Maintaining relative child position after applying QGraphicsItem::ItemIgnoresTransformations

Question

I have a QGraphicsTextItem parented to a QGraphicsItem. I want the QGraphicsTextItem to always reside directly above the QGraphicsItem, but I also want the text to remain the same size when the scale factor goes below 1, i.e. the text remains the size it is at a scale factor of 1 even when the parent graphics item is scaled smaller. I have found that setting the QGraphicsItem::ItemIgnoresTransformations flag to true when the scale factor is below 1 does the trick for retaining the size.

But I can’t seem to find a way to get the position of the text to always remain above the QGraphicsItem. Is there a way to do this? I tried using deviceTransform () function, but the text still moved off of the QGraphicsItem as I scrolled out. What was worse is that some of the text items started “jiggling”, i.e. they started continuously changing their position ever so slightly, so that it looked like they were shaking. If this is the function I need to use, I guess I don’t know how to use it properly.

In the constructor of my QGraphicsItem I’ve added a QGraphicsTextItem:

fTextItem = new QGraphicsTextItem(getName(), this);
fTextItem->setFlag(QGraphicsItem::ItemIgnoresTransformations);

Here is code snippet from paint function of QGraphicsItem

qreal lod = painter->worldTransform().m22();
if(lod <= 1.0) {
     fTextItem-setFlag(QGraphicsItem::ItemIgnoresTransformations);
     fTextItem->setPos(fTextItem->deviceTransform(view-viewportTransform()).inverted().map(view->mapFromScene(mapToScene(0,0))));
} else {
     fTextItem->setFlag(QGraphicsItem::ItemIgnoresTransformations, false);
     fTextItem->setPos(0, 0);
}

Torrlow

shouldn't you set the QGraphicsTextItem objects flag to ignore parent transformation instead of the the QGraphicsItem? – Cowgill

Answer 1

Disclaimer: this may be overkill for what you are trying to do. We had some additional restrictions in our project that made this solution the easiest for us.

We had to do something similar in a project, and it ended up being easiest for us to not use ItemIgnoresTransformations and instead roll our own transform. Here is the main function we use to create a translation-only (no scaling) transform for drawing an item at a specific location. You might be able to modify it for your usage.

static QTransform GenerateTranslationOnlyTransform(
    const QTransform &original_transform,
    const QPointF &target_point) {
  // To draw the unscaled icons, we desire a transform with scaling factors
  // of 1 and shearing factors of 0 and the appropriate translation such that
  // our icon center ends up at the same point. According to the
  // documentation, QTransform transforms a point in the plane to another
  // point using the following formulas:
  // x' = m11*x + m21*y + dx
  // y' = m22*y + m12*x + dy
  //
  // For our new transform, m11 and m22 (scaling) are 1, and m21 and m12
  // (shearing) are 0. Since we want x' and y' to be the same, we have the
  // following equations:
  // m11*x + m21*y + dx = x + dx[new]
  // m22*y + m12*x + dy = y + dy[new]
  //
  // Thus,
  // dx[new] = m11*x - x + m21*y + dx
  // dy[new] = m22*y - y + m12*x + dy
  qreal dx = original_transform.m11() * target_point.x()
             - target_point.x()
             + original_transform.m21() * target_point.y()
             + original_transform.m31();
  qreal dy = original_transform.m22() * target_point.y()
             - target_point.y()
             + original_transform.m12() * target_point.x()
             + original_transform.m32();

  return QTransform::fromTranslate(dx, dy);
}

To use, take the QPainter transform that is passed to the paint method and do something like:

painter->save();
painter->setTransform(GenerateTranslationOnlyTransform(painter->transform(),
                                                       some_point));
// Draw your item.
painter->restore();

Bougie

At this point looks like overkill for my problem :) But solid ground up approach if there is no easy way. Thanks a lot! – Torrlow

