In two experiments the depth effect induced by motion of two dots was investigated by estimations of length and angle of inclination of the perceived rod that moves in 3-D space. A third control experiment assessed the corresponding 2-D percepts with static stimuli. In Experiment 1 the dots moved on straight paths, and in Experiment 2 they moved on elliptical paths. The results were compared with predictions from Johansson's frontal-parallel principle. The principle was confirmed for dots moving on convergent paths, the results were inconclusive for straight paths, and the principle was disconfirmed for divergent paths.