when I read the paper "Structure-from-Motion Revisited". I met a few words I didn't understand， "track" and "track length". what exactly these terms mean?
A track is a sequence of image positions of a feature (usually an image point) in subsequent images. For example, the track of an eye in a video showing a person is the sequences of pixel positions of that eye in the video stream
Track length is the number of frames that feature is successfully tracked for - say, from the frame the feature is detected to the one it is lost, e.g. due to occlusion, blurring, going out of the field of view or very far away, etc.