I tried using this custom transformation in my application. It works as far transformation is concern but still my item positions are off. I want my child item(has custom transformation) to stay at (0,0) point of parent. I pass some_point as (0,0). – Torrlow

Answer 2

My suggestion is to subclass QGraphicsSimpleTextItem in this manner:

class TextItem
    : public QGraphicsSimpleTextItem
{
public:
    TextItem(const QString &text)
        : QGraphicsSimpleTextItem(text)
    {

    }
    void paint(QPainter *painter, 
        const QStyleOptionGraphicsItem *option, QWidget *widget)
    {
        painter->translate(boundingRect().topLeft());
        QGraphicsSimpleTextItem::paint(painter, option, widget);
        painter->translate(-boundingRect().topLeft());
    }
    QRectF boundingRect() const
    {
        QRectF b = QGraphicsSimpleTextItem::boundingRect();
        return QRectF(b.x()-b.width()/2.0, b.y()-b.height()/2.0, 
            b.width(), b.height());
    }
};
QGraphicsSimpleTextItem *mText = new TextItem("Item");
scene()->addItem(mText);
mText->setFlag(QGraphicsItem::ItemIgnoresTransformations, true);
mText->setPos(itemToFollow->pos());

Naturalism

Note that KD07 asked about QGraphicsTextItem (which supports HTML/rich text formatting). Another take on QGraphicsSimpleTextItem can be found on Qt Centre. – Behindhand

Answer 3

I've found another solution, which does not involve messing with any transformations or by hand scaling/positioning. There is a hint in QGraphicsItem::ItemIgnoresTransformations flag description:

QGraphicsItem::ItemIgnoresTransformations

The item ignores inherited transformations (i.e., its position is still anchored to its parent, but the parent or view rotation, zoom or shear transformations are ignored). [...]

And that's the key! We need two items: a parent that will keep the relative position (without any flags set) and a child item that will do the drawing at parent's (0,0) point (with QGraphicsItem::ItemIgnoresTransformations flag set). Simple as that!

I've encapsulated this functionality into a single class - here is some code:

#include <QGraphicsItem>
#include <QPainter>

class SampleShape : public QGraphicsItem
{
private:
    /* This class implements shape drawing */
    class SampleShapeImpl : public QGraphicsItem
    {
    public:
        SampleShapeImpl (qreal len, QGraphicsItem *parent = nullptr)
            : QGraphicsItem(parent), m_len(len)
        {
            /* ignore transformations (!) */
            setFlag(QGraphicsItem::ItemIgnoresTransformations);
        }

        QRectF boundingRect (void) const override
        {
            /* sample bounding rectangle */
            return QRectF(-m_len, -m_len, m_len*2, m_len*2);
        }

        void paint (QPainter *painter, const QStyleOptionGraphicsItem *, QWidget *) override
        {
            /* draw a shape, (0,0) is an anchor */
            painter->drawLine(0, -m_len, 0, m_len);
            painter->drawLine(-m_len, 0, m_len, 0);
            // ...
        }

    private:
        qreal m_len;  // sample shape parameter
    };

public:
    /* This is actually almost an empty class, you only need to set
     * a position and pass any parameters to a SampleShapeImpl class.
     */
    SampleShape (qreal x, qreal y, qreal len, QGraphicsItem *parent = nullptr)
        : QGraphicsItem(parent), m_impl(len, this) // <-- IMPORTANT!!!
    {
        /* set position at (x, y), view transformations will apply */
        setPos(x, y);
    }

    QRectF boundingRect (void) const override
    {
        return QRectF(); // it's just a point, no size
    }

    void paint (QPainter *, const QStyleOptionGraphicsItem *, QWidget *) override
    {
        // empty, drawing is done in SampleShapeImpl
    }

private:
    SampleShapeImpl m_impl;
};

