qt-everywhere-src-5.15.1/qtdeclarative/examples/quick/quickwidgets/quickwidget/fbitem.cpp

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#include "fbitem.h"
#include <QtGui/QOpenGLFramebufferObject>
#include <QtGui/QOpenGLContext>
#include <QtGui/QOpenGLFunctions>
#include <QtCore/QDebug>

#if QT_CONFIG(opengl)
class FbRenderer : public QQuickFramebufferObject::Renderer
{
public:
    FbRenderer() { }

    // The lifetime of the FBO and this class depends on how QQuickWidget
    // manages the scenegraph and context when it comes to showing and hiding
    // the widget. The actual behavior is proven by the debug prints.
    ~FbRenderer() override {
        qDebug("FbRenderer destroyed");
    }

    void render() override {
        QOpenGLFunctions *f = QOpenGLContext::currentContext()->functions();
        f->glClearColor(c, 0, 0, 1);
        f->glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT | GL_STENCIL_BUFFER_BIT);
        c += 0.01f * dir;
        if (c >= 1.0f || c <= 0.0f)
            dir *= -1;
        update();
    }

    QOpenGLFramebufferObject *createFramebufferObject(const QSize &size) override {
        qDebug() << "Creating FBO" << size;
        QOpenGLFramebufferObjectFormat format;
        format.setAttachment(QOpenGLFramebufferObject::CombinedDepthStencil);
        return new QOpenGLFramebufferObject(size, format);
    }

private:
    float c = 0;
    int dir = 1;
};
#endif

QQuickFramebufferObject::Renderer *FbItem::createRenderer() const
{
#if QT_CONFIG(opengl)
    return new FbRenderer;
#else
    return nullptr;
#endif
}