Zoom in on the current mouse position of a QGraphicsView without losing quality

emiliathewhite

I know this is possible by using the scale method of the QGraphicsView and setting the anchor using setTransformationAnchor(QGraphicsView.AnchorUnderMouse). However, doing so would result to the image quality being compromised when zooming.

So what I did instead is to scale the pixmap, and set it as the QGraphicsPixmapItem of the scene. To zoom in on the current mouse position, I use the mapToScene method and position of the wheel event, but this is still not as smooth as the "traditional" implementation. Any idea how I can improve this behavior?

Code:

class Zoom_View(QGraphicsView):

    def __init__(self):
        super().__init__()

        self.scene = QGraphicsScene()
        self.setScene(self.scene)
        self.pix_map = QPixmap("path/to/file")
        self.pix_map_item = self.scene.addPixmap(self.pix_map)
        self.global_factor = 1

    def scaleImage(self, factor):
        _pixmap = self.pix_map.scaledToHeight(int(factor*self.viewport().geometry().height()), Qt.SmoothTransformation)
        self.pix_map_item.setPixmap(_pixmap)
        self.scene.setSceneRect(QRectF(_pixmap.rect()))

    def wheelEvent(self, event):
        factor = 1.5

        if QApplication.keyboardModifiers() == Qt.ControlModifier:
            view_pos = event.pos()
            scene_pos = self.mapToScene(view_pos)
            self.centerOn(scene_pos)

            if event.angleDelta().y() > 0 and self.global_factor < 20:
                self.global_factor *= factor
                self.scaleImage(self.global_factor)
            elif event.angleDelta().y() < 0 and self.global_factor > 0.2:
                self.global_factor /= factor
                self.scaleImage(self.global_factor)
        else:
            return super().wheelEvent(event)

SGaist

Hi and welcome to devnet,

What about scaling the view rather than the pixmap ?

See this wiki article for a possible implementation.

emiliathewhite

@SGaist said in Zoom in on the current mouse position of a QGraphicsView without losing quality:

What about scaling the view rather than the pixmap ?

Hello @SGaist do you mean using QGraphicsView scale method? I have already tried that and it zooms in properly on the current mouse position when I set the anchor to AnchorUnderMouse. However the quality of the image is awful since the scale method has no smooth transformation mode

SGaist

By the way, what is the resolution of your original image ?

emiliathewhite

@SGaist 300 dpi. Even at high resolution, the image will be pixelated if I use the scale method

SGaist

Depending on your end goal, you might want to consider using a library like OpenCV to do the zooming part and Qt to display the result.