Low Frame Rate in OpenGL When Opening Multiple Windows, Works Fine in DX11

[imkcy9]

Version/Branch of Dear ImGui:

Version 1.91.X, Branch: master

Back-ends:

imgui_impl_glfw.h.cpp + imgui_impl_XXX.cpp

Compiler, OS:

Win11 + MSVC2022

Full config/build information:

No response

Details:

Hi everyone,

I encountered a performance issue when using ImGui with OpenGL on Windows. Specifically, when rendering multiple windows, the frame rate drops significantly in OpenGL mode. However, the same setup works perfectly fine with DirectX 11, and the frame rate remains stable.

Here’s a summary of the problem:

Frame rate drops noticeably when multiple ImGui windows are rendered in OpenGL.
Switching to DirectX 11 solves the issue, even with the same number of windows and content.
I am using the latest version of ImGui and my GPU drivers are up to date.
Could this be a bug in ImGui’s OpenGL implementation, or might it be related to OpenGL context handling or resource management? Is there any recommended way to optimize OpenGL performance in this scenario?

Any advice or insights would be greatly appreciated.

Thank you!

Screenshots/Video:

No response

Minimal, Complete and Verifiable Example code:

#include "imguiwidget.h"

ImGuiWidget::ImGuiWidget(QWidget *parent) : QOpenGLWidget(parent)
{

}

ImGuiWidget::~ImGuiWidget()
{

}

void ImGuiWidget::initializeGL() {
initializeOpenGLFunctions();
ref = QtImGui::initialize(this, false);

// Set ImGui style to light theme
ImGui::StyleColorsLight();

// Update at 60 fps
auto* timer = new QTimer(this);
connect(timer, SIGNAL(timeout()), this, SLOT(update()));
timer->start(16);

}

void ImGuiWidget::paintGL() {
QtImGui::newFrame(ref);
auto& io = ImGui::GetIO();
static bool use_work_area = false;

const ImGuiViewport* viewport = ImGui::GetMainViewport();
ImGui::SetNextWindowPos(use_work_area ? viewport->WorkPos : viewport->Pos);
ImGui::SetNextWindowSize(use_work_area ? viewport->WorkSize : viewport->Size);

onFrame(io.DeltaTime);

// Do render before ImGui UI is rendered
glViewport(0, 0, width(), height());
glClearColor(clear_color.x, clear_color.y, clear_color.z, clear_color.w);
glClear(GL_COLOR_BUFFER_BIT);

ImGui::Render();
QtImGui::render(ref);

}

ImGuiWindowFlags ImGuiWidget::defaultWindowFlags() {
return ImGuiWindowFlags_NoDecoration | ImGuiWindowFlags_NoMove | ImGuiWindowFlags_NoResize | ImGuiWindowFlags_NoSavedSettings;
}

[imkcy9]

1.Framerate is 120/fps when only one window open,

2.Framerate drops to 67/fps when opening over 40 windows.

[imkcy9]

Frame rate remains stable in 120/fps even over 100 windows opened.