Zooming in / out on images - QT c++

问题

I am currently using QT to load images into a graphicsview scene. Essentially, I am looking for a way to be able to zoom in and out on the loaded image using a slider. The image loads successfully into the pane, but whatever I try to implement to zoom in / out, the image would disappear.

This is how it looks:

GraphicsView

This is my implemented function, but the image disappears:

    void MainWindow::on_horizontalSlider_valueChanged(int value)
{
    float pool ;
        if(value==0 )
            pool=0.1;
        else
            pool  = value*0.01;

        scene->update();
        ui->graphicsView->transform();
        ui->graphicsView->scale(pool,pool);

}

This is how I'm loading the images:

    void MainWindow::on_BrowseImages_clicked()
{
    QString imagePath = QFileDialog::getOpenFileName(
                this,
                tr("Open File"),
                "",
                tr("Images (*.jpg *.jpeg *.png)" )
                );

    imageObject = new QImage();
    imageObject->load(imagePath);

    image = QPixmap::fromImage(*imageObject);

    scene = new QGraphicsScene(this);
    scene->addPixmap(image);
    scene->setSceneRect(image.rect());
    ui->graphicsView->setScene(scene);
    //ui->graphicsView->fitInView(scene->sceneRect(),Qt::KeepAspectRatio);

}

回答1:

When scaling is applied to transformation it always scales around origin.

Scaling with a certain center (that is not the origin) means that the center has been translated to the origin before scaling.

This could be

v' = translate_O→C(scale(translate_C→O(v)))

or with matrix operations

v' = M_{translate_O→C} • M_scale • M_{translate_C→O} • v

However, the Grahics View Framework provides something where combining transformations is actually built-in by default:

Every item provides its own local transformation.
The transformation of a group item is applied to the child items as well forming something which can be imagined as a local coordinate system.

This in mind, I came up with the following MCVE where - translation to center is applied to the pixmap item - scaling is applied to a group item which becomes parent of the pixmap item.

testQGraphicsViewScaleItem.cc:

// Qt header:
#include <QtWidgets>

// main application
int main(int argc, char **argv)
{
  qDebug() << "Qt Version:" << QT_VERSION_STR;
  QApplication app(argc, argv);
  // setup data
  QGraphicsScene qGScene;
  QGraphicsItemGroup qGItemGrp;
  QImage qImgCat("cat.jpg");
  QGraphicsPixmapItem qGItemImg(QPixmap::fromImage(qImgCat));
  qGItemImg.setTransform(
    QTransform().translate(-0.5 * qImgCat.width(), -0.5 * qImgCat.height()));
  qGItemGrp.addToGroup(&qGItemImg);
  qGScene.addItem(&qGItemGrp);
  // setup GUI
  QWidget qWinMain;
  qWinMain.setWindowTitle("QGraphicsView - Scale Image");
  QVBoxLayout qVBox;
  QGraphicsView qGView;
  qGView.setScene(&qGScene);
  qVBox.addWidget(&qGView, 1);
  QSlider qSlider(Qt::Horizontal);
  qSlider.setRange(-100, 100);
  qVBox.addWidget(&qSlider);
  qWinMain.setLayout(&qVBox);
  qWinMain.show();
  // install signal handlers
  auto scaleImg = [&](int value) {
    const double exp = value * 0.01;
    const double scl = pow(10.0, exp);
    qGItemGrp.setTransform(QTransform().scale(scl, scl));
  };
  QObject::connect(&qSlider, &QSlider::valueChanged,
    scaleImg);
  // runtime loop
  return app.exec();
}

and a qmake project file testQGraphicsViewScaleItem.pro:

SOURCES = testQGraphicsViewScaleItem.cc

QT += widgets

Output:

来源：https://stackoverflow.com/questions/60240192/zooming-in-out-on-images-qt-c

标签

c++