Having trouble calculating scroll for viewport when zooming in

[NeilAgg]

I would like to keep the center of the viewport in the same place when I zoom in, right now the viewport stays at the same absolute top and left coordinate and the center moves.

I am attaching my zoom in click handler showing my calculations, but the center moves way too far over when I hit the zoom button.
Anyone have any insight into this?

[NeilAgg]

Someone on another forum helped me figure out the calculation. It is:

    double oldZoom = drawPane.getZoomLevelAsDouble();
    double oldLeft = drawPane.getScrollLeft();
    double oldTop = drawPane.getScrollTop();
    int drawPaneWidth = drawPane.getWidth();
    int drawPaneHeight = drawPane.getHeight();
    int newLeft = (int)((oldLeft/oldZoom + drawPaneWidth/oldZoom/2.0 - drawPaneWidth/newZoom/2.0)*newZoom);
    int newTop = (int)((oldTop/oldZoom + drawPaneHeight/oldZoom/2.0 - drawPaneHeight/newZoom/2.0)*newZoom);
    drawPane.setZoomLevel(newZoom);
    drawPane.scrollTo(newLeft,newTop);