How QGraphicsScene::setSceneRect exactly works?

[rippa]

Hello.

I have 3 examples:

1.

mgw = MyGW::_self(this);
    ui->setupUi(this);
    this->resize(300,300);
    ui->verticalLayout->addWidget(mgw);
 
    scene.setSceneRect(-700,-700,700,700);
 
    mgw->setScene(&scene);
    scene.addLine(QLineF(mgw->mapToScene(0,0),mgw->mapToScene(100,0)),axisPen);

mgw = MyGW::_self(this);
    ui->setupUi(this);
    this->resize(300,300);
    ui->verticalLayout->addWidget(mgw);

    scene.setSceneRect(-700,-700,700,700);

    mgw->setScene(&scene);
    scene.addLine(QLineF(mgw->mapToScene(0,0),mgw->mapToScene(100,0)),axisPen);

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In this case graphicsView has long scrolls and my line starts not in top left corner of the view (as i expected) but somewhere around 80,200 point in graphicsView coordinates.

2.

mgw = MyGW::_self(this);
    ui->setupUi(this);
    this->resize(300,300);
    ui->verticalLayout->addWidget(mgw);
 
   scene.setSceneRect(-100,-100,100,100);
 
    mgw->setScene(&scene);
    scene.addLine(QLineF(mgw->mapToScene(0,0),mgw->mapToScene(100,0)),axisPen);

mgw = MyGW::_self(this);
    ui->setupUi(this);
    this->resize(300,300);
    ui->verticalLayout->addWidget(mgw);

   scene.setSceneRect(-100,-100,100,100);

    mgw->setScene(&scene);
    scene.addLine(QLineF(mgw->mapToScene(0,0),mgw->mapToScene(100,0)),axisPen);

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No scrolls in this case and line is not visible

3.

mgw = MyGW::_self(this);
    ui->setupUi(this);
    this->resize(300,300);
    ui->verticalLayout->addWidget(mgw);
 
   // scene.setSceneRect(-100,-100,100,100);
 
    mgw->setScene(&scene);
    scene.addLine(QLineF(mgw->mapToScene(0,0),mgw->mapToScene(100,0)),axisPen);

mgw = MyGW::_self(this);
    ui->setupUi(this);
    this->resize(300,300);
    ui->verticalLayout->addWidget(mgw);

   // scene.setSceneRect(-100,-100,100,100);

    mgw->setScene(&scene);
    scene.addLine(QLineF(mgw->mapToScene(0,0),mgw->mapToScene(100,0)),axisPen);

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Line starts at top left corner but graphicsView has scrolls


So I need correct painting in external coordinates and no unnecessary scrolls. But most of all i want to understand how exactly should i use setSceneRect.

Thank you.

[amleto]

have you looked at the values returned by mapToScene in a debugger?