QQuickRenderControl Example

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#include "window_singlethreaded.h"
#include "cuberenderer.h"
#include <QOpenGLContext>
#include <QOpenGLFunctions>
#include <QOpenGLFramebufferObject>
#include <QOpenGLShaderProgram>
#include <QOpenGLVertexArrayObject>
#include <QOpenGLBuffer>
#include <QOpenGLVertexArrayObject>
#include <QOffscreenSurface>
#include <QScreen>
#include <QQmlEngine>
#include <QQmlComponent>
#include <QQuickItem>
#include <QQuickWindow>
#include <QQuickRenderControl>
#include <QCoreApplication>

class RenderControl : public QQuickRenderControl
{
public:
    RenderControl(QWindow *w) : m_window(w) { }
    QWindow *renderWindow(QPoint *offset) override;

private:
    QWindow *m_window;
};

QWindow *RenderControl::renderWindow(QPoint *offset)
{
    if (offset)
        *offset = QPoint(0, 0);
    return m_window;
}

WindowSingleThreaded::WindowSingleThreaded()
    : m_rootItem(nullptr),
      m_fbo(nullptr),
      m_quickInitialized(false),
      m_quickReady(false),
      m_dpr(0)
{
    setSurfaceType(QSurface::OpenGLSurface);

    // The rendercontrol does not necessarily need an FBO. Demonstrate this
    // when requested.
    m_onscreen = QCoreApplication::arguments().contains(QStringLiteral("--onscreen"));

    QSurfaceFormat format;
    // Qt Quick may need a depth and stencil buffer. Always make sure these are available.
    format.setDepthBufferSize(16);
    format.setStencilBufferSize(8);
    setFormat(format);

    m_context = new QOpenGLContext;
    m_context->setFormat(format);
    m_context->create();

    m_offscreenSurface = new QOffscreenSurface;
    // Pass m_context->format(), not format. Format does not specify and color buffer
    // sizes, while the context, that has just been created, reports a format that has
    // these values filled in. Pass this to the offscreen surface to make sure it will be
    // compatible with the context's configuration.
    m_offscreenSurface->setFormat(m_context->format());
    m_offscreenSurface->create();

    m_cubeRenderer = new CubeRenderer(m_offscreenSurface);

    m_renderControl = new RenderControl(this);

    // Create a QQuickWindow that is associated with out render control. Note that this
    // window never gets created or shown, meaning that it will never get an underlying
    // native (platform) window.
    m_quickWindow = new QQuickWindow(m_renderControl);

    // Create a QML engine.
    m_qmlEngine = new QQmlEngine;
    if (!m_qmlEngine->incubationController())
        m_qmlEngine->setIncubationController(m_quickWindow->incubationController());

    // When Quick says there is a need to render, we will not render immediately. Instead,
    // a timer with a small interval is used to get better performance.
    m_updateTimer.setSingleShot(true);
    m_updateTimer.setInterval(5);
    connect(&m_updateTimer, &QTimer::timeout, this, &WindowSingleThreaded::render);

    // Now hook up the signals. For simplicy we don't differentiate between
    // renderRequested (only render is needed, no sync) and sceneChanged (polish and sync
    // is needed too).
    connect(m_quickWindow, &QQuickWindow::sceneGraphInitialized, this, &WindowSingleThreaded::createFbo);
    connect(m_quickWindow, &QQuickWindow::sceneGraphInvalidated, this, &WindowSingleThreaded::destroyFbo);
    connect(m_renderControl, &QQuickRenderControl::renderRequested, this, &WindowSingleThreaded::requestUpdate);
    connect(m_renderControl, &QQuickRenderControl::sceneChanged, this, &WindowSingleThreaded::requestUpdate);

    // Just recreating the FBO on resize is not sufficient, when moving between screens
    // with different devicePixelRatio the QWindow size may remain the same but the FBO
    // dimension is to change regardless.
    connect(this, &QWindow::screenChanged, this, &WindowSingleThreaded::handleScreenChange);
}

WindowSingleThreaded::~WindowSingleThreaded()
{
    // Make sure the context is current while doing cleanup. Note that we use the
    // offscreen surface here because passing 'this' at this point is not safe: the
    // underlying platform window may already be destroyed. To avoid all the trouble, use
    // another surface that is valid for sure.
    m_context->makeCurrent(m_offscreenSurface);

    // Delete the render control first since it will free the scenegraph resources.
    // Destroy the QQuickWindow only afterwards.
    delete m_renderControl;

    delete m_qmlComponent;
    delete m_quickWindow;
    delete m_qmlEngine;
    delete m_fbo;

    m_context->doneCurrent();

    delete m_cubeRenderer;

    delete m_offscreenSurface;
    delete m_context;
}

void WindowSingleThreaded::createFbo()
{
    // The scene graph has been initialized. It is now time to create an FBO and associate
    // it with the QQuickWindow.
    m_dpr = devicePixelRatio();
    if (!m_onscreen) {
        m_fbo = new QOpenGLFramebufferObject(size() * m_dpr, QOpenGLFramebufferObject::CombinedDepthStencil);
        m_quickWindow->setRenderTarget(m_fbo);
    } else {
        // Special case: No FBO. Render directly to the window's default framebuffer.
        m_onscreenSize = size() * m_dpr;
        m_quickWindow->setRenderTarget(0, m_onscreenSize);
    }
}

void WindowSingleThreaded::destroyFbo()
{
    delete m_fbo;
    m_fbo = nullptr;
}

void WindowSingleThreaded::render()
{
    QSurface *surface = m_offscreenSurface;
    if (m_onscreen)
        surface = this;
    if (!m_context->makeCurrent(surface))
        return;

