How to implement a shape resizer with QGraphicsPolygonItem?

[d_stranz]

Maybe I don't understand your issue. I thought you were trying to implement resizing of graphics items using resize handles on the bounding rectangles of the items.

What you implemented in your initial post works fine for rectangles and ellipses because they are defined by their bounding rectangles. Polygons and other items are defined by an arbitrary closed set of points connected by lines, but they do have a bounding rectangle, the smallest rectangle that contains all of the points. QPolygon (on which QGraphicsPolygonItem is based) has a constructor that takes a QRect argument, but that creates a rectangle polygon. That is the problem you ran into when you tried to implement the same code for resizing polygons that you used for the rectangle and ellipse.

The Wikipedia article I pointed you to shows how to construct a scale transformation using homogenous coordinates, which is what QTransform uses. Your resize code needs to look something like this (and of course you will add checks to avoid divide by zero):

float sx = float( rect.width() ) / float( boundingRect.width() );
float sy = float( rect.height() ) / float( boundingRect.height() );
 
QTransform transform;
transform.scale( sx, sy );
 
QPolygonF  oldPolygon = item->polygon();
QPolygonF newPolygon = oldPolygon * transform;
 
item->setPolygon( newPolygon );

float sx = float( rect.width() ) / float( boundingRect.width() );
float sy = float( rect.height() ) / float( boundingRect.height() );

QTransform transform;
transform.scale( sx, sy );

QPolygonF  oldPolygon = item->polygon();
QPolygonF newPolygon = oldPolygon * transform;

item->setPolygon( newPolygon );

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This code will change the aspect ratio. If you want to keep the same aspect ratio, then you will need to change one of the scale factors so that the new width and height preserve the same ratio as the old width and height.

But like I said earlier, the scale transformation scales the object around its (0,0) point. If the polygon's origin (0,0) is not at the center of the polygon, then scaling will probably also move it. If that happens to you, you will have to scale in three steps: translate your polygon to (0,0), scale, translate back. But by default, QGraphicsItem coordinates have the origin at their center (except for QGraphicsTextItem, QGraphicsPixmapItem, and maybe a few others that have their origin at topLeft).

[xd123]

Thank you so much. I now completely see what you meant, I'm sorry for not understanding it the first time :D working with Qt for just about 4 months now. I implemented your idea and it works just as I wanted it to. Really appreciate your help and willingness to explain your solution.

If anybody stumbles on this issue too here is the working polygon resizer class:

class PolygonResizer : public SizeGripItem::Resizer // resize class for polygon
        {
            public:
                virtual void operator()(QGraphicsItem* item, const QRectF& rect)
                {
                    QGraphicsPolygonItem* polygonItem =
                        dynamic_cast<QGraphicsPolygonItem*>(item);
 
                    QRectF itemBoundingRect = item->boundingRect();
                    float sx = float( rect.width() ) / float( itemBoundingRect.width() );
                    float sy = float( rect.height() ) / float( itemBoundingRect.height() );
 
                    QTransform transform;
                    transform.scale( sx, sy );
 
                    QPolygonF  oldPolygon = polygonItem->polygon();
                    QPolygonF newPolygon = oldPolygon * transform;
 
                    if (polygonItem)
                    {
                        polygonItem->setPolygon( newPolygon );
                    }
                }
        };

class PolygonResizer : public SizeGripItem::Resizer // resize class for polygon
        {
            public:
                virtual void operator()(QGraphicsItem* item, const QRectF& rect)
                {
                    QGraphicsPolygonItem* polygonItem =
                        dynamic_cast<QGraphicsPolygonItem*>(item);

                    QRectF itemBoundingRect = item->boundingRect();
                    float sx = float( rect.width() ) / float( itemBoundingRect.width() );
                    float sy = float( rect.height() ) / float( itemBoundingRect.height() );

                    QTransform transform;
                    transform.scale( sx, sy );

                    QPolygonF  oldPolygon = polygonItem->polygon();
                    QPolygonF newPolygon = oldPolygon * transform;

                    if (polygonItem)
                    {
                        polygonItem->setPolygon( newPolygon );
                    }
                }
        };

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[d_stranz]

Happy to help. A couple of suggestions:

- Since itemBoundingRect and rect are already QRectF (which is floating point based), there is no need to cast width() and height() to float. They already are. I was assuming they were QRect, which is integer based and thus needed casting to avoid rounding errors on division. (if either height or width of the rect is less than the height or width of the bounding rect, integer division would always yield zero as the scale factor).

- You should move the check on polygonItem validity to right after the dynamic_cast. If it is null, then retrieving the oldPolygon a few lines later will blow up.

- You should also use qgraphicsitem_cast<> instead of dynamic_cast<> in all of your resizing methods.