Re: Zoom relative to mouse position

From:

Amir Kouchekinia <amir_nospam@pyrus_nospam.us>

Newsgroups:

comp.lang.java.programmer

Date:

Tue, 20 May 2008 07:18:47 -0700

Message-ID:

<J4BYj.9256$nl7.3109@flpi146.ffdc.sbc.com>

Hi John,

Thank you very much for your reply.

I have tried your code, and the behavior is not what I am looking for.

The red rectangle in my code was located at 100,100 point in the initial
view (Scale 1.0, No Translate). I am not sure why you have centered the
rectangle to the 0,0 point. Also, where ever I try to zoom, the red
rectangle ends up being centered around that point.

This is the behavior I want to see: When I place the mouse pointer over
the lower-left corner of the rectangle and zoom, I want the lower-left
corner of the rectangle to remain at the original mouse pointer location
relative to the screen. Now if I move the mouse pointer to the
upper-left corner of the rectangle and zoom, I want the upper-left
corner of the rectangle to remain at the new mouse pointer location
relative to the screen. If there where other shapes on the canvas, I'd
expect similar behavior when the mouse pointer is located over one of
those other shapes.

I have found an example of this behavior on my Mac OSX 10.5 desktop:
If I hold the 'control' key and use the mouse wheel to zoom in, the
point in the object displayed under the mouse pointer stays under the
mouse pointer relative to the screen.

I have looked at the Arrow thread in this group, but have not been able
to extract the answer I am looking for.

Thanks again.

Amir

John B. Matthews wrote:

In article <IOqYj.2253$l97.606@flpi144.ffdc.sbc.com>,
Amir Kouchekinia <amir_nospam@pyrus_nospam.us> wrote:

Hi,

I am trying to figure out how to zoom relative to the mouse pointer
position. Below, please find my sample code.

When I initially position the mouse pointer on one corner of the red
rectangle and use the mouse wheel, I am able to zoom in and out as
expected. As soon as I move the pointer position, let's say to another
corner of the red rectangle, and try to zoom in or out, my position
relative to the rectangle shifts around.

I think I may be missing a transform or two. In another attempt, in the
mouseWheelMoved method, I inverted tx from the previous iteration and
transformed the mouse position p with the inverted transform before
recalculating the new transform; but the behavior got more erratic.

Any help is greatly appreciated. What am I missing here?

You want to transform a Shape's coordinates, but the normal
concatenation of transformations is arranged for a different goal.
Recalling that matrix multiplication is _not_ commutative, if you want
to scale then translate, you have to apply then in reverse order. See
concatenate() and preConcatenate(), as well as the scaled arrow thread
in this group.

Is this the effect you wanted? [Path2D replaced by Shape for 5.0 users.]

import java.awt.Color;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.Point;
import java.awt.Shape;
import java.awt.event.MouseWheelEvent;
import java.awt.event.MouseWheelListener;
import java.awt.geom.AffineTransform;
import java.awt.geom.Rectangle2D;
import javax.swing.JFrame;
import javax.swing.JPanel;

@SuppressWarnings("serial")
public class ZoomDemo extends JPanel {

    AffineTransform tx = new AffineTransform();

    Rectangle2D.Double rect = new Rectangle2D.Double(-15, -30, 30, 60);

    public ZoomDemo() {
        this.addMouseWheelListener(new ZoomHandler());
    }

    @Override
    public void paint(Graphics g) {
        super.paint(g);
        Graphics2D g2 = (Graphics2D) g;
        g2.setColor(Color.RED);
        Shape shape = tx.createTransformedShape(rect);
        g2.draw(shape);
    }

    private class ZoomHandler implements MouseWheelListener {

        double scale = 1.0;

        public void mouseWheelMoved(MouseWheelEvent e) {
            if (e.getScrollType() == MouseWheelEvent.WHEEL_UNIT_SCROLL) {

                scale += (.1 * e.getWheelRotation());
                scale = Math.max(0.1, scale);
                Point p = e.getPoint();

                tx.setToIdentity();
                tx.translate(p.getX(), p.getY());
                tx.scale(scale, scale);

                ZoomDemo.this.revalidate();
                ZoomDemo.this.repaint();
            }
        }
    }

    public static void main(String[] args) {

        JFrame f = new JFrame("ZoomDemo");
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        ZoomDemo zoomDemo = new ZoomDemo();
        f.getContentPane().add(zoomDemo);
        f.setSize(500, 500);
        f.setLocationRelativeTo(null);
        f.setVisible(true);
    }
}

John