QQuickFramebufferObject: When using an Item, whose parent has "visible: false", as texture source, nothing is displayed

[Stefan Monov76]

This works fine in Canvas3D, but not in QQuickFramebufferObject.

My code

I've based my testcase on the textureinsgnode sample app bundled with Qt. Since mine is a derived work, I have to first post the license terms here:

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And now for the actual source.

main.cpp:

#include <QGuiApplication>
#include <QQmlApplicationEngine>
#include <QOpenGLFramebufferObject>
#include <QSGTextureProvider>
#include <QQuickFramebufferObject>
#include <QOpenGLShaderProgram>
#include <QOpenGLFunctions>
#include <QQuickWindow>
// propertyhelper.h is taken from http://syncor.blogspot.bg/2014/11/qt-auto-property.html
#include "propertyhelper.h"

class MyItem : public QQuickFramebufferObject {
    Q_OBJECT

    AUTO_PROPERTY(QQuickItem*, sourceItem)

public:
    Renderer *createRenderer() const;
};

class MyItemRenderer : public QQuickFramebufferObject::Renderer, protected QOpenGLFunctions {
public:
    MyItemRenderer() {
        initializeOpenGLFunctions();

        m_program.addShaderFromSourceCode(QOpenGLShader::Vertex,
            "attribute highp vec4 aPos;"
            "attribute highp vec2 aTexCoord;"
            "varying mediump vec2 vTexCoord;"
            ""
            "void main() {"
            "    gl_Position = aPos;"
            "    vTexCoord = aTexCoord;"
            "}");

        m_program.addShaderFromSourceCode(QOpenGLShader::Fragment,
            "varying mediump vec2 vTexCoord;"
            "uniform sampler2D uTex;"
            ""
            "void main() {"
            "   gl_FragColor = texture2D(uTex, vTexCoord);"
            "}");

        m_program.link();

        m_program.bind();

        // Setup texture sampler uniform
        glActiveTexture(GL_TEXTURE0);
        m_program.setUniformValue("uTex", 0);

        createGeometry();
    }

    void render() {
        glClearColor(0.0f, 0.0f, 0.0f, 0.0f);

        glClear(GL_COLOR_BUFFER_BIT);

        glDisable(GL_CULL_FACE);
        glDisable(GL_DEPTH_TEST);

        m_program.bind();
        m_tex->bind();
        paintGeometry();

        m_window->resetOpenGLState();

        update();
    }


    void paintGeometry() {
        m_program.enableAttributeArray("aPos");
        m_program.enableAttributeArray("aTexCoord");
        m_program.setAttributeArray("aPos", m_vertices.constData());
        m_program.setAttributeArray("vTexCoord", m_texCoords.constData());
        glDrawArrays(GL_TRIANGLES, 0, m_vertices.size());
        m_program.disableAttributeArray("aPos");
        m_program.disableAttributeArray("aTexCoord");
    }

    QOpenGLFramebufferObject *createFramebufferObject(const QSize &size) {
        QOpenGLFramebufferObjectFormat format;
        format.setAttachment(QOpenGLFramebufferObject::CombinedDepthStencil);
        return new QOpenGLFramebufferObject(size, format);
    }

protected:
    void synchronize(QQuickFramebufferObject* qqfbo) {
        MyItem* parentItem = (MyItem*)qqfbo;
        QQuickItem* sourceItem = parentItem->sourceItem();
        QQuickItem* texSource = sourceItem->childItems()[0];
        QSGTextureProvider* texProvider = texSource->textureProvider();
        m_tex = texProvider->texture();

        m_window = parentItem->window();
    }

private:
    QVector<QVector3D> m_vertices;
    QVector<QVector2D> m_texCoords;
    QOpenGLShaderProgram m_program;
    QSGTexture* m_tex;
    QQuickWindow* m_window;

    void createGeometry() {
        m_vertices << QVector3D(0, 0, 0.0f);
        m_vertices << QVector3D(1, 0, 0.0f);
        m_vertices << QVector3D(0, 1, 0.0f);

        m_texCoords << QVector2D(0, 0);
        m_texCoords << QVector2D(1, 0);
        m_texCoords << QVector2D(0, 1);
    }
};

QQuickFramebufferObject::Renderer *MyItem::createRenderer() const {
    return new MyItemRenderer();
}

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

    qmlRegisterType<MyItem>("MyItem", 1, 0, "MyItem");

    QQmlApplicationEngine engine;
    engine.load(QUrl(QStringLiteral("qrc:/main.qml")));

    return app.exec();
}

#include "main.moc"

main.qml:

import QtQuick 2.0

import MyItem 1.0
import QtQuick.Window 2.2

Window {
    visible: true
    width: 400
    height: 400

    Rectangle {
        anchors.fill: parent
        color: "lightblue"
    }

    Item {
        id: test
        // visible: false

        Text {
            layer.enabled: true

            font.pointSize: 30

            text: "hello\nhello\nhello"
        }
    }

    MyItem {
        anchors.fill: parent
        sourceItem: test
    }
}

If you uncomment the visible: false in main.qml, the QQFBO stops displaying anything. Why is that, and what is a proper solution?

I've tried setting opacity: 0 instead of visible: false but this causes the same problem.

I tried replacing layer: true with a separate ShaderEffectSource element with hideSource: true, but then I have to set visible: false on the ShaderEffectSource, and that causes the same problem.

I've also tried setting x: -10000; y: -10000 instead of visible: false and that works fine but is a hack.

[sierdzio]

Both in QtWidgets and QtQuick "visible" property is propagated to children. So if you make "test" invisible, all it's children will become invisible as well - so your text will cease showing up. And since your MyItem uses that as source, it will paint invisible item == nothing is displayed.

[Stefan Monov76]

@sierdzio: Sure, the text will cease to be rendered to the window by Qt Quick, and that's ok. But I want it to keep being rendered to the texture. Surely that's a common use case - they even included a hideSource option in ShaderEffectSource, but neglected this use case in the QQFBO case.

[sierdzio]

It's a bug/ missing feature then, maybe? Have you checked the bugtracker? https://bugreports.qt.io