1. A method for determining three-dimensional structure in a road environment, the method performed by a system mountable in a host vehicle, wherein the system includes a camera operatively connectible to a processor, the method comprising: capturing a plurality of image frames in the field of view of the camera; in the image frames selecting a line below which the road is imaged, wherein the line separates between upper images essentially excluding images of the road and lower images essentially including images of the road; and warping at least one of the lower images according to a road homography to produce at least one warped lower image.