Why does QGraphicsItem::scenePos() keep returning (0,0)

Question

I have been toying with this piece of code:

QGraphicsLineItem * anotherLine = this->addLine(50,50, 100, 100);
qDebug() << anotherLine->scenePos();

QGraphicsLineItem * anotherLine2 = this->addLine(80,10, 300, 300);
qDebug() << anotherLine2->scenePos();

Where the this pointer refers to a QGraphicsScene. In both cases, I get QPointF(0,0) for both output.
From reading the document, I thought scenePos() is supposed to return the position of the line within the scene, not where it is within its local coordinate system. What am I doing wrong?

Lecher

Answer 1

After reading the QT 4.5 documentation carefully on addLine, I realize what I have been doing wrong. According to the doc:

Note that the item's geometry is provided in item coordinates, and its position is initialized to (0, 0)

So if I specify addLine(50,50, 100, 100), I am actually modifying its local item coordinate. The assumption I made that it will be treated as a scene coordinate is wrong or unfounded. What I should be doing is this

// Create a line of length 100
QGraphicsItem * anotherLine = addLine(0,0, 100, 100); 

// move it to where I want it to be within the scene
anotherLine->setPos(50,50);

So if I am adding a line by drawing within the scene, I need to reset its centre to (0,0) then use setPos() to move it to where I want it to be in the scene.

Hope this helps anyone who stumble upon the same problem.

Lecher

I've spent one afternoon hunting this down! Thank you so much! (even if this thread is 3 years old) – Blandish

Still very helpful after almost 15 years :) To understand a bit better why this works, I also found this post enlightening: forum.qt.io/post/403605 – Disheveled