How to compute a QPainterPath from a pixmap - qt

Question

I have a QGraphicsPixmapItem that rotates through different pixmaps to simulate animation. I need to accurately implement the shape() function so the scene can properly determine collision with other objects. Each pixmap obviously has slightly different collision paths. Is there a simple way to create a QPainterPath from a pixmap by outlining the colored pixels of the actual image that border the alpha background of the bounding rect without having to write my own complex algorithm that tries to create that path manually?
I plan on having these paths pre-drawn and cycle through them the same way I do as the pixmaps.

Answer 1

You can use QGraphicsPixmapItem::setShapeMode() with either QGraphicsPixmapItem::MaskShape or QGraphicsPixmapItem::HeuristicMaskShape for this:
#include <QtGui>
#include <QtWidgets>
class Item : public QGraphicsPixmapItem
{
public:
Item() {
setShapeMode(QGraphicsPixmapItem::MaskShape);
QPixmap pixmap(100, 100);
pixmap.fill(Qt::transparent);
QPainter painter(&pixmap);
painter.setBrush(Qt::gray);
painter.setPen(Qt::NoPen);
painter.drawEllipse(0, 0, 100 - painter.pen().width(), 100 - painter.pen().width());
setPixmap(pixmap);
}
enum { Type = QGraphicsItem::UserType };
int type() const {
return Type;
}
};
int main(int argc, char *argv[])
{
QApplication app(argc, argv);
QGraphicsView view;
view.setScene(new QGraphicsScene());
Item *item = new Item();
view.scene()->addItem(item);
// Comment out to see the item.
QGraphicsPathItem *shapeItem = view.scene()->addPath(item->shape());
shapeItem->setBrush(Qt::red);
shapeItem->setPen(Qt::NoPen);
view.show();
return app.exec();
}