    // Polish, synchronize and render the next frame (into our fbo).  In this example
    // everything happens on the same thread and therefore all three steps are performed
    // in succession from here. In a threaded setup the render() call would happen on a
    // separate thread.
    m_renderControl->polishItems();
    m_renderControl->sync();
    m_renderControl->render();

    m_quickWindow->resetOpenGLState();
    QOpenGLFramebufferObject::bindDefault();

    m_context->functions()->glFlush();

    m_quickReady = true;

    // Get something onto the screen.
    if (!m_onscreen)
        m_cubeRenderer->render(this, m_context, m_quickReady ? m_fbo->texture() : 0);
    else
        m_context->swapBuffers(this);
}

void WindowSingleThreaded::requestUpdate()
{
    if (!m_updateTimer.isActive())
        m_updateTimer.start();
}

void WindowSingleThreaded::run()
{
    disconnect(m_qmlComponent, &QQmlComponent::statusChanged, this, &WindowSingleThreaded::run);

    if (m_qmlComponent->isError()) {
        const QList<QQmlError> errorList = m_qmlComponent->errors();
        for (const QQmlError &error : errorList)
            qWarning() << error.url() << error.line() << error;
        return;
    }

    QObject *rootObject = m_qmlComponent->create();
    if (m_qmlComponent->isError()) {
        const QList<QQmlError> errorList = m_qmlComponent->errors();
        for (const QQmlError &error : errorList)
            qWarning() << error.url() << error.line() << error;
        return;
    }

    m_rootItem = qobject_cast<QQuickItem *>(rootObject);
    if (!m_rootItem) {
        qWarning("run: Not a QQuickItem");
        delete rootObject;
        return;
    }

    // The root item is ready. Associate it with the window.
    m_rootItem->setParentItem(m_quickWindow->contentItem());

    // Update item and rendering related geometries.
    updateSizes();

    // Initialize the render control and our OpenGL resources.
    QSurface *surface = m_offscreenSurface;
    if (m_onscreen)
        surface = this;
    m_context->makeCurrent(surface);
    m_renderControl->initialize(m_context);
    m_quickInitialized = true;
}

void WindowSingleThreaded::updateSizes()
{
    // Behave like SizeRootObjectToView.
    m_rootItem->setWidth(width());
    m_rootItem->setHeight(height());

    m_quickWindow->setGeometry(0, 0, width(), height());

    m_cubeRenderer->resize(width(), height());
}

void WindowSingleThreaded::startQuick(const QString &filename)
{
    m_qmlComponent = new QQmlComponent(m_qmlEngine, QUrl(filename));
    if (m_qmlComponent->isLoading())
        connect(m_qmlComponent, &QQmlComponent::statusChanged, this, &WindowSingleThreaded::run);
    else
        run();
}

void WindowSingleThreaded::exposeEvent(QExposeEvent *)
{
    if (isExposed()) {
        if (!m_quickInitialized) {
            if (!m_onscreen)
                m_cubeRenderer->render(this, m_context, m_quickReady ? m_fbo->texture() : 0);
            startQuick(QStringLiteral("qrc:/rendercontrol/demo.qml"));
        }
    }
}

void WindowSingleThreaded::resizeFbo()
{
    QSurface *surface = m_offscreenSurface;
    if (m_onscreen)
        surface = this;
    if (m_rootItem && m_context->makeCurrent(surface)) {
        delete m_fbo;
        createFbo();
        m_context->doneCurrent();
        updateSizes();
        render();
    }
}

void WindowSingleThreaded::resizeEvent(QResizeEvent *)
{
    // If this is a resize after the scene is up and running, recreate the fbo and the
    // Quick item and scene.
    if (!m_onscreen) {
        if (m_fbo && m_fbo->size() != size() * devicePixelRatio())
            resizeFbo();
    } else {
        if (m_onscreenSize != size() * devicePixelRatio())
            resizeFbo();
    }
}

void WindowSingleThreaded::handleScreenChange()
{
    if (m_dpr != devicePixelRatio())
        resizeFbo();
}

void WindowSingleThreaded::mousePressEvent(QMouseEvent *e)
{
    // Use the constructor taking localPos and screenPos. That puts localPos into the
    // event's localPos and windowPos, and screenPos into the event's screenPos. This way
    // the windowPos in e is ignored and is replaced by localPos. This is necessary
    // because QQuickWindow thinks of itself as a top-level window always.
    QMouseEvent mappedEvent(e->type(), e->localPos(), e->screenPos(), e->button(), e->buttons(), e->modifiers());
    QCoreApplication::sendEvent(m_quickWindow, &mappedEvent);
}

void WindowSingleThreaded::mouseReleaseEvent(QMouseEvent *e)
{
    QMouseEvent mappedEvent(e->type(), e->localPos(), e->screenPos(), e->button(), e->buttons(), e->modifiers());
    QCoreApplication::sendEvent(m_quickWindow, &mappedEvent);
}