Question about QGraphicsRectItem

[wysota]

In QWidget, we never have to worry about where is the parent's location, we don't care, because we know the child position is only related to its parent...

This is exactly the case with graphics items as well.

But not in QGraphicsItem, becasue all item's cordinates have the same origin, which is the scene's origin...

No, this is not true. You are simply using the item in a wrong way. QGraphicsRectItem::setRect() is not equivalent to QWidget::setGeometry()!

Substitute your testGraphicsRect() with this one:

static void testGraphicsRect()
{
  // test graphics
  QGraphicsScene* scene = new QGraphicsScene;
  scene->setSceneRect( 0, 0, 500, 500 );
 
  QGraphicsRectItem* outerRect = new QGraphicsRectItem;
  outerRect->setRect( QRectF( 0, 0, 400, 400 ) );
  outerRect->setBrush( QBrush( Qt::black ) );
 
  QGraphicsRectItem* innerRect = new QGraphicsRectItem;
  innerRect->setRect( QRectF( 0, 0, 200, 400 ) );
  innerRect->setParentItem( outerRect );
  innerRect->setPos(5,5);
  innerRect->setPen( QPen( Qt::red ) );
  innerRect->setBrush( QBrush( Qt::red ) );
 
  scene->addItem( outerRect );
  outerRect->setPos(20,20);
  QGraphicsView* view = new QGraphicsView(scene);
 
  view->setWindowTitle( "testGraphicsRect" );
  view->show();
 
}

static void testGraphicsRect()
{
  // test graphics
  QGraphicsScene* scene = new QGraphicsScene;
  scene->setSceneRect( 0, 0, 500, 500 );
 
  QGraphicsRectItem* outerRect = new QGraphicsRectItem;
  outerRect->setRect( QRectF( 0, 0, 400, 400 ) );
  outerRect->setBrush( QBrush( Qt::black ) );
 
  QGraphicsRectItem* innerRect = new QGraphicsRectItem;
  innerRect->setRect( QRectF( 0, 0, 200, 400 ) );
  innerRect->setParentItem( outerRect );
  innerRect->setPos(5,5);
  innerRect->setPen( QPen( Qt::red ) );
  innerRect->setBrush( QBrush( Qt::red ) );
 
  scene->addItem( outerRect );
  outerRect->setPos(20,20);
  QGraphicsView* view = new QGraphicsView(scene);
 
  view->setWindowTitle( "testGraphicsRect" );
  view->show();
 
}

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[lni]

You are simply using the item in a wrong way. QGraphicsRectItem::setRect() is not equivalent to QWidget::setGeometry()!

Maybe the document for QGraphicsRectItem::setRect() should be more clear, I did think it was the same as setGeometry and use it as setGeometry, and now I have to toss my codes...

Another question about setPos and translate: it appears that both functions have exactly the same visual effect. I did scale and rotate after setPos and translate, both produce identical view. Can you give example where they will produce different result after some operations? I will be using a lot setPos and/or translate, I hope you can give more detailed info on them so I don't make mistake again...

Also, what is the chance to have QGraphicsAbstractBuilder and QGraphicsBuilder, just as QFormBuilder? Serializing QGraphicsScene and its content is very useful...

Many thanks

[wysota]

Maybe the document for QGraphicsRectItem::setRect() should be more clear, I did think it was the same as setGeometry and use it as setGeometry, and now I have to toss my codes...

Please don't blame the docs, they are very clear about this, it's just you didn't read them carefully enough.

The Graphics View coordinate system.

Another question about setPos and translate: it appears that both functions have exactly the same visual effect.

Yes, that's correct.

I did scale and rotate after setPos and translate, both produce identical view. Can you give example where they will produce different result after some operations?

QGraphicsItem::pos() will probably return a different value which can be very important in many situations.

I will be using a lot setPos and/or translate, I hope you can give more detailed info on them so I don't make mistake again...

Use setPos() and forget about translate(), it's purely for completeness.

Also, what is the chance to have QGraphicsAbstractBuilder and QGraphicsBuilder, just as QFormBuilder? Serializing QGraphicsScene and its content is very useful...

None, I guess. There is no data format for storing graphics items. The easiest way to do serialization is to use QDataStream.

[lni]

Please don't blame the docs, they are very clear about this, it's just you didn't read them carefully enough.

Yes, it is my careless. People tend to skip most part of document.

Use setPos() and forget about translate(), it's purely for completeness.

setPos and translate are very very different and produce totally different results after several scale and translate operations. It took me 4 hours to finally find out that I should be using setPos!


You are very helpful! Thank you!