I am trying to overlay a transparent PNG on a Slippy map using stretchdraw. That works fine until I change the zoom level - the map zooms in and the weather radar overlay stays the same size. Is there someway that I can lock the PNG overlay to the map? Perhaps I should check the bounding rectangle coordinates because they are fixed lat/lons before I do the stretch draw.
SlippyMaps use a virtual bitmap, so you're going to have to repaint it each time the view changes (and scale it by the zoom level).
Alternatively, if you don't need panning (just zooming), we have support for Google Maps in our current beta, which would avoid using a virtual bitmap (though Google does charge for the service).