Lack of direct 3-D information. Most of them use contour tracing and the search complexity increased exponentially with image complexity. Some of them are working on nadir views only. 3-D cues in monocular images are not fully utilized. Our previous system used perceptual grouping and shadow techniques, but limited to nadir views.
Projection is locally weak perspective.
Viewpoint and sun angles are known. Roofs are flat and rectilinear, walls are vertical. Shadows fall on flat ground. Hypothesize roof candidates. Select good hypotheses based on image evidence.