A8_UNORM;
        desc.ViewDimension = D3D12_RTV_DIMENSION_TEXTURE2D;
        g_pd3dDevice->CreateRenderTargetView(*framebuffer, &desc, ColorTargetDescHeap->GetCPUDescriptorHandleForHeapStart());
    }
    {
        D3D12_SHADER_RESOURCE_VIEW_DESC desc{};
        desc.Format = DXGI_FORMAT_R8G8B8A8_UNORM;
        desc.ViewDimension = D3D12_SRV_DIMENSION_TEXTURE2D;
        desc.Texture2D.MipLevels = 1;
        desc.Shader4ComponentMapping = D3D12_DEFAULT_SHADER_4_COMPONENT_MAPPING;
        g_pd3dDevice->CreateShaderResourceView(*framebuffer, &desc, TextureDescHeap->GetCPUDescriptorHandleForHeapStart());
    }
}

// Helper functions
bool CreateDeviceD3D(HWND hWnd)
{
    // Setup swap chain
    DXGI_SWAP_CHAIN_DESC1 sd;
    {
        ZeroMemory(&sd, sizeof(sd));
        sd.BufferCount = NUM_BACK_BUFFERS;
        sd.Width = 0;
        sd.Height = 0;
        sd.Format = DXGI_FORMAT_R8G8B8A8_UNORM;
        sd.Flags = DXGI_SWAP_CHAIN_FLAG_FRAME_LATENCY_WAITABLE_OBJECT;
        sd.BufferUsage = DXGI_USAGE_RENDER_TARGET_OUTPUT;
        sd.SampleDesc.Count = 1;
        sd.SampleDesc.Quality = 0;
        sd.SwapEffect = DXGI_SWAP_EFFECT_FLIP_DISCARD;
        sd.AlphaMode = DXGI_ALPHA_MODE_UNSPECIFIED;
        sd.Scaling = DXGI_SCALING_STRETCH;
        sd.Stereo = FALSE;
    }

    // [DEBUG] Enable debug interface
#ifdef DX12_ENABLE_DEBUG_LAYER
    ID3D12Debug* pdx12Debug = nullptr;
    if (SUCCEEDED(D3D12GetDebugInterface(IID_PPV_ARGS(&pdx12Debug))))
        pdx12Debug->EnableDebugLayer();
#endif

    // Create device
    D3D_FEATURE_LEVEL featureLevel = D3D_FEATURE_LEVEL_11_0;
    if (D3D12CreateDevice(nullptr, featureLevel, IID_PPV_ARGS(&g_pd3dDevice)) != S_OK)
        return false;

    // [DEBUG] Setup debug interface to break on any warnings/errors
#ifdef DX12_ENABLE_DEBUG_LAYER
    if (pdx12Debug != nullptr)
    {
        ID3D12InfoQueue* pInfoQueue = nullptr;
        g_pd3dDevice->QueryInterface(IID_PPV_ARGS(&pInfoQueue));
        pInfoQueue->SetBreakOnSeverity(D3D12_MESSAGE_SEVERITY_ERROR, true);
        pInfoQueue->SetBreakOnSeverity(D3D12_MESSAGE_SEVERITY_CORRUPTION, true);
        pInfoQueue->SetBreakOnSeverity(D3D12_MESSAGE_SEVERITY_WARNING, true);
        pInfoQueue->Release();
        pdx12Debug->Release();
    }
#endif

    {
        D3D12_DESCRIPTOR_HEAP_DESC desc = {};
        desc.Type = D3D12_DESCRIPTOR_HEAP_TYPE_RTV;
        desc.NumDescriptors = NUM_BACK_BUFFERS;
        desc.Flags = D3D12_DESCRIPTOR_HEAP_FLAG_NONE;
        desc.NodeMask = 1;
        if (g_pd3dDevice->CreateDescriptorHeap(&desc, IID_PPV_ARGS(&g_pd3dRtvDescHeap)) != S_OK)
            return false;

        SIZE_T rtvDescriptorSize = g_pd3dDevice->GetDescriptorHandleIncrementSize(D3D12_DESCRIPTOR_HEAP_TYPE_RTV);
        D3D12_CPU_DESCRIPTOR_HANDLE rtvHandle = g_pd3dRtvDescHeap->GetCPUDescriptorHandleForHeapStart();
        for (UINT i = 0; i < NUM_BACK_BUFFERS; i++)
        {
            g_mainRenderTargetDescriptor[i] = rtvHandle;
            rtvHandle.ptr += rtvDescriptorSize;
        }
    }

