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