examples/example_freeglut_opengl2/main.cpp

// ImGui - standalone example application for FreeGLUT + OpenGL2, using legacy fixed pipeline
// If you are new to ImGui, see examples/README.txt and documentation at the top of imgui.cpp.
// (Using GLUT or FreeGLUT is not recommended unless you really miss the 90's)

#include "imgui.h"
#include "../imgui_impl_freeglut.h"
#include "../imgui_impl_opengl2.h"
#include <GL/freeglut.h>

#ifdef _MSC_VER
#pragma warning (disable: 4505) // unreferenced local function has been removed
#endif

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

void my_display_code()
{
    // 1. Show a simple window.
    // Tip: if we don't call ImGui::Begin()/ImGui::End() the widgets automatically appears in a window called "Debug".
    {
        static float f = 0.0f;
        static int counter = 0;
        ImGui::Text("Hello, world!");                           // Display some text (you can use a format string too)
        ImGui::SliderFloat("float", &f, 0.0f, 1.0f);            // Edit 1 float using a slider from 0.0f to 1.0f    
        ImGui::ColorEdit3("clear color", (float*)&clear_color); // Edit 3 floats representing a color

        ImGui::Checkbox("Demo Window", &show_demo_window);      // Edit bools storing our windows open/close state
        ImGui::Checkbox("Another Window", &show_another_window);

        if (ImGui::Button("Button"))                            // Buttons return true when clicked (NB: most widgets return true when edited/activated)
            counter++;
        ImGui::SameLine();
        ImGui::Text("counter = %d", counter);

        ImGui::Text("Application average %.3f ms/frame (%.1f FPS)", 1000.0f / ImGui::GetIO().Framerate, ImGui::GetIO().Framerate);
    }

    // 2. Show another simple window. In most cases you will use an explicit Begin/End pair to name your windows.
    if (show_another_window)
    {
        ImGui::Begin("Another Window", &show_another_window);
        ImGui::Text("Hello from another window!");
        if (ImGui::Button("Close Me"))
            show_another_window = false;
        ImGui::End();
    }

    // 3. Show the ImGui demo window. Most of the sample code is in ImGui::ShowDemoWindow(). Read its code to learn more about Dear ImGui!
    if (show_demo_window)
    {
        ImGui::SetNextWindowPos(ImVec2(650, 20), ImGuiCond_FirstUseEver); // Normally user code doesn't need/want to call this because positions are saved in .ini file anyway. Here we just want to make the demo initial state a bit more friendly!
        ImGui::ShowDemoWindow(&show_demo_window);
    }
}

void glut_display_func()
{
    ImGui_ImplOpenGL2_NewFrame();
    ImGui_ImplFreeGLUT_NewFrame();

    my_display_code();

    ImGui::Render();

    ImGuiIO& io = ImGui::GetIO();
    glViewport(0, 0, (GLsizei)io.DisplaySize.x, (GLsizei)io.DisplaySize.y);
    glClearColor(clear_color.x, clear_color.y, clear_color.z, clear_color.w);
    glClear(GL_COLOR_BUFFER_BIT);
    //glUseProgram(0); // You may want this if using this code in an OpenGL 3+ context where shaders may be bound, but prefer using the GL3+ code.
    ImGui_ImplOpenGL2_RenderDrawData(ImGui::GetDrawData());

    glutSwapBuffers();
    glutPostRedisplay();
}

// 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.
// - When io.WantCaptureKeyboard is true, do not dispatch keyboard input data to your main application.
// Generally you may always pass all inputs to dear imgui, and hide them from your application based on those two flags.

int main(int argc, char** argv)
{ 
    // Create GLUT window
    glutInit(&argc, argv);
    glutSetOption(GLUT_ACTION_ON_WINDOW_CLOSE, GLUT_ACTION_GLUTMAINLOOP_RETURNS);
    glutInitDisplayMode(GLUT_RGBA | GLUT_DOUBLE | GLUT_MULTISAMPLE);
    glutInitWindowSize(1280, 720);
    glutCreateWindow("ImGui FreeGLUT+OpenGL2 Example");

    // Setup GLUT display function
    // We will also call ImGui_ImplFreeGLUT_InstallFuncs() to get all the other functions installed for us, 
    // otherwise it is possible to install our own functions and call the imgui_impl_freeglut.h functions ourselves.
    glutDisplayFunc(glut_display_func);

    // Setup ImGui binding
    ImGui::CreateContext();
    ImGuiIO& io = ImGui::GetIO(); (void)io;
    //io.ConfigFlags |= ImGuiConfigFlags_NavEnableKeyboard;  // Enable Keyboard Controls

    ImGui_ImplFreeGLUT_Init();
    ImGui_ImplFreeGLUT_InstallFuncs();
    ImGui_ImplOpenGL2_Init();

    // Setup style
    ImGui::StyleColorsDark();
    //ImGui::StyleColorsClassic();

    // 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 NULL. 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.
    // - Read 'misc/fonts/README.txt' 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("../../misc/fonts/Roboto-Medium.ttf", 16.0f);
    //io.Fonts->AddFontFromFileTTF("../../misc/fonts/Cousine-Regular.ttf", 15.0f);
    //io.Fonts->AddFontFromFileTTF("../../misc/fonts/DroidSans.ttf", 16.0f);
    //io.Fonts->AddFontFromFileTTF("../../misc/fonts/ProggyTiny.ttf", 10.0f);
    //ImFont* font = io.Fonts->AddFontFromFileTTF("c:\\Windows\\Fonts\\ArialUni.ttf", 18.0f, NULL, io.Fonts->GetGlyphRangesJapanese());
    //IM_ASSERT(font != NULL);

    glutMainLoop();

    // Cleanup
    ImGui_ImplOpenGL2_Shutdown();
    ImGui_ImplFreeGLUT_Shutdown();
    ImGui::DestroyContext();

    return 0;
}