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.