How to update QGraphicsScene in a QMainWindow - c++

Question

I'm trying to create a simple Mandlebrot viewer in Qt, I have a Mainwindow with a QGraphicsScene in it, I generate the QImage with the picture and then I have some buttons that I'd like to use to navigate the image (Move, zoom, etc.)
I can get the initial image to appear, however I'm not sure how to tell it to rerender after I've changed any of the coordinates.
For the life of me I can't work out how to refresh the QMainWindow, or alternatively remove the QGraphicsScene from the MainWindow and make a call to render it.
QImage renderImage(//loads in global variables)
{
//calculates the image and returns a QImage
}
MainWindow::MainWindow(QWidget *parent) :
QMainWindow(parent),
ui(new Ui::MainWindow)
{
ui->setupUi(this);
QGraphicsScene *graphic = new QGraphicsScene( this );
graphic->addPixmap(QPixmap::fromImage(renderImage()));
ui->graphicsView->setScene(graphic);
}
void MainWindow::on_Left_clicked()
{
// Changes global variables and try to rerender the scene.
update(); //does nothing
}
UPDATE: Solved!
Thank you so much goug, that helped a lot. I'm new to Qt, so couldn't work out where the loop was that would update things. I added the code you suggested and it worked perfectly. Thanks :)
MainWindow::MainWindow(QWidget *parent) :
QMainWindow(parent),
ui(new Ui::MainWindow)
{
ui->setupUi(this);
QGraphicsScene *graphic = new QGraphicsScene( this );
pixmap_item = graphic->addPixmap(QPixmap::fromImage(renderImage()));
ui->graphicsView->setScene(graphic);
}
void MainWindow::on_Left_clicked()
{
// Changes global variables and try to rerender the scene.
centerR -= 0.1;
pixmap_item->setPixmap(QPixmap::fromImage(renderImage()));
}

Answer 1

You don't show any code that changes any coordinates, or anything for that matter. For the built-in graphics items such as QGraphicsPixmapItem, which is what you create by call addPixmap, you generally don't have to force anything. Those objects repaint themselves as needed when you change something via their member functions.
I suspect that where you're going wrong is that you may be believing that there's a connection between the pixmap and the QGraphicsPixmapItem that you created in the constructor. There isn't; so if it's the pixmap that you're changing, then you need to reapply that pixmap to the pixmap item. You'll need a new member in your class to track:
QGraphicsPixmapItem *pixmap_item_;
And change your constructor code to:
pixmap_item_ = graphic->addPixmap(QPixmap::fromImage(renderImage()));
Then whenever you've updated your pixmap, reapply that pixmap to the graphics item you've created in the constructor:
pixmap_item_->setPixmap (QPixmap::fromImage(renderImage()));
The setPixmap call will trigger the pixmap item to repaint itself; you don't have to call update() separately. If this isn't the issue, then we need to see more of your code.