physics

Structure from motion (psychophysics)

Structure from motion (psychophysics) is a physics topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Structure from motion (psychophysics) rather than just read about it. In short: In visual perception, structure from motion (SFM) refers to how humans (and other living creatures) recover depth structure from object's motion. The human visual field has an important function: capturing the three-dimensional structures of an object using different kinds of visual cues.

Key takeaways

Structure from motion (psychophysics) belongs to physics; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Structure from motion (psychophysics) to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Structure from motion (psychophysics) from memory before moving on to harder problems.

Reference excerpt

In visual perception, structure from motion (SFM) refers to how humans (and other living creatures) recover depth structure from object's motion. The human 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 cue that uses motion of two-dimensional surfaces to demonstrate 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.

Psychophysical studies

In a 1953 study on SFM done by Wallach and O'Connell the kinetic depth effect was tested. They found that by turning shadow images of a three dimensional object can be used as a cue to recover the structure of the physical object quite well. Johansson's study conducted in 1973 discovered our ability to perceive human form of walking or dancing simply from projected motion of several points on the body, this motion pattern was later termed as biological motion. A proposition for how we generate a 3D surface representation of an object is that our visual system uses the spatial and temporal integration of information to detect the structure. Other studies agree that SFM is a process which contains several aspects: the perception of rotating direction, perceived orientation of rotation axis, space interpolation effects and object recognition. Given its complexity, 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-dimensions also confirms the close relationship between MT area and SFM. However, V1 neuron 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 studies. 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 approximations, 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 (meaning SFM is partly a top down process). Combination of all orders of visual cues gives the best estimate of 3D objects.

See also Kinetic depth effect Depth perception Structure from motion

References

External links "demo of a rotating cylinder". YouTube. 4 September 2009.

Worked examples

Example 1 — a first encounter with Structure from motion (psychophysics)

Start with the simplest possible case. Write down what Structure from motion (psychophysics) claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In physics, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Structure from motion (psychophysics) before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Structure from motion (psychophysics) ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Structure from motion (psychophysics)

In research: Structure from motion (psychophysics) appears in physics research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Structure from motion (psychophysics) in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Structure from motion (psychophysics) is common in secondary-school and first-year university syllabi. It links to neighbouring topics Vision, Visual perception, so understanding it makes those chapters shorter.
In everyday life: Look for Structure from motion (psychophysics) outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Structure from motion (psychophysics) in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Structure from motion (psychophysics) means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Structure from motion (psychophysics) out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Structure from motion (psychophysics) in simple terms?

