JavaFX 8 - Zooming Relative to Mouse Pointer

Question

I have been trying to use this example to learn about zooming and scaling. It does a great job of demonstrating scaling.

I've been trying to figure out how to change it so that rather than simply zooming on the current center of the view, it will zoom in on the position of the mouse pointer. I have failed. I'm not sure which values to use to do the math. I've found some examples in other languages, but none for JavaFX.

Answer 1

I had the same question here on StackOverflow. The following code is a modified version of the question/answer there.

You can use

• right mouse button = panning
• left mouse button = dragging of the nodes
• scroll wheel = zoom in on mouse cursor location

The relevant part for you is the onScrollEventHandler.

import javafx.application.Application;
import javafx.beans.property.DoubleProperty;
import javafx.beans.property.SimpleDoubleProperty;
import javafx.event.EventHandler;
import javafx.scene.Group;
import javafx.scene.Node;
import javafx.scene.Scene;
import javafx.scene.canvas.Canvas;
import javafx.scene.canvas.GraphicsContext;
import javafx.scene.control.Label;
import javafx.scene.input.MouseEvent;
import javafx.scene.input.ScrollEvent;
import javafx.scene.layout.Pane;
import javafx.scene.paint.Color;
import javafx.scene.shape.Circle;
import javafx.scene.shape.Rectangle;
import javafx.stage.Stage;

class PannableCanvas extends Pane {

    DoubleProperty myScale = new SimpleDoubleProperty(1.0);

    public PannableCanvas() {
        setPrefSize(600, 600);
        setStyle("-fx-background-color: lightgrey; -fx-border-color: blue;");

        // add scale transform
        scaleXProperty().bind(myScale);
        scaleYProperty().bind(myScale);
    }

    /**
     * Add a grid to the canvas, send it to back
     */
    public void addGrid() {

        double w = getBoundsInLocal().getWidth();
        double h = getBoundsInLocal().getHeight();

        // add grid
        Canvas grid = new Canvas(w, h);

        // don't catch mouse events
        grid.setMouseTransparent(true);

        GraphicsContext gc = grid.getGraphicsContext2D();

        gc.setStroke(Color.GRAY);
        gc.setLineWidth(1);

        // draw grid lines
        double offset = 50;
        for( double i=offset; i < w; i+=offset) {
            gc.strokeLine( i, 0, i, h);
            gc.strokeLine( 0, i, w, i);
        }

        getChildren().add( grid);

        grid.toBack();
    }

    public double getScale() {
        return myScale.get();
    }

    public void setScale( double scale) {
        myScale.set(scale);
    }

    public void setPivot( double x, double y) {
        setTranslateX(getTranslateX()-x);
        setTranslateY(getTranslateY()-y);
    }
}


/**
 * Mouse drag context used for scene and nodes.
 */
class DragContext {

    double mouseAnchorX;
    double mouseAnchorY;

    double translateAnchorX;
    double translateAnchorY;

}

/**
 * Listeners for making the nodes draggable via left mouse button. Considers if parent is zoomed.
 */
class NodeGestures {

    private DragContext nodeDragContext = new DragContext();

    PannableCanvas canvas;

    public NodeGestures( PannableCanvas canvas) {
        this.canvas = canvas;

    }

    public EventHandler<MouseEvent> getOnMousePressedEventHandler() {
        return onMousePressedEventHandler;
    }

    public EventHandler<MouseEvent> getOnMouseDraggedEventHandler() {
        return onMouseDraggedEventHandler;
    }

    private EventHandler<MouseEvent> onMousePressedEventHandler = new EventHandler<MouseEvent>() {

        public void handle(MouseEvent event) {

            // left mouse button => dragging
            if( !event.isPrimaryButtonDown())
                return;

            nodeDragContext.mouseAnchorX = event.getSceneX();
            nodeDragContext.mouseAnchorY = event.getSceneY();

            Node node = (Node) event.getSource();

            nodeDragContext.translateAnchorX = node.getTranslateX();
            nodeDragContext.translateAnchorY = node.getTranslateY();

        }

    };

    private EventHandler<MouseEvent> onMouseDraggedEventHandler = new EventHandler<MouseEvent>() {
        public void handle(MouseEvent event) {

            // left mouse button => dragging
            if( !event.isPrimaryButtonDown())
                return;

            double scale = canvas.getScale();

            Node node = (Node) event.getSource();

            node.setTranslateX(nodeDragContext.translateAnchorX + (( event.getSceneX() - nodeDragContext.mouseAnchorX) / scale));
            node.setTranslateY(nodeDragContext.translateAnchorY + (( event.getSceneY() - nodeDragContext.mouseAnchorY) / scale));

            event.consume();

        }
    };
}

/**
 * Listeners for making the scene's canvas draggable and zoomable
 */
class SceneGestures {

    private static final double MAX_SCALE = 10.0d;
    private static final double MIN_SCALE = .1d;

    private DragContext sceneDragContext = new DragContext();

    PannableCanvas canvas;

    public SceneGestures( PannableCanvas canvas) {
        this.canvas = canvas;
    }

    public EventHandler<MouseEvent> getOnMousePressedEventHandler() {
        return onMousePressedEventHandler;
    }

    public EventHandler<MouseEvent> getOnMouseDraggedEventHandler() {
        return onMouseDraggedEventHandler;
    }

    public EventHandler<ScrollEvent> getOnScrollEventHandler() {
        return onScrollEventHandler;
    }

    private EventHandler<MouseEvent> onMousePressedEventHandler = new EventHandler<MouseEvent>() {

        public void handle(MouseEvent event) {

            // right mouse button => panning
            if( !event.isSecondaryButtonDown())
                return;

            sceneDragContext.mouseAnchorX = event.getSceneX();
            sceneDragContext.mouseAnchorY = event.getSceneY();

            sceneDragContext.translateAnchorX = canvas.getTranslateX();
            sceneDragContext.translateAnchorY = canvas.getTranslateY();

