I am trying to create an app, where the user can run a (time-consuming) backend physics simulation with the press of a button. But at the same time, I would like the rest of the GUI to remain functional and not to “freeze” until the simulation is over. Now I know that it is not a good idea to combine many threads for rendering in OpenGL, so my thought is rather simple : 1) Use one thread (I suppose the default one) to run everything apart from the physics. 2) Use another thread, only to run the physics which I have limited in one function as you will see. But even like this, the whole GUI still “freezes” and as a result it is useless until the physics ends. How can I fix that issue?

Below follows the code that I thought it would work. It renders a “Test button” in order to check if I can actually press it while the physics is running and the “Run physics” button that triggers a heavy computation.

Thank you in advance.

#include"imgui_impl_glfw.h"
#include"imgui_impl_opengl3.h"

#include<GL/glew.h>
#include<GLFW/glfw3.h>

#include<cstdio>
#include<cmath>
#include<thread>

//Hypothetical "heavy" computation
void run_physics()
{
    for (double z = 0.0; z < 10000.0; z += 0.001)
    {
        double y = exp(sin(sqrt(z*abs(z)+ cos(z))));
    }
    return;
}

int main()
{
    glfwInit();
    glfwWindowHint(GLFW_CONTEXT_VERSION_MAJOR, 3);
    glfwWindowHint(GLFW_CONTEXT_VERSION_MINOR, 3);
    glfwWindowHint(GLFW_OPENGL_PROFILE, GLFW_OPENGL_CORE_PROFILE);

    GLFWwindow *window = glfwCreateWindow(800,600, "Test", NULL, NULL);
    if (window == NULL)
    {
        printf("Failed to open a glfw window. Exiting...n");
        return 0;
    }

    glfwMakeContextCurrent(window);

    glewExperimental = GL_TRUE;
    if (glewInit() != GLEW_OK)
    {
        printf("Failed to initialize glew. Exiting...n");
        return 0;
    }

    IMGUI_CHECKVERSION();
    ImGui::CreateContext();
    ImGuiIO &io = ImGui::GetIO();
    (void)io;
    ImGui::StyleColorsDark();
    ImGui_ImplGlfw_InitForOpenGL(window, true);
    ImGui_ImplOpenGL3_Init("#version 330");

    glClearColor(0.0f, 0.0f, 0.0f, 1.0f);
    while (!glfwWindowShouldClose(window))
    {
        glClear(GL_COLOR_BUFFER_BIT);

        ImGui_ImplOpenGL3_NewFrame();
        ImGui_ImplGlfw_NewFrame();
        ImGui::NewFrame();
        ImGui::Begin("Test");
        ImGui::Button("Test button");
        if (ImGui::Button("Run physics"))
        {
            //This pauses the render loop until run_physics is done. Good.
            //run_physics();

            //This also pauses the render loop. Why?
            std::thread thr(run_physics);
            thr.join();
        }
        ImGui::End();
        ImGui::Render();
        ImGui_ImplOpenGL3_RenderDrawData(ImGui::GetDrawData());

        glfwSwapBuffers(window);
        glfwPollEvents();
    }
    ImGui_ImplOpenGL3_Shutdown();
    ImGui_ImplGlfw_Shutdown();
    ImGui::DestroyContext();
    glfwTerminate();
    return 0;
}