imgui_cache_layer.cpp

#include <functional>
#include <iostream>

#define IMGUI_DEFINE_MATH_OPERATORS
#include "imgui.h"
#include "imgui_internal.h"

#include "imgui_impl_sdl2.h"
#include "imgui_impl_opengl3.h"

#include <OpenGL/gl3.h>
#include <SDL.h>

// Function to create a texture
inline GLuint createTexture(int width, int height) {
    std::cout << "create texture size " << width << "x" << height << std::endl;
    GLuint texture;
    glGenTextures(1, &texture);
    // glActiveTexture(GL_TEXTURE1);
    glBindTexture(GL_TEXTURE_2D, texture);
    glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, width, height, 0, GL_RGBA, GL_UNSIGNED_BYTE, NULL);
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
    glBindTexture(GL_TEXTURE_2D, 0); // Unbind the texture
    return texture;
}

// Function to create a framebuffer and attach the texture to it
inline GLuint createFramebuffer(GLuint texture) {
    GLuint fbo;
    glGenFramebuffers(1, &fbo);
    glBindFramebuffer(GL_FRAMEBUFFER, fbo);

    glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D, texture, 0);

    if (glCheckFramebufferStatus(GL_FRAMEBUFFER) != GL_FRAMEBUFFER_COMPLETE) {
        std::cerr << "Framebuffer not complete!" << std::endl;
    }

    glBindFramebuffer(GL_FRAMEBUFFER, 0);
    return fbo;
}

struct OffscreenRender {
    struct Render {
        ImGuiID id;
        GLuint fbo{0};
        GLuint texture{0};
        bool needsRender = true;
        ImVec2 size{0, 0};
        ImVec2 frameBufferSize{0, 0};
        std::function<void()> callback;
        long long everyMS = 0;
        long long lastRender = 0;

        explicit Render(
            ImGuiID id,
            const std::function<void()> &callback
        ): id(id), callback(callback) {
        }

        Render &every(long long ms) {
            everyMS = ms;
            return *this;
        }

        Render &withSize(float width, float height) {
            size = ImVec2(width, height);
            return *this;
        }

        Render &withSize(ImVec2 s) {
            size = s;
            return *this;
        }
    };

    ImGuiContext *context = nullptr;
    std::unordered_map<ImGuiID, Render> renders;

    void init() {
        if (context) return;
        auto currentContext = ImGui::GetCurrentContext();
        auto &oldIo = currentContext->IO;
        context = ImGui::CreateContext(ImGui::GetIO().Fonts);
        auto &io = context->IO;
        io.BackendRendererUserData = oldIo.BackendRendererUserData;
        io.BackendPlatformUserData = oldIo.BackendPlatformUserData;
        io.BackendPlatformName = oldIo.BackendPlatformName;
        io.BackendFlags = oldIo.BackendFlags;
        io.FontGlobalScale = oldIo.FontGlobalScale;
        io.FontAllowUserScaling = oldIo.FontAllowUserScaling;
        io.FontDefault = oldIo.FontDefault;
        io.DisplaySize = oldIo.DisplaySize;
        io.DisplayFramebufferScale = oldIo.DisplayFramebufferScale;
        ImGui::SetCurrentContext(currentContext);
    }

    ~OffscreenRender() {
        for (auto &render: renders) {
            if (render.second.texture) glDeleteTextures(1, &render.second.texture);
            if (render.second.fbo) glDeleteFramebuffers(1, &render.second.fbo);
        }
    }

    /**
     * Creates the textures so that next render() call uses the texture instead of the rendering callback.
     * This needs to be called outside of the main ImGui loop (not between ImGui::NewFrame() and ImGui::Render()).
     * After ImGui::Render() is a good place.
     */
    void createTextures() {
        for (auto &render: renders) {
            doRender(render.second);
        }
    }

