I have implemented a 3D map display for the game I am currently working on. The map can be scrolled and zoomed as required so the player can either have a zoomed out view of the whole game world or zoom right in to get close to the action.

For the landmarks (buildings) on the map I have played around with using fully 3D models, but have eventually gone with a billboarding system. This works great when zoomed right in with the texturing on the billboards nice and crisp with each building being surrounded with a border (worked into the texture) to maximise visibility. However, when zooming right out the billboards obviously get smaller and the buildings more indistinct. This is okay up to a point but one thing that annoys me is that the border around the buildings gets finer and finer until it looses its clarity. To give an example the image below shows one building at maximum zoom and minimum zoom.

I would like to improve on this and as a minimum ensure that a clear border is visible around each building. As an idea I am thinking about creating additional textures for use when zoomed further out, the basic texture being unchanged but a progressively thicker and thicker border being applied to ensure this. However, before I start down this road I wanted to check if there is any alternative out there (one point to note is that certain buildings have 'cutouts' meaning that a border may be needed inside the outline of the building).

Any help appreciated ... thanks.