David Walend's Blog

January 2006 Archives

Affine Frustration Transformed - New! Fewer Bugs!

I put together a generic ZoomPane that holds other Swing components. Hand ZomePane's transformChanged() method a new AffineTransform to show a new portion of the underlying view. ZoomPane uses a little linear animation to glide into the new view gracefully.

I set up ZoomButtonPane as a simple interface. (I want to play with something more complex and transparent. It's been a while since I've worked with Swing, so I wanted to warm up with some easy buttons.) ZoomButtonPane hands off AffineTransforms to a ZoomPane.

Panning worked great. Zooming in and out was a pain to get right. I found similar old questions in some forums, with no answers. I asked for help in other forums, and got a few responses that pointed to example code. Unfortunately, none of the examples solved the whole problem; in each case, some controller held on to some of the state information used by ZoomPane. I figured out how to get those details from just the old transform and size of the window.

    private void zoom(double newScale)
    {
        AffineTransform oldDestinationTransform = zoomPane.getCopyOfDestionationTransform();

//Unwind the centering from the old transform.
        double centerX = zoomPane.getWidth()/2;
        double centerY = zoomPane.getHeight()/2;

        double oldScale = oldDestinationTransform.getScaleX();

        oldDestinationTransform.translate(centerX,centerY);
        oldDestinationTransform.scale(1.0/oldScale,1.0/oldScale);
        oldDestinationTransform.translate(-centerX,-centerY);
        
        oldDestinationTransform.translate((oldDestinationTransform.getTranslateX()/oldScale)
                                            -oldDestinationTransform.getTranslateX(),
                                            (oldDestinationTransform.getTranslateY()/oldScale)
                                            -oldDestinationTransform.getTranslateY());
        
        AffineTransform newDestinationTransform = new AffineTransform();

        newDestinationTransform.translate(centerX,centerY);
        newDestinationTransform.scale(newScale,newScale);
        newDestinationTransform.translate(-centerX,-centerY);

        newDestinationTransform.concatenate(oldDestinationTransform);
        
        zoomPane.transformChanged(newDestinationTransform);
    }

Here's an applet showing it working:

Kick the code around yourself at https://tokenarranger.dev.java.net.

This work is licensed under a Creative Commons License.