    /**
     * Initially calls the callback to render the content and measures the size of the content.
     * Places the callback into a queue to be rendered later via createTextures() to turn the callback
     * rendering pipeline into a static texture.
     */
    Render &render(
        const char *label,
        const std::function<void()> &callback
    ) {
        auto id = ImGui::GetID(label);
        auto found = renders.find(id);
        if (found == renders.end()) {
            found = renders.insert_or_assign(id, Render(id, callback)).first;
        }
        auto &render = found->second;

        if (render.texture) {
            auto size = render.frameBufferSize.x == 0 ? context->IO.DisplaySize : render.frameBufferSize;

            ImGuiWindow *window = ImGui::GetCurrentWindow();
            ImVec2 pos = window->DC.CursorPos;
            const ImRect bb(pos, pos + render.size);
            ImGui::ItemSize(bb);
            if (!ImGui::ItemAdd(bb, 0)) return render;
            auto texture = (void *) (intptr_t) render.texture;
            auto uv0 = ImVec2(0, 1);
            auto uv1 = ImVec2(1, 0);
            static auto col = ImGui::GetColorU32(ImVec4(1, 1, 1, 1));
            window->DrawList->AddImage(texture, pos, pos + size, uv0, uv1, col);
        } else {
            callback();
        }

        return render;
    }

    void doRender(Render &render) {
        auto currentMS = std::chrono::duration_cast<std::chrono::milliseconds>(
            std::chrono::system_clock::now().time_since_epoch()
        ).count();

        auto diff = currentMS - render.lastRender;
        if (!render.everyMS || diff < render.everyMS) return;
        render.lastRender = currentMS;

        init();
        auto size = render.frameBufferSize.x == 0 ? context->IO.DisplaySize : render.frameBufferSize;
        if (!render.fbo) {
            render.texture = createTexture(
                size.x * context->IO.DisplayFramebufferScale.x,
                size.y * context->IO.DisplayFramebufferScale.y
            );
            render.fbo = createFramebuffer(render.texture);
        }
        auto currentContext = ImGui::GetCurrentContext();
        ImGui::SetCurrentContext(context);

        glBindFramebuffer(GL_FRAMEBUFFER, render.fbo);
        // glViewport(0, 0, render.size.x * 2, render.size.y * 2);
        // glClearColor(1.0f, 0.0f, 0.0f, 1.0f);
        glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

        context->IO.DisplaySize = ImVec2(size.x, size.y);
        // context->IO.DisplayFramebufferScale = ImVec2(2, 2);
        // ImGui_ImplSDL2_NewFrame();
        // ImGui_ImplOpenGL3_NewFrame();
        ImGui::NewFrame();

        ImGui::PushID(render.id);
        ImGui::SetNextWindowPos(ImVec2(0, 0));
        ImGui::SetNextWindowSize(context->IO.DisplaySize);
        //set WindowPadding to 0
        ImGui::PushStyleVar(ImGuiStyleVar_WindowPadding, ImVec2(0, 0));

        ImGui::Begin("##offscreen", nullptr, ImGuiWindowFlags_NoTitleBar | ImGuiWindowFlags_NoResize |
                                             ImGuiWindowFlags_NoMove | ImGuiWindowFlags_NoCollapse |
                                             ImGuiWindowFlags_NoBringToFrontOnFocus |
                                             ImGuiWindowFlags_NoBackground);
        ImVec2 initialCursorPos = ImGui::GetCursorPos();
        render.callback();
        auto second = ImGui::GetItemRectMax();
        render.size = ImVec2(second.x - initialCursorPos.x, second.y - initialCursorPos.y);
        ImGui::End();
        ImGui::PopID();
        ImGui::PopStyleVar();
        ImGui::Render();
        ImGui_ImplOpenGL3_RenderDrawData(ImGui::GetDrawData());

        glBindFramebuffer(GL_FRAMEBUFFER, 0); // Unbind the framebuffer
        ImGui::SetCurrentContext(currentContext);
    }
};

int main() {
    if (SDL_Init(SDL_INIT_VIDEO | SDL_INIT_TIMER | SDL_INIT_GAMECONTROLLER) != 0) {
        std::cerr << "Error: " << SDL_GetError() << std::endl;
        return 2;
    }