    {
        D3D12_DESCRIPTOR_HEAP_DESC desc = {};
        desc.Type = D3D12_DESCRIPTOR_HEAP_TYPE_CBV_SRV_UAV;
        //desc.NumDescriptors = 1;
        desc.NumDescriptors = 1'000'000;
        desc.Flags = D3D12_DESCRIPTOR_HEAP_FLAG_SHADER_VISIBLE;
        if (g_pd3dDevice->CreateDescriptorHeap(&desc, IID_PPV_ARGS(&g_pd3dSrvDescHeap)) != S_OK)
            return false;
    }

    {
        D3D12_COMMAND_QUEUE_DESC desc = {};
        desc.Type = D3D12_COMMAND_LIST_TYPE_DIRECT;
        desc.Flags = D3D12_COMMAND_QUEUE_FLAG_NONE;
        desc.NodeMask = 1;
        if (g_pd3dDevice->CreateCommandQueue(&desc, IID_PPV_ARGS(&g_pd3dCommandQueue)) != S_OK)
            return false;
    }

    for (UINT i = 0; i < NUM_FRAMES_IN_FLIGHT; i++)
        if (g_pd3dDevice->CreateCommandAllocator(D3D12_COMMAND_LIST_TYPE_DIRECT, IID_PPV_ARGS(&g_frameContext[i].CommandAllocator)) != S_OK)
            return false;

    if (g_pd3dDevice->CreateCommandList(0, D3D12_COMMAND_LIST_TYPE_DIRECT, g_frameContext[0].CommandAllocator, nullptr, IID_PPV_ARGS(&g_pd3dCommandList)) != S_OK ||
        g_pd3dCommandList->Close() != S_OK)
        return false;

    if (g_pd3dDevice->CreateFence(0, D3D12_FENCE_FLAG_NONE, IID_PPV_ARGS(&g_fence)) != S_OK)
        return false;

    g_fenceEvent = CreateEvent(nullptr, FALSE, FALSE, nullptr);
    if (g_fenceEvent == nullptr)
        return false;

    {
        IDXGIFactory4* dxgiFactory = nullptr;
        IDXGISwapChain1* swapChain1 = nullptr;
        if (CreateDXGIFactory1(IID_PPV_ARGS(&dxgiFactory)) != S_OK)
            return false;
        if (dxgiFactory->CreateSwapChainForHwnd(g_pd3dCommandQueue, hWnd, &sd, nullptr, nullptr, &swapChain1) != S_OK)
            return false;
        if (swapChain1->QueryInterface(IID_PPV_ARGS(&g_pSwapChain)) != S_OK)
            return false;
        swapChain1->Release();
        dxgiFactory->Release();
        g_pSwapChain->SetMaximumFrameLatency(NUM_BACK_BUFFERS);
        g_hSwapChainWaitableObject = g_pSwapChain->GetFrameLatencyWaitableObject();
    }

    CreateRenderTarget();
    return true;
}

void CleanupDeviceD3D()
{
    CleanupRenderTarget();
    if (g_pSwapChain) { g_pSwapChain->SetFullscreenState(false, nullptr); g_pSwapChain->Release(); g_pSwapChain = nullptr; }
    if (g_hSwapChainWaitableObject != nullptr) { CloseHandle(g_hSwapChainWaitableObject); }
    for (UINT i = 0; i < NUM_FRAMES_IN_FLIGHT; i++)
        if (g_frameContext[i].CommandAllocator) { g_frameContext[i].CommandAllocator->Release(); g_frameContext[i].CommandAllocator = nullptr; }
    if (g_pd3dCommandQueue) { g_pd3dCommandQueue->Release(); g_pd3dCommandQueue = nullptr; }
    if (g_pd3dCommandList) { g_pd3dCommandList->Release(); g_pd3dCommandList = nullptr; }
    if (g_pd3dRtvDescHeap) { g_pd3dRtvDescHeap->Release(); g_pd3dRtvDescHeap = nullptr; }
    if (g_pd3dSrvDescHeap) { g_pd3dSrvDescHeap->Release(); g_pd3dSrvDescHeap = nullptr; }
    if (g_fence) { g_fence->Release(); g_fence = nullptr; }
    if (g_fenceEvent) { CloseHandle(g_fenceEvent); g_fenceEvent = nullptr; }
    if (g_pd3dDevice) { g_pd3dDevice->Release(); g_pd3dDevice = nullptr; }

