I am wondering if anyone would have any idea of how to go about creating a scanning line effect. I have some footage of a downtown cityscape that I would like to have a bright line pass over, where the line molds to the shape of the buildings (like they are being scanned in a photocopier, although that is not the context I am going for). I want to have the line "turn" a black and white cityscape into color. I can handle the transition from black and white to color easy enough with masking, but I can't figure out how I would get a line to appear to move along the z-axis and mold to the shape of buildings of a 2D image without some serious rotorscoping.
I suggest you give the Displacement Map a shot - it offsets pixels based on the value of pixels in another layer. While it won't nail the effect you want perfectly, the bright line just might be bright enough that it takes the viewers' attention away from the fact that it's not perfect...
You might also want to do some hand work in Photoshop to get the line closer to conforming to your shot in advance of the Displacement Map effect.
I think I know the type of effect you're shooting for and it's not going to be that easy to pull off if you want the line to actually look like it's being projected onto the building scapes. Using the displacement effect is only really going to conform your line to the subtle details of the shots, for example, if you use the displacement effect it will help conform the line to the subtle insets (bricks, windows, ledges) and facades of the buildings but it's not going to give you that scanner effect because it's not actually being projected onto 3D geometry.
You can either rotoscope it out by hand in PS or AE. (Not Fun)
You can try creating 3D geometry to match the buildings in the shot and then actually project a real line onto them using a 3D program. Or you can create some simple 3D boxes in AE that match the size and shape of your buildings, then use an After Effects light with projection enabled and project a line onto the 3D planes which you can then either turn into shadow catchers or make them black and make the line white and set them to screen so they blend in. But you need some sort of actual 3D geometry to project the line onto to actually get it to conform to the surfaces much like a scanner does.