JavaFx 8 - Scaling / zooming ScrollPane relative to mouse position

Question

I need to zoom in / out on a scroll pane, relative to the mouse position.

I currently achieve the zooming functionality by wrapping my content in a Group, and scaling the group itself. I create a new Scale object with a custom pivot. (Pivot is set to the mouse position)

This works perfectly for where the Group's initial scale is 1.0, however scaling afterwards does not scale in the correct direction - I believe this is because the relative mouse position changes when the Group has been scaled.

My code:

@Override
public void initialize(URL location, ResourceBundle resources) {

    Delta initial_mouse_pos = new Delta();

    anchorpane.setOnScrollStarted(event -> {
        initial_mouse_pos.x = event.getX();
        initial_mouse_pos.y = event.getY();
    });

    anchorpane.setOnScroll(event -> {
        double zoom_fac = 1.05;
        double delta_y = event.getDeltaY();

        if(delta_y < 0) {
            zoom_fac = 2.0 - zoom_fac;
        }

        Scale newScale = new Scale();
        newScale.setPivotX(initial_mouse_pos.x);
        newScale.setPivotY(initial_mouse_pos.y);
        newScale.setX( content_group.getScaleX() * zoom_fac );
        newScale.setY( content_group.getScaleY() * zoom_fac );

        content_group.getTransforms().add(newScale);

        event.consume();
    });
}

private class Delta { double x, y; }

How do I get the correct mouse position at different levels of scaling? Is there a completely different way to zooming the ScrollPane that is easier?

Answer 1

This is a scalable, pannable JavaFX ScrollPane :

import javafx.geometry.Bounds;
import javafx.geometry.Point2D;
import javafx.geometry.Pos;
import javafx.scene.Group;
import javafx.scene.Node;
import javafx.scene.control.ScrollPane;
import javafx.scene.layout.VBox;

public class ZoomableScrollPane extends ScrollPane {
    private double scaleValue = 0.7;
    private double zoomIntensity = 0.02;
    private Node target;
    private Node zoomNode;

    public ZoomableScrollPane(Node target) {
        super();
        this.target = target;
        this.zoomNode = new Group(target);
        setContent(outerNode(zoomNode));

        setPannable(true);
        setHbarPolicy(ScrollPane.ScrollBarPolicy.NEVER);
        setVbarPolicy(ScrollPane.ScrollBarPolicy.NEVER);
        setFitToHeight(true); //center
        setFitToWidth(true); //center

        updateScale();
    }

    private Node outerNode(Node node) {
        Node outerNode = centeredNode(node);
        outerNode.setOnScroll(e -> {
            e.consume();
            onScroll(e.getTextDeltaY(), new Point2D(e.getX(), e.getY()));
        });
        return outerNode;
    }

    private Node centeredNode(Node node) {
        VBox vBox = new VBox(node);
        vBox.setAlignment(Pos.CENTER);
        return vBox;
    }

    private void updateScale() {
        target.setScaleX(scaleValue);
        target.setScaleY(scaleValue);
    }

    private void onScroll(double wheelDelta, Point2D mousePoint) {
        double zoomFactor = Math.exp(wheelDelta * zoomIntensity);

        Bounds innerBounds = zoomNode.getLayoutBounds();
        Bounds viewportBounds = getViewportBounds();

        // calculate pixel offsets from [0, 1] range
        double valX = this.getHvalue() * (innerBounds.getWidth() - viewportBounds.getWidth());
        double valY = this.getVvalue() * (innerBounds.getHeight() - viewportBounds.getHeight());

        scaleValue = scaleValue * zoomFactor;
        updateScale();
        this.layout(); // refresh ScrollPane scroll positions & target bounds

        // convert target coordinates to zoomTarget coordinates
        Point2D posInZoomTarget = target.parentToLocal(zoomNode.parentToLocal(mousePoint));

        // calculate adjustment of scroll position (pixels)
        Point2D adjustment = target.getLocalToParentTransform().deltaTransform(posInZoomTarget.multiply(zoomFactor - 1));

        // convert back to [0, 1] range
        // (too large/small values are automatically corrected by ScrollPane)
        Bounds updatedInnerBounds = zoomNode.getBoundsInLocal();
        this.setHvalue((valX + adjustment.getX()) / (updatedInnerBounds.getWidth() - viewportBounds.getWidth()));
        this.setVvalue((valY + adjustment.getY()) / (updatedInnerBounds.getHeight() - viewportBounds.getHeight()));
    }
}

Answer 2

Did you try to remove the setOnScrollStarted-event and move its content to the setOnScroll-event?

Doing so reduces the need of your extra Delta-class and the computations of your mouse-positions are always on par with the current zoom factor.

I implemented the same thing and it works the way you are describing it.

Somehow like this:

@Override
public void initialize(URL location, ResourceBundle resources) {

    anchorpane.setOnScroll(event -> {
        double zoom_fac = 1.05;

        if(delta_y < 0) {
            zoom_fac = 2.0 - zoom_fac;
        }

        Scale newScale = new Scale();
        newScale.setPivotX(event.getX);
        newScale.setPivotY(event.getY);
        newScale.setX( content_group.getScaleX() * zoom_fac );
        newScale.setY( content_group.getScaleY() * zoom_fac );

        content_group.getTransforms().add(newScale);

        event.consume();
    });
}