    SDL_GL_SetAttribute(SDL_GL_CONTEXT_FLAGS, SDL_GL_CONTEXT_FORWARD_COMPATIBLE_FLAG);
    SDL_GL_SetAttribute(SDL_GL_CONTEXT_PROFILE_MASK, SDL_GL_CONTEXT_PROFILE_CORE);
    SDL_GL_SetAttribute(SDL_GL_CONTEXT_MAJOR_VERSION, 3);
    SDL_GL_SetAttribute(SDL_GL_CONTEXT_MINOR_VERSION, 2);

    SDL_Window *window = SDL_CreateWindow("test", SDL_WINDOWPOS_CENTERED, SDL_WINDOWPOS_CENTERED, 1280, 720,
                                          SDL_WINDOW_OPENGL | SDL_WINDOW_RESIZABLE | SDL_WINDOW_ALLOW_HIGHDPI);
    SDL_GLContext gl_context = SDL_GL_CreateContext(window);
    SDL_GL_MakeCurrent(window, gl_context);
    SDL_GL_SetSwapInterval(1);

    // Setup Dear ImGui context
    IMGUI_CHECKVERSION();
    ImGui::CreateContext();
    ImGuiIO &io = ImGui::GetIO();
    ImGui::StyleColorsDark();
    ImGui_ImplSDL2_InitForOpenGL(window, gl_context);
    ImGui_ImplOpenGL3_Init("#version 150");

    OffscreenRender offscreenRender;

    // Main loop
    bool running = true;
    auto maxFPS = 60;

    while (running) {
        auto startingTick = SDL_GetTicks();
        SDL_Event event;
        while (SDL_PollEvent(&event)) {
            ImGui_ImplSDL2_ProcessEvent(&event);
            if (event.type == SDL_WINDOWEVENT && event.window.event == SDL_WINDOWEVENT_CLOSE && event.window.windowID ==
                SDL_GetWindowID(window)) {
                running = false;
            }
            if (event.type == SDL_QUIT) {
                running = false;
            }
        }

        ImGui_ImplSDL2_NewFrame();
        ImGui_ImplOpenGL3_NewFrame();

        // Start a new ImGui frame
        ImGui::NewFrame();

        // Clear the screen
        glClearColor(0.45f, 0.55f, 0.60f, 1.00f);
        glClear(GL_COLOR_BUFFER_BIT);

        // Display the texture
        ImGui::SetNextWindowPos(ImVec2(0, 0));
        ImGui::SetNextWindowSize(ImGui::GetIO().DisplaySize);
        ImGui::Begin("mainWindow", nullptr,
                     ImGuiWindowFlags_NoTitleBar | ImGuiWindowFlags_NoResize | ImGuiWindowFlags_NoMove |
                     ImGuiWindowFlags_NoCollapse | ImGuiWindowFlags_NoBringToFrontOnFocus |
                     ImGuiWindowFlags_NoBackground);

        static int i = 0;
        ImGui::Text("Onscreen renderer %d", i++);

        static int j = 0;
        offscreenRender.render("offscreen", [&]() {
            ImGui::Text("Offscreen renderer %d", j++);
        }).every(1000);

        ImGui::End();

        // Render ImGui
        ImGui::Render();
        ImGui_ImplOpenGL3_RenderDrawData(ImGui::GetDrawData());

        // Swap buffers
        SDL_GL_SwapWindow(window);

        // After ImGui::Render() is a good place to create the textures
        offscreenRender.createTextures();

        if ((1000 / maxFPS) > SDL_GetTicks() - startingTick) {
            SDL_Delay(1000 / maxFPS - (SDL_GetTicks() - startingTick));
        }
    }

    // Cleanup
    ImGui_ImplOpenGL3_Shutdown();
    ImGui_ImplSDL2_Shutdown();
    ImGui::DestroyContext();
    SDL_GL_DeleteContext(gl_context);
    SDL_DestroyWindow(window);
    SDL_Quit();

    return 0;
}