        }

    };

    private EventHandler<MouseEvent> onMouseDraggedEventHandler = new EventHandler<MouseEvent>() {
        public void handle(MouseEvent event) {

            // right mouse button => panning
            if( !event.isSecondaryButtonDown())
                return;

            canvas.setTranslateX(sceneDragContext.translateAnchorX + event.getSceneX() - sceneDragContext.mouseAnchorX);
            canvas.setTranslateY(sceneDragContext.translateAnchorY + event.getSceneY() - sceneDragContext.mouseAnchorY);

            event.consume();
        }
    };

    /**
     * Mouse wheel handler: zoom to pivot point
     */
    private EventHandler<ScrollEvent> onScrollEventHandler = new EventHandler<ScrollEvent>() {

        @Override
        public void handle(ScrollEvent event) {

            double delta = 1.2;

            double scale = canvas.getScale(); // currently we only use Y, same value is used for X
            double oldScale = scale;

            if (event.getDeltaY() < 0)
                scale /= delta;
            else
                scale *= delta;

            scale = clamp( scale, MIN_SCALE, MAX_SCALE);

            double f = (scale / oldScale)-1;

            double dx = (event.getSceneX() - (canvas.getBoundsInParent().getWidth()/2 + canvas.getBoundsInParent().getMinX()));
            double dy = (event.getSceneY() - (canvas.getBoundsInParent().getHeight()/2 + canvas.getBoundsInParent().getMinY()));

            canvas.setScale( scale);

            // note: pivot value must be untransformed, i. e. without scaling
            canvas.setPivot(f*dx, f*dy);

            event.consume();

        }

    };


    public static double clamp( double value, double min, double max) {

        if( Double.compare(value, min) < 0)
            return min;

        if( Double.compare(value, max) > 0)
            return max;

        return value;
    }
}



/**
 * An application with a zoomable and pannable canvas.
 */
public class ZoomAndScrollApplication extends Application {
    public static void main(String[] args) {
        launch(args);
    }

    @Override
    public void start(Stage stage) {

        Group group = new Group();

        // create canvas
        PannableCanvas canvas = new PannableCanvas();

        // we don't want the canvas on the top/left in this example => just
        // translate it a bit
        canvas.setTranslateX(100);
        canvas.setTranslateY(100);

        // create sample nodes which can be dragged
        NodeGestures nodeGestures = new NodeGestures( canvas);

        Label label1 = new Label("Draggable node 1");
        label1.setTranslateX(10);
        label1.setTranslateY(10);
        label1.addEventFilter( MouseEvent.MOUSE_PRESSED, nodeGestures.getOnMousePressedEventHandler());
        label1.addEventFilter( MouseEvent.MOUSE_DRAGGED, nodeGestures.getOnMouseDraggedEventHandler());

        Label label2 = new Label("Draggable node 2");
        label2.setTranslateX(100);
        label2.setTranslateY(100);
        label2.addEventFilter( MouseEvent.MOUSE_PRESSED, nodeGestures.getOnMousePressedEventHandler());
        label2.addEventFilter( MouseEvent.MOUSE_DRAGGED, nodeGestures.getOnMouseDraggedEventHandler());

        Label label3 = new Label("Draggable node 3");
        label3.setTranslateX(200);
        label3.setTranslateY(200);
        label3.addEventFilter( MouseEvent.MOUSE_PRESSED, nodeGestures.getOnMousePressedEventHandler());
        label3.addEventFilter( MouseEvent.MOUSE_DRAGGED, nodeGestures.getOnMouseDraggedEventHandler());

        Circle circle1 = new Circle( 300, 300, 50);
        circle1.setStroke(Color.ORANGE);
        circle1.setFill(Color.ORANGE.deriveColor(1, 1, 1, 0.5));
        circle1.addEventFilter( MouseEvent.MOUSE_PRESSED, nodeGestures.getOnMousePressedEventHandler());
        circle1.addEventFilter( MouseEvent.MOUSE_DRAGGED, nodeGestures.getOnMouseDraggedEventHandler());

        Rectangle rect1 = new Rectangle(100,100);
        rect1.setTranslateX(450);
        rect1.setTranslateY(450);
        rect1.setStroke(Color.BLUE);
        rect1.setFill(Color.BLUE.deriveColor(1, 1, 1, 0.5));
        rect1.addEventFilter( MouseEvent.MOUSE_PRESSED, nodeGestures.getOnMousePressedEventHandler());
        rect1.addEventFilter( MouseEvent.MOUSE_DRAGGED, nodeGestures.getOnMouseDraggedEventHandler());

        canvas.getChildren().addAll(label1, label2, label3, circle1, rect1);

        group.getChildren().add(canvas);

        // create scene which can be dragged and zoomed
        Scene scene = new Scene(group, 1024, 768);

        SceneGestures sceneGestures = new SceneGestures(canvas);
        scene.addEventFilter( MouseEvent.MOUSE_PRESSED, sceneGestures.getOnMousePressedEventHandler());
        scene.addEventFilter( MouseEvent.MOUSE_DRAGGED, sceneGestures.getOnMouseDraggedEventHandler());
        scene.addEventFilter( ScrollEvent.ANY, sceneGestures.getOnScrollEventHandler());

        stage.setScene(scene);
        stage.show();

        canvas.addGrid();

    }
}