In active sensing, the object is sensed by emitting a special source such as laser or ultrasonic waves, while in the passive technique, the scene illumination is provided by ambient light. The other seven monocular methods are texture, linear perspective, motion, focus, occlusion, etc. The monocular methods yield low accuracy and the dynamic range is usually narrow. Active sensing is straight-forward and accurate, but its application domain is limited by security and environmental constraints due to their use of specialized energy source. The passive sensing has wider applicability, but the depth extraction is relatively more involved and complicated.
Binocular method has the most accuracy and more promising future.
However, stereo matching is a long recognized complex problem. The monocular method, either focus or motion, provide an initial guess of the depth field, which guides the stereo matching in an approximate direction, and the binocular stereo in turn yields a better accuracy and resolution. Structural manner means that the plane of laser light and the camera geometry are setup so that they have fixed relation to each other. The camera will always see the object reflection at a known fixed distance intersecting with the laser light plane. The contours of the object vary in the other two dimensions.