OpenGlOffscreenSurface.h

// #ifndef OPENGLOFFSCREENSURFACE_H
// #define OPENGLOFFSCREENSURFACE_H

// #pragma once

#include <QObject>
#include <QScreen>
#include <QOffscreenSurface>
#include <QPaintEvent>
#include <QResizeEvent>
#include <QOpenGLPaintDevice>
#include <QOpenGLFunctions>
#include <QOpenGLFunctions_3_0>
#include <QOpenGLVersionFunctions>
#include <QOpenGLFramebufferObject>
#include <QSurfaceFormat>
#include <QWidget>
#include <QOpenGLShaderProgram>

#include <atomic>
#include <mutex>

class OpenGlOffscreenSurface
    : public QOffscreenSurface
{
    Q_OBJECT

public:
    /// @brief Constructor. Creates a render window.
    /// @param targetScreen Target screen.
    /// @param size Initial size of a surface buffer.
    /// this is because before the FBO and off-screen surface haven't been created.
    /// By default this uses the QWindow::requestedFormat() for OpenGL context and off-screen
    /// surface.
    explicit OpenGlOffscreenSurface(
            QScreen* targetScreen = nullptr,
            const QSize& size = QSize (1, 1));

    /// @brief Destructor.
    virtual ~OpenGlOffscreenSurface();

    /// @brief Check if the window is initialized and can be used for rendering.
    /// @return Returns true if context, surface and FBO have been set up to start rendering.
    bool isValid() const;

    /// @brief Return the context used in this window.
    /// @return The context used in this window or nullptr if it hasn't been created yet.
    QOpenGLContext* context() const;

    /// @brief Return the OpenGL function object that can be used the issue OpenGL commands.
    /// @return The functions for the context or nullptr if it the context hasn't been created yet.
    QOpenGLFunctions* functions() const;

    /// @brief Return the OpenGL off-screen frame buffer object identifier.
    /// @return The OpenGL off-screen frame buffer object identifier or 0 if no FBO has been created
    /// yet.
    /// @note This changes on every resize!
    GLuint framebufferObjectHandle() const;

    /// @brief Return the OpenGL off-screen frame buffer object.
    /// @return The OpenGL off-screen frame buffer object or nullptr if no FBO has been created yet.
    /// @note This changes on every resize!
    const QOpenGLFramebufferObject* getFramebufferObject() const;

    /// @brief Return the QPaintDevice for paint into it.
    QPaintDevice* getPaintDevice() const;

    /// @brief Return the OpenGL off-screen frame buffer object identifier.
    /// @return The OpenGL off-screen frame buffer object identifier or 0 if no FBO has been created
    /// yet.
    void bindFramebufferObject();

    /// @brief Return the current contents of the FBO.
    /// @return FBO content as 32bit QImage. You might need to swap RGBA to BGRA or vice-versa.
    QImage grabFramebuffer();

    /// @brief Makes the OpenGL context current for rendering.
    /// @note Make sure to bindFramebufferObject() if you want to render to this widgets FBO.
    void makeCurrent();

    /// @brief Release the OpenGL context.
    void doneCurrent();

    /// @brief Copy content of framebuffer to back buffer and swap buffers if the surface is
    /// double-buffered.
    /// If the surface is not double-buffered, the frame buffer content is blitted to the front
    /// buffer.
    /// If the window is not exposed, only the OpenGL pipeline is glFlush()ed so the framebuffer can
    /// be read back.
    void swapBuffers();

    /// @brief Use bufferSize() instead size() for get a size of a surface buffer. We can't override size() as it is not virtual.
    QSize bufferSize() const;

    /// @brief Resize surface buffer to newSize.
    void resize(const QSize& newSize);

    /// @brief Resize surface buffer to size with width w and height h.
    /// @param w Width.
    /// @param h Height.
    void resize(int w, int h);

public slots:
    /// @brief Lazy update routine like QWidget::update().
    void update();

    /// @brief Immediately render the widget contents to framebuffer.
    void render();

signals:
    /// @brief Emitted when swapBuffers() was called and bufferswapping is done.
    void frameSwapped();

    /// @brief Emitted after a resizeEvent().
    void resized();

protected:
    virtual void exposeEvent(QExposeEvent* e);
    virtual void resizeEvent(QResizeEvent* e);
    virtual bool event(QEvent* e) override;

//    virtual int metric(QPaintDevice::PaintDeviceMetric metric) const override;

    /// @brief Called exactly once when the window is first exposed OR render() is called when the
    /// widget is invisible.
    /// @note After this the off-screen surface and FBO are available.
    virtual void initializeGL() = 0;

    /// @brief Called whenever the window size changes.
    /// @param width New window width.
    /// @param height New window height.
    virtual void resizeGL(
            int width,
            int height) = 0;

    /// @brief Called whenever the window needs to repaint itself. Override to draw OpenGL content.
    /// When this function is called, the context is already current and the correct framebuffer is
    /// bound.
    virtual void paintGL() = 0;

    //      /// @brief Called whenever the window needs to repaint itself. Override to draw QPainter
    // content.
    //      /// @brief This is called AFTER paintGL()! Only needed when painting using a QPainter.
    //      virtual void paintEvent(QPainter & painter) = 0;

private:
    Q_DISABLE_COPY(OpenGlOffscreenSurface)
    /// @brief Initialize the window.
    void initializeInternal();

    /// @brief Internal method that does the actual swap work, NOT using a mutex.
    void swapBuffersInternal();

    /// @brief Internal method that checks state and makes the context current, NOT using a mutex.
    void makeCurrentInternal();

    /// @brief Internal method to grab content of a specific framebuffer.
    QImage grabFramebufferInternal(QOpenGLFramebufferObject* fbo);

    /// @brief (Re-)allocate FBO and paint device if needed due to size changes etc.
    void recreateFBOAndPaintDevice();

    /// @brief False before the window was first exposed OR render() was called.
    std::atomic_bool m_initialized;
    /// @brief False before the overridden initializeGL() was first called.
    bool m_initializedGL = false;
    /// @brief True when currently a window update is pending.
    std::atomic_bool m_updatePending;
    /// @brief Mutex making sure not grabbing while drawing etc.
    std::mutex m_mutex;

    /// @brief OpenGL render context.
    QOpenGLContext* m_context = nullptr;
    /// @brief The OpenGL 2.1 / ES 2.0 function object that can be used the issue OpenGL commands.
    QOpenGLFunctions* m_functions = nullptr;
    /// @brief The OpenGL 3.0 function object that can be used the issue OpenGL commands.
    QOpenGLFunctions_3_0* m_functions_3_0 = nullptr;
    /// @brief OpenGL paint device for painting with a QPainter.
    QOpenGLPaintDevice* m_paintDevice = nullptr;
    /// @brief Background FBO for off-screen rendering when the window is not exposed.
    QOpenGLFramebufferObject* m_fbo = nullptr;
    /// @brief Background FBO resolving a multi sampling frame buffer in m_fbo to a frame buffer
    /// that can be grabbed to a QImage.
    QOpenGLFramebufferObject* m_resolvedFbo = nullptr;

    /// @brief Shader used for blitting m_fbo to screen if glBlitFrameBuffer is not available.
    QOpenGLShaderProgram* m_blitShader;

    QSize m_size;
};

// #endif  // OPENGLOFFSCREENSURFACE_H