Why doesn't setPos() work when using addPolygon() method? - c++

Question

Consider this class called Hex to draw a hexagon :
// Hex.h
#include <QGraphicsPolygonItem>
#include <QPointF>
#include <QVector>
#include <QImage>
#include <QBrush>
#include <QPen>
class Hex : public QGraphicsPolygonItem {
public:
// constructor
Hex(QGraphicsItem* parent = NULL);
// getters/setters
QPolygonF *getHexagon() {return hexagon;}
QBrush *getBrush() {return brush;}
protected:
QBrush *brush;
QPolygonF *hexagon;
};
and
// Hex.cpp
#include "hex.h"
Hex::Hex(QGraphicsItem *parent) {
// set Points
QVector<QPointF> hexPoints;
hexPoints << QPointF(1, 0) << QPointF(0, 1) << QPointF(0, 2)
<< QPointF(1, 3) << QPointF(2, 2) << QPointF(2 ,1);
// scale the poly
int SCALE_BY = 40;
for (size_t i = 0, n = hexPoints.size(); i < n ; i++) {
hexPoints[i] *= SCALE_BY;
}
//create a QPyolygon with the scaled points
hexagon = new QPolygonF(hexPoints);
// draw the polygon
setPolygon(*hexagon);
}
In main.cpp :
#include <QGraphicsView>
#include <QGraphicsScene>
#include "hex.h"
int main(int argc, char *argv[]){
QApplication a(argc, argv);
QGraphicsScene *scene = new QGraphicsScene();
QGraphicsView *view = new QGraphicsView();
Hex *hex0 = new Hex();
Hex *hex1 = new Hex();
hex0->setPos(400, 400);
hex1->setPos(300, 300);
scene->addItem(hex0);
scene->addItem(hex1);
scene->setSceneRect(0,0 , 1024, 768);
view->setScene(scene);
view->show();
return a.exec();
}
Ok. this is ok and setPos works fine.
But using addPolygon instead of addItem to set picture for the hexagons results in setPos not working and all the hexagons cover each other at the first coordinates given to them.
Like this :
// main.cpp
#include <QApplication>
#include <QGraphicsView>
#include <QGraphicsScene>
#include "hex.h"
int main(int argc, char *argv[]){
QApplication a(argc, argv);
QGraphicsScene *scene = new QGraphicsScene();
QGraphicsView *view = new QGraphicsView();
Hex *hex0 = new Hex();
Hex *hex1 = new Hex();
hex0->setPos(400, 400);
hex1->setPos(300, 300);
// set picture for hexagons:
QBrush* brush = new QBrush(QImage("image.png"));
scene->addPolygon(*hex0->getHexagon(), QPen(Qt::black) ,*brush);
scene->addPolygon(*hex1->getHexagon(), QPen(Qt::black) ,*brush);
scene->setSceneRect(0,0 , 1024, 768);
view->setScene(scene);
view->show();
return a.exec();
}
Why does setPos() has this behavior and how to move the pictured hexagons.
Thanks in advance.

Answer 1

setPos () does not modify the QPolygonF but moves the item in scene coordinates, instead QPolygonF are drawn with respect to the internal coordinate system of the item. Therefore, if you want to observe the initial behavior, you have 2 options:
Move the QGraphicsPolygonItem:
QGraphicsPolygonItem *p0 = scene->addPolygon(*hex0->getHexagon(), QPen(Qt::black) ,*brush);
QGraphicsPolygonItem *p1 = scene->addPolygon(*hex1->getHexagon(), QPen(Qt::black) ,*brush);
p0->setPos(400, 400);
p1->setPos(300, 300);
Move the QPolygonF:
scene.addPolygon((*hex0->getHexagon()).translated(400, 400), QPen(Qt::black));
scene.addPolygon((*hex0->getHexagon()).translated(300, 300), QPen(Qt::black));

Answer 2

Your confusion comes from the fact that QGraphicsPolygonItem already has a QPolygonF and a QBrush member, you don't need extra ones.
Not only that, but addPolygon constructs a whole new QGraphicsPolygonItem:
Creates and adds a polygon item to the scene, and returns the item pointer. The polygon is defined by polygon, and its pen and brush are initialized to pen and brush.
I would suggest that Hex shouldn't be a class, it should be a function:
QGraphicsSceneItem * makeHex(QPointF pos, int scale = 40) {
QVector<QPointF> hexPoints{ QPointF(1, 0), QPointF(0, 1), QPointF(0, 2), QPointF(1, 3), QPointF(2, 2), QPointF(2 ,1) };
// scale the poly
for (auto & point : hexpoints) {
point *= scale;
}
auto * hex = new QGraphicsPolygonItem();
hex->setPolygon(hexPoints);
hex->setPos(pos);
hex->setBrush(QImage("image.png"));
return hex;
}
Or
QGraphicsSceneItem * addHex(QGraphicsScene *scene, int scale = 40) {
QVector<QPointF> hexPoints{ QPointF(1, 0), QPointF(0, 1), QPointF(0, 2), QPointF(1, 3), QPointF(2, 2), QPointF(2 ,1) };
// scale the poly
for (auto & point : hexpoints) {
point *= scale;
}
return scene->addPolygon(hexpoints, QPen(Qt::black), QImage("image.png"));
}