imgui/examples/example_emscripten_opengl3/main.cpp

// Dear ImGui: standalone example application for Emscripten + OpenGL 3, using programmable pipeline
// (Emscripten is a complete Open Source compiler toolchain to WebAssembly.)

// 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

#include "imgui.h"
#include "imgui_impl_emscripten.h"
#include "imgui_impl_opengl3.h"

#include <EGL/egl.h>
#include <GLES2/gl2.h>          // Use GL ES 2

#include "../libs/emscripten/emscripten_mainloop_stub.h"

#include <emscripten/html5.h>
#include <emscripten/console.h>

#define TARGET_ELEMENT_ID "#canvas"

static EMSCRIPTEN_WEBGL_CONTEXT_HANDLE g_ContextHandle;
static int g_CanvasWidth;
static int g_CanvasHeight;

static void ResizeCanvas(int width, int height)
{
    g_CanvasWidth = width;
    g_CanvasHeight = height;
    emscripten_set_canvas_element_size(TARGET_ELEMENT_ID, g_CanvasWidth, g_CanvasHeight);
}

static bool CreateWebGLContext()
{
    EmscriptenWebGLContextAttributes attr{};
    emscripten_webgl_init_context_attributes(&attr);
    attr.majorVersion = 1;
    attr.minorVersion = 0;
    attr.antialias = false;
    attr.depth = false;
    attr.stencil = false;
    attr.alpha = true;

    g_ContextHandle = emscripten_webgl_create_context(TARGET_ELEMENT_ID, &attr);
    if (g_ContextHandle != 0)
    {
        double width, height;
        emscripten_get_element_css_size(TARGET_ELEMENT_ID, &width, &height);
        ResizeCanvas((int)width, (int)height);
    }

    return g_ContextHandle != 0;
}

static bool WindowResizeCallback(int type, const EmscriptenUiEvent* event, void* user_data)
{
    ResizeCanvas(event->windowInnerWidth, event->windowInnerHeight);
    ImGui_ImplEmscripten_UpdateCanvasSize(g_CanvasWidth, g_CanvasHeight);
    return false;
}

// Main code
int main(int, char**)
{
    const float main_scale = 1.f;

    // Decide GL+GLSL versions
#if defined(IMGUI_IMPL_OPENGL_ES2)
    // GL ES 2.0 + GLSL 100 (WebGL 1.0)
    const char* glsl_version = "#version 100";
#elif defined(IMGUI_IMPL_OPENGL_ES3)
    // GL ES 3.0 + GLSL 300 es (WebGL 2.0)
    const char* glsl_version = "#version 300 es";
#else
    // GL 3.0 + GLSL 130
    const char* glsl_version = "#version 130";
#endif

    if (!CreateWebGLContext())
    {
        emscripten_console_error("Failed to create webgl context");
        return 1;
    }

    emscripten_webgl_make_context_current(g_ContextHandle);
    emscripten_set_resize_callback(EMSCRIPTEN_EVENT_TARGET_WINDOW, NULL, false, (em_ui_callback_func)&WindowResizeCallback);

    // 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

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

    // Setup scaling
    ImGuiStyle& style = ImGui::GetStyle();
    style.ScaleAllSizes(main_scale);        // Bake a fixed style scale. (until we have a solution for dynamic style scaling, changing this requires resetting Style + calling this again)
    style.FontScaleDpi = main_scale;        // Set initial font scale. (using io.ConfigDpiScaleFonts=true makes this unnecessary. We leave both here for documentation purpose)

    // Setup Platform/Renderer backends
    ImGui_ImplEmscripten_Init(TARGET_ELEMENT_ID);
    ImGui_ImplOpenGL3_Init(glsl_version);

    // 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).
    // - 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. If you like the default font but want it to scale better, consider using the 'ProggyVector' from the same author!
    // - Remember that in C/C++ if you want to include a backslash \ in a string literal you need to write a double backslash \\ !
    // - Our Emscripten build process allows embedding fonts to be accessible at runtime from the "fonts/" folder. See Makefile.emscripten for details.
    //style.FontSizeBase = 20.0f;
    //io.Fonts->AddFontDefault();
    //io.Fonts->AddFontFromFileTTF("c:\\Windows\\Fonts\\segoeui.ttf");
    //io.Fonts->AddFontFromFileTTF("../../misc/fonts/DroidSans.ttf");
    //io.Fonts->AddFontFromFileTTF("../../misc/fonts/Roboto-Medium.ttf");
    //io.Fonts->AddFontFromFileTTF("../../misc/fonts/Cousine-Regular.ttf");
    //ImFont* font = io.Fonts->AddFontFromFileTTF("c:\\Windows\\Fonts\\ArialUni.ttf");
    //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);

    // Main loop
    // For an Emscripten build we are disabling file-system access, so let's not attempt to do a fopen() of the imgui.ini file.
    // You may manually call LoadIniSettingsFromMemory() to load settings from your own storage.
    io.IniFilename = nullptr;
    EMSCRIPTEN_MAINLOOP_BEGIN
    {
        // Poll and handle events (inputs, window resize, etc.)
        // 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.
        // Emscripten backend uses callbacks to handle events so there is no need in this!

        // Start the Dear ImGui frame
        ImGui_ImplOpenGL3_NewFrame();
        ImGui_ImplEmscripten_NewFrame();
        ImGui::NewFrame();

        // 1. Show the big demo window (Most of the sample code is in ImGui::ShowDemoWindow()! You can browse its code to learn more about Dear ImGui!).
        if (show_demo_window)
            ImGui::ShowDemoWindow(&show_demo_window);

        // 2. Show a simple window that we create ourselves. We use a Begin/End pair to create a named window.
        {
            static float f = 0.0f;
            static int counter = 0;

            ImGui::Begin("Hello, world!");                          // Create a window called "Hello, world!" and append into it.

            ImGui::Text("This is some useful text.");               // Display some text (you can use a format strings too)
            ImGui::Checkbox("Demo Window", &show_demo_window);      // Edit bools storing our window open/close state
            ImGui::Checkbox("Another Window", &show_another_window);

            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

            if (ImGui::Button("Button"))                            // Buttons return true when clicked (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 / io.Framerate, io.Framerate);
            ImGui::End();
        }

        // 3. Show another simple window.
        if (show_another_window)
        {
            ImGui::Begin("Another Window", &show_another_window);   // Pass a pointer to our bool variable (the window will have a closing button that will clear the bool when clicked)
            ImGui::Text("Hello from another window!");
            if (ImGui::Button("Close Me"))
                show_another_window = false;
            ImGui::End();
        }

        // Rendering
        ImGui::Render();
        int display_w, display_h;
        emscripten_webgl_get_drawing_buffer_size(g_ContextHandle, &display_w, &display_h);
        glViewport(0, 0, display_w, display_h);
        glClearColor(clear_color.x * clear_color.w, clear_color.y * clear_color.w, clear_color.z * clear_color.w, clear_color.w);
        glClear(GL_COLOR_BUFFER_BIT);
        ImGui_ImplOpenGL3_RenderDrawData(ImGui::GetDrawData());
    }
    EMSCRIPTEN_MAINLOOP_END;

    // Cleanup
    ImGui_ImplOpenGL3_Shutdown();
    ImGui_ImplEmscripten_NewFrame();
    ImGui::DestroyContext();

    return 0;
}