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Thread: setTransformOriginPoint

  1. 16th August 2010, 22:25 #1
    brianc
    brianc is offline Beginner
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    Default setTransformOriginPoint

    Can someone please help me figure out how setTransformationOriginPoint works?

    I have a simple program where I am trying to rotate an image using the image edges as the rotation axis. If you press the up arrow, it should rotate along the top axis. The bottom arrow should rotate along the bottom axis, etc.

    Of course, that's not what happens. For each case, it appears that the rotation axis is 0,0. Can someone please show my what I'm doing wrong, or let me know if there is a better way to rotate images along different axis.

    Qt Code: 
    1. #include "mainwindow.h"
    2.  
    3. #include <QGraphicsScene>
    4. #include <QHBoxLayout>
    5. #include <QGraphicsView>
    6. #include <QKeyEvent>
    7.  
    8. MainWindow::MainWindow(QWidget *parent)
    9.     : QWidget(parent)
    10. {
    11.  
    13.     scene->setSceneRect(-150, -150, 300, 300);
    14.     scene->setBackgroundBrush(Qt::white);
    15.  
    16.     item = new QGraphicsPixmapItem(QPixmap(":/images/0.png"));
    17.     item->setPos(QPointF(-item->boundingRect().width()/2,-item->boundingRect().height()/2));
    18.     scene->addItem(item);
    19.  
    20.  
    22.     view->setRenderHints(QPainter::Antialiasing | QPainter::SmoothPixmapTransform);
    23.     view->setScene(scene);
    24.     view->setSceneRect(-150, -150, 300, 300);
    25.     view->setFocus();
    26.  
    27.     QHBoxLayout *layout = new QHBoxLayout;
    28.     layout->addWidget(view);
    29.     setLayout(layout);
    30.  
    31.     rotateTimeLine = new QTimeLine(1500, this);
    32.     rotateTimeLine->setCurveShape(QTimeLine::LinearCurve);
    33.  
    34.     connect(rotateTimeLine, SIGNAL(valueChanged(qreal)),
    35.             this, SLOT(updateRotate(qreal)));
    36.  
    37. }
    38.  
    39. MainWindow::~MainWindow()
    40. {
    41.  
    42. }
    43.  
    44.  
    45. void MainWindow::keyPressEvent(QKeyEvent *event)
    46. {
    47.     qDebug("MainWindow::keyPressEvent");
    48.  
    49.     switch (event->key()) {
    50.  
    51.     case Qt::Key_Up:
    52.         rotate(0);
    53.         break;
    54.  
    55.     case Qt::Key_Right:
    56.         rotate(1);
    57.         break;
    58.  
    59.     case Qt::Key_Left:
    60.         rotate(2);
    61.         break;
    62.  
    63.     case Qt::Key_Down:
    64.         rotate(3);
    65.         break;
    66.  
    67.     }
    68. }
    69.  
    70. void MainWindow::rotate(int axis)
    71. {
    72.     rotateAxis = axis;
    73.     rotateAngle = 0;
    74.     rotateTimeLine->start();
    75.  
    76. }
    77.  
    78. void MainWindow::updateRotate(qreal value)
    79. {
    80.     QTransform txf = QTransform();
    81.  
    82.     rotateAngle = 360*value;
    83.  
    84.     switch (rotateAxis)
    85.     {
    86.     case 0:
    87.         // Up arrow - Rotate along top axis
    88.         txf.rotate(rotateAngle, Qt::XAxis);
    89.         item->setTransformOriginPoint(item->boundingRect().width()/2, item->boundingRect().height()/2);
    90.         break;
    91.  
    92.     case 1:
    93.         // Right arrow - Rotate along rigth axis
    94.         txf.rotate(rotateAngle, Qt::YAxis);
    95.         item->setTransformOriginPoint(item->boundingRect().width(), item->boundingRect().height()/2);
    96.         break;
    97.  
    98.     case 2:
    99.         // Left arrow - Rotate along left axis
    100.         txf.rotate(rotateAngle, Qt::YAxis);
    101.         item->setTransformOriginPoint(0, item->boundingRect().height()/2);
    102.         break;
    103.  
    104.     case 3:
    105.         // Down arrow - Rotate along bottom axis
    106.         txf.rotate(rotateAngle, Qt::XAxis);
    107.         item->setTransformOriginPoint(item->boundingRect().width()/2,item->boundingRect().height());
    108.         break;
    109.     }
    110.  
    111.     item->setTransform(txf, false);
    112.     update();
    113. }
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  2. 16th August 2010, 23:06 #2
    Urthas
    Urthas is offline Intermediate user
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    Default Re: setTransformOriginPoint

    Well, your updateRotate logic isn't doing what you think it's doing. By analogy, you are pushing a thumb tack into a piece of construction paper on a cork board, and then rotating the paper around the tack.

    I understand what you *want* to do, but not really how to do it. However, perhaps you should check out the HelloGL example, ignoring the gl-related code and concentrating on the rotation code. Worth a shot?
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  3. 17th August 2010, 00:26 #3
    brianc
    brianc is offline Beginner
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    Unix/X11 Windows

    Default Re: setTransformOriginPoint

    I have a work around now. I can get the behavior I'm looking for if I do the following:

    * Translate the item to the rotation center point
    * Rotate the item
    * Translate the item back to the starting position

    See the code below for details. I still don't understand why setTransformOriginPoint() doesn't work. Can any Qt experts out there shed some light on this.

    Brian

    Qt Code: 
    1. void MainWindow::updateRotate(qreal value)
    2. {
    3.     QTransform txf = QTransform();
    4.  
    5.     rotateAngle = 360*value;
    6.  
    7.     switch (rotateAxis)
    8.     {
    9.     case 0:
    10.         // Up arrow - Rotate along top axis
    11.         txf.translate(item->boundingRect().width()/2, 0);
    12.         txf.rotate(rotateAngle, Qt::XAxis);
    13.         txf.translate(-item->boundingRect().width()/2, 0);
    14.         //item->setTransformOriginPoint(item->boundingRect().width()/2, item->boundingRect().height()/2);
    15.         break;
    16.  
    17.     case 1:
    18.         // Right arrow - Rotate along rigth axis
    19.         txf.translate(item->boundingRect().width(), item->boundingRect().height()/2);
    20.         txf.rotate(rotateAngle, Qt::YAxis);
    21.         txf.translate(-item->boundingRect().width(), -item->boundingRect().height()/2);
    22.         //item->setTransformOriginPoint(item->boundingRect().width(), item->boundingRect().height()/2);
    23.         break;
    24.  
    25.     case 2:
    26.         // Left arrow - Rotate along left axis
    27.         txf.translate(0, item->boundingRect().height()/2);
    28.         txf.rotate(rotateAngle, Qt::YAxis);
    29.         txf.translate(0, -item->boundingRect().height()/2);
    30.         //item->setTransformOriginPoint(0, item->boundingRect().height()/2);
    31.         break;
    32.  
    33.     case 3:
    34.         // Down arrow - Rotate along bottom axis
    35.         txf.translate(item->boundingRect().width()/2,item->boundingRect().height());
    36.         txf.rotate(rotateAngle, Qt::XAxis);
    37.         txf.translate(-item->boundingRect().width()/2,-item->boundingRect().height());
    38.         //item->setTransformOriginPoint(item->boundingRect().width()/2,item->boundingRect().height());
    39.         break;
    40.     }
    41.  
    42.     item->setTransform(txf, false);
    43. }
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