#ifdef DX12_ENABLE_DEBUG_LAYER
    IDXGIDebug1* pDebug = nullptr;
    //if (SUCCEEDED(DXGIGetDebugInterface1(0, IID_PPV_ARGS(&pDebug))))
    //{
    //    pDebug->ReportLiveObjects(DXGI_DEBUG_ALL, DXGI_DEBUG_RLO_SUMMARY);
    //    pDebug->Release();
    //}
#endif
}

void CreateRenderTarget()
{
    for (UINT i = 0; i < NUM_BACK_BUFFERS; i++)
    {
        ID3D12Resource* pBackBuffer = nullptr;
        g_pSwapChain->GetBuffer(i, IID_PPV_ARGS(&pBackBuffer));
        g_pd3dDevice->CreateRenderTargetView(pBackBuffer, nullptr, g_mainRenderTargetDescriptor[i]);
        g_mainRenderTargetResource[i] = pBackBuffer;
    }
}

void CleanupRenderTarget()
{
    WaitForLastSubmittedFrame();

    for (UINT i = 0; i < NUM_BACK_BUFFERS; i++)
        if (g_mainRenderTargetResource[i]) { g_mainRenderTargetResource[i]->Release(); g_mainRenderTargetResource[i] = nullptr; }
}

void WaitForLastSubmittedFrame()
{
    FrameContext* frameCtx = &g_frameContext[g_frameIndex % NUM_FRAMES_IN_FLIGHT];

    UINT64 fenceValue = frameCtx->FenceValue;
    if (fenceValue == 0)
        return; // No fence was signaled

    frameCtx->FenceValue = 0;
    if (g_fence->GetCompletedValue() >= fenceValue)
        return;

    g_fence->SetEventOnCompletion(fenceValue, g_fenceEvent);
    WaitForSingleObject(g_fenceEvent, INFINITE);
}

FrameContext* WaitForNextFrameResources()
{
    UINT nextFrameIndex = g_frameIndex + 1;
    g_frameIndex = nextFrameIndex;

    HANDLE waitableObjects[] = { g_hSwapChainWaitableObject, nullptr };
    DWORD numWaitableObjects = 1;

    FrameContext* frameCtx = &g_frameContext[nextFrameIndex % NUM_FRAMES_IN_FLIGHT];
    UINT64 fenceValue = frameCtx->FenceValue;
    if (fenceValue != 0) // means no fence was signaled
    {
        frameCtx->FenceValue = 0;
        g_fence->SetEventOnCompletion(fenceValue, g_fenceEvent);
        waitableObjects[1] = g_fenceEvent;
        numWaitableObjects = 2;
    }

    WaitForMultipleObjects(numWaitableObjects, waitableObjects, TRUE, INFINITE);

    return frameCtx;
}

// Forward declare message handler from imgui_impl_win32.cpp
extern IMGUI_IMPL_API LRESULT ImGui_ImplWin32_WndProcHandler(HWND hWnd, UINT msg, WPARAM wParam, LPARAM lParam);

// Win32 message handler
// You can read the io.WantCaptureMouse, io.WantCaptureKeyboard flags to tell if dear imgui wants to use your inputs.
// - When io.WantCaptureMouse is true, do not dispatch mouse input data to your main application, or clear/overwrite your copy of the mouse data.
// - When io.WantCaptureKeyboard is true, do not dispatch keyboard input data to your main application, or clear/overwrite your copy of the keyboard data.
// Generally you may always pass all inputs to dear imgui, and hide them from your application based on those two flags.
LRESULT WINAPI WndProc(HWND hWnd, UINT msg, WPARAM wParam, LPARAM lParam)
{
    if (ImGui_ImplWin32_WndProcHandler(hWnd, msg, wParam, lParam))
        return true;

    switch (msg)
    {
    case WM_SIZE:
        if (g_pd3dDevice != nullptr && wParam != SIZE_MINIMIZED)
        {
            WaitForLastSubmittedFrame();
            CleanupRenderTarget();
            HRESULT result = g_pSwapChain->ResizeBuffers(0, (UINT)LOWORD(lParam), (UINT)HIWORD(lParam), DXGI_FORMAT_UNKNOWN, DXGI_SWAP_CHAIN_FLAG_FRAME_LATENCY_WAITABLE_OBJECT);
            assert(SUCCEEDED(result) && "Failed to resize swapchain.");
            CreateRenderTarget();
        }
        return 0;
    case WM_SYSCOMMAND:
        if ((wParam & 0xfff0) == SC_KEYMENU) // Disable ALT application menu
            return 0;
        break;
    case WM_DESTROY:
        ::PostQuitMessage(0);
        return 0;
    }
    return ::DefWindowProcW(hWnd, msg, wParam, lParam);
}

[shinji12345]

I append.

I assume it is a CommandQueues synchronization issue.

For a single window, there is one CommandQueue.

Therefore, GUI is drawed after drawing of a triangle.

On the other hand, for multiple windows, I think it is not guaranteed.

The CommandQueue of ImGuiViewport should wait for a signal from my CommandQueue.

But the CommandQueue of ImGuiViewport is hidden in imgui_impl_dx12.cpp