A 3d map of a city is a bunch of polygons. A skyline is a perspective on that 3d model, so you need occlusion/etc. The single line is then a postprocess on that view of the skyline.
Maybe there's some 2.5D cheats like DOOM did since the 3d model is all on one plane.
Agreed, a ridiculous question.
These questions where you either know the algorithms of giants, or are expected to reverse-engineer algorithms that people did research on for months or years... The height of arrogance.
