User:XiaochenM/sandbox

(change "Structure from motion (SFM) refers to how humans recover depth structure from rotating objects" to "In visual perception, structure from motion (SFM) refers to how humans recover depth structure from object's motion.")

In visual perception, structure from motion (SFM) refers to how humans recover depth structure from object's motion. Human's visual field has an important function: capturing the three-dimensional structures of an object using different kinds of visual cues. SFM is a kind of motion visual cues that uses motion of two-dimensional surfaces to demonstrate the three-dimensional objects, and this visual cue works really well even independent of other depth cues. Psychological, especially psychophysical studies have been focused on this topic for decades.

(add a new section named "Psychophysical Studies")

Psychophysical studies
One of the most representative studies about SFM was done by Wallach and O'Connell in 1953. In this study, they tested the kinetic depth effect and found that the turning shadow images of the three dimensional object can be used as a cue to recover the structure of physical object quiet well. After, Johansson's study discovered our ability to perceive human form of walking or dancing from projected motion of several points on the body, and this motion pattern was later termed as biological motion.

It is proposed that our visual system uses the spatial and temporal integration of information to detect the structure, and this process is achieved by generating a 3D surface representation of the object. Other studies also agree on the fact that SFM is complex which contains several aspects : the perception of rotating direction, perceived orientation of rotation axis , space interpolation effects and object recognition. Given its complexity, it is reasonable to say that SFM involves very high-level of visual processing. Studies have shown that MT, rather than V1 (the primary visual cortex), is directly involved in the generation of the SFM perception. Neurons in MT are also triggered by motion parallax and show depth signs independent of other depth cues, and MT's representation of three-dimensional also confirms the close relationship between MT area and SFM. However, V1 neurons activities are indirectly related to SFM perception, which receives general feedback from MT.

The importance of motion perception of SFM in detecting three-dimensional structure is also demonstrated by several researches. It is studied that the 3D objects can be perceived from the 2D projections of the moving object on a screen, but not the stationary 2D images. Also, one essential condition for SFM perception to occur accurately is that the projection of the object must has simultaneously changing contour and lines. A relatively invariant point lifetime threshold of SFM (50-85 msec) was found, and it turns out that this threshold is close to the threshold of velocity measurement, which suggests that velocity measurement is involved in the SFM processing procedure. Given such mechanism, human visual system can derive an accurate model of SFM even with the presence of noise.

Being a complex process, SFM requires more than orthographic projections approximation though many experiments used orthographic projections. Studies have found that higher order visual cues like acceleration and perspective projection are involved in this process rather than just first order flow. Combination of all orders of visual cues gives the best estimate of 3D objects.