The user need to see the image at the same direction he see the real world,

So we are looking for a fast algorithm to rotate bitmaped map (scanned map) so that the user views the north rotated to any angle on the screen. On big maps the regular approach tends to last very long (too long). Does Manifold contain that feature built in? If so, how would one set it? If not, what are the alternatives. We were thinking about placing a copy of the viewed area only (with some margins) and rotate only this portion, which will make the software more complex.

Could you misuse the terrain of a completely white surface overlayed by the image? Just an idea.