Szenengraph – OpenGL unter QML

class Squircle : public QQuickItem
{
    Q_OBJECT
    Q_PROPERTY(qreal t READ t WRITE setT NOTIFY tChanged)
    QML_ELEMENT

public:
    Squircle();

    qreal t() const { return m_t; }
    void setT(qreal t);

signals:
    void tChanged();

public slots:
    void sync();
    void cleanup();

private slots:
    void handleWindowChanged(QQuickWindow *win);

private:
    void releaseResources() override;

    qreal m_t;
    SquircleRenderer *m_renderer;
};

class SquircleRenderer : public QObject, protected QOpenGLFunctions
{
    Q_OBJECT
public:
    ~SquircleRenderer();

    void setT(qreal t) { m_t = t; }
    void setViewportSize(const QSize &size) { m_viewportSize = size; }
    void setWindow(QQuickWindow *window) { m_window = window; }

public slots:
    void init();
    void paint();

private:
    QSize m_viewportSize;
    qreal m_t = 0.0;
    QOpenGLShaderProgram *m_program = nullptr;
    QQuickWindow *m_window = nullptr;
    QOpenGLBuffer m_vbo;
};

Squircle::Squircle()
    : m_t(0)
    , m_renderer(nullptr)
{
    connect(this, &QQuickItem::windowChanged, this, &Squircle::handleWindowChanged);
}

void Squircle::handleWindowChanged(QQuickWindow *win)
{
    if (win) {
        connect(win, &QQuickWindow::beforeSynchronizing, this, &Squircle::sync, Qt::DirectConnection);
        connect(win, &QQuickWindow::sceneGraphInvalidated, this, &Squircle::cleanup, Qt::DirectConnection);

        // Ensure we start with cleared to black. The squircle's blend mode relies on this.
        win->setColor(Qt::black);
    }
}

void Squircle::sync()
{
    if (!m_renderer) {
        m_renderer = new SquircleRenderer();
        connect(window(), &QQuickWindow::beforeRendering, m_renderer, &SquircleRenderer::init, Qt::DirectConnection);
        connect(window(), &QQuickWindow::beforeRenderPassRecording, m_renderer, &SquircleRenderer::paint, Qt::DirectConnection);
    }
    m_renderer->setViewportSize(window()->size() * window()->devicePixelRatio());
    m_renderer->setT(m_t);
    m_renderer->setWindow(window());
}

void Squircle::cleanup()
{
    delete m_renderer;
    m_renderer = nullptr;
}

class CleanupJob : public QRunnable
{
public:
    CleanupJob(SquircleRenderer *renderer) : m_renderer(renderer) { }
    void run() override { delete m_renderer; }
private:
    SquircleRenderer *m_renderer;
};

void Squircle::releaseResources()
{
    window()->scheduleRenderJob(new CleanupJob(m_renderer), QQuickWindow::BeforeSynchronizingStage);
    m_renderer = nullptr;
}

SquircleRenderer::~SquircleRenderer()
{
    delete m_program;
}

void Squircle::setT(qreal t)
{
    if (t == m_t)
        return;
    m_t = t;
    emit tChanged();
    if (window())
        window()->update();
}

void SquircleRenderer::init()
{
    if (!m_program) {
        QSGRendererInterface *rif = m_window->rendererInterface();
        Q_ASSERT(rif->graphicsApi() == QSGRendererInterface::OpenGL);

        initializeOpenGLFunctions();

        const float values[] = { -1, -1, 1, -1, -1, 1, 1, 1 };

        m_vbo.create();
        m_vbo.bind();
        m_vbo.allocate(values, sizeof(values));

        glEnableVertexAttribArray(0);
        glVertexAttribPointer(0, 2, GL_FLOAT, GL_FALSE, 2 * sizeof(GLfloat), nullptr);

        m_program = new QOpenGLShaderProgram();
        m_program->addCacheableShaderFromSourceCode(QOpenGLShader::Vertex,
                                                    "attribute highp vec4 vertices;"
                                                    "varying highp vec2 coords;"
                                                    "void main() {"
                                                    "    gl_Position = vertices;"
                                                    "    coords = vertices.xy;"
                                                    "}");
        m_program->addCacheableShaderFromSourceCode(QOpenGLShader::Fragment,
                                                    "uniform lowp float t;"
                                                    "varying highp vec2 coords;"
                                                    "void main() {"
                                                    "    lowp float i = 1. - (pow(abs(coords.x), 4.) + pow(abs(coords.y), 4.));"
                                                    "    i = smoothstep(t - 0.8, t + 0.8, i);"
                                                    "    i = floor(i * 20.) / 20.;"
                                                    "    gl_FragColor = vec4(coords * .5 + .5, i, i);"
                                                    "}");

        m_program->bindAttributeLocation("vertices", 0);
        m_program->link();

    }
}

void SquircleRenderer::paint()
{
    // Play nice with the RHI. Not strictly needed when the scenegraph uses
    // OpenGL directly.
    m_window->beginExternalCommands();

    m_vbo.bind();
    m_program->bind();
    m_program->setUniformValue("t", (float)m_t);

    glEnableVertexAttribArray(0);
    glVertexAttribPointer(0, 2, GL_FLOAT, GL_FALSE, 2 * sizeof(GLfloat), nullptr);

    glViewport(0, 0, m_viewportSize.width(), m_viewportSize.height());

    glDisable(GL_DEPTH_TEST);

    glEnable(GL_BLEND);
    glBlendFunc(GL_SRC_ALPHA, GL_ONE);

    glDrawArrays(GL_TRIANGLE_STRIP, 0, 4);

    glDisableVertexAttribArray(0);
    m_program->release();

    m_window->endExternalCommands();
}

int main(int argc, char **argv)
{
    QGuiApplication app(argc, argv);

    QQuickWindow::setGraphicsApi(QSGRendererInterface::OpenGL);

    QQuickView view;
    view.setResizeMode(QQuickView::SizeRootObjectToView);
    view.setSource(QUrl("qrc:///scenegraph/openglunderqml/main.qml"));
    view.show();

    return QGuiApplication::exec();
}

import QtQuick
import OpenGLUnderQML

Item {

    width: 320
    height: 480

    Squircle {
        SequentialAnimation on t {
            NumberAnimation { to: 1; duration: 2500; easing.type: Easing.InQuad }
            NumberAnimation { to: 0; duration: 2500; easing.type: Easing.OutQuad }
            loops: Animation.Infinite
            running: true
        }
    }

    Rectangle {
        color: Qt.rgba(1, 1, 1, 0.7)
        radius: 10
        border.width: 1
        border.color: "white"
        anchors.fill: label
        anchors.margins: -10
    }

    Text {
        id: label
        color: "black"
        wrapMode: Text.WordWrap
        text: qsTr("The background here is a squircle rendered with raw OpenGL using the 'beforeRender()' signal in QQuickWindow. This text label and its border is rendered using QML")
        anchors.right: parent.right
        anchors.left: parent.left
        anchors.bottom: parent.bottom
        anchors.margins: 20
    }
}