Kallick

Thanks for the solution, picked this and is working as a charm. – Interact

Answer 4

Great answer by Dave Mateer! I had the problem that I wanted to define a different scale factor at different zoom levels. This is how I did it:

void MyGraphicsItem::paint(QPainter * painter, const QStyleOptionGraphicsItem* option, QWidget* widget)
{
    //save painter for later operations
    painter->save();
    QTransform originalTransform = painter->transform();
    QPointF originalCenter = rect().center();
    qreal dx = originalTransform.m11() * originalCenter.x() + originalTransform.m21() * originalCenter.y() + originalTransform.m31();
    qreal dy = originalTransform.m22() * originalCenter.y() + originalTransform.m12() * originalCenter.x() + originalTransform.m32();
    //normally our target scale factor is 1, meaning the item has keeps its size, regardless of zoom
    //we adjust the scale factor though when the item is smaller than one pixel in comparison to the background image
    qreal factor = 1.0;
    //check if scale factor if bigger that the item size, and thus it occupies less that a pixel in comparision to the background image
    if (rect().width() < originalTransform.m11()) {
        //calculate adjusted scale factor
        factor = originalTransform.m11() / rect().width();
    }
    //adjust position according to scale factor
    dx -= factor * originalCenter.x();
    dy -= factor * originalCenter.y();
    //set the new transform for painting
    painter->setTransform(QTransform::fromScale(factor, factor) * QTransform::fromTranslate(dx, dy));
    //now paint...
    QGraphicsXYZItem::paint(painter, option, widget);
    //restore original painter
    painter->restore();
}

You do need to adjust the bounding rectangle too in that case:

QRectF MyGraphicsItem::boundingRect() const
{
    QRectF rect = QGraphicsEllipseItem::boundingRect();
    //this is a bit hackish, let me know if you know another way...
    if (scene() != NULL && scene()->views().at(0) != NULL)
    {
        //get viewport transform
        QTransform itemTransform = scene()->views().at(0)->transform();
        QPointF originalCenter = rect.center();
        //calculate back-projected original size of item
        qreal realSizeX = rect.width() / itemTransform.m11();
        qreal realSizeY = rect.height() / itemTransform.m11();
        //check if scale factor is bigger that the item size, and thus it occupies less that a pixel in comparison 
        //to the background image and adjust size back to equivalent of 1 pixel
        realSizeX = realSizeX < 1.0 ? 1.0 : realSizeX;
        realSizeY = realSizeY < 1.0 ? 1.0 : realSizeY;
        //set adjusted position and size according to scale factor
        rect = QRectF(rect.center().x() - realSizeX / 2.0, rect.center().y() - realSizeY / 2.0, realSizeX, realSizeY);
    }
    return rect;
}

With this solution the item work very well in my case.

Brothers

Answer 5

Adding to Dave Mateer's answer, I think it'd be helpful to add that in some scenario, you should also maintain proper bounding rectangle (as well as shape) of the object. For me, I need to modify boundingRect() a little too for proper object selection behavior. Remember that the bounding rect of the object will be scaled and transformed as usual if we do NOT use ItemIgnoresTransformations flag. So we also need to rescale the boundingRect to maintain the view independence effect.

To maintain the view-independent bounding rectangle turns out to be quite easy: just grab the scaling factor from deviceTransform(m_view->viewportTransform()).inverted().m11() and multiply this constant to your local coordinate bounding rectangle. For example:

qreal m = this->deviceTransform(m_view->viewportTransform()).inverted().m11();
return QRectF(m*(m_shapeX), m*(m_shapeY),
              m*(m_shapeR), m*(m_shapeR)); 

Henrique

Answer 6

here is a solution I devised of very moderate complexity :

1) Get the boundingRect() of the parent and map it to scene 2) take the minimum X and Y of this list of points, this is the real origin of your item, in scene coordinates 3) set the position of the child

In Pyside :

    br = parent.mapToScene(parent.boundingRect())
    realX = min([item.x() for item in br])
    realY = min([item.y() for item in br])
    child.setPos(parent.mapFromScene(realX, realY)) #modify according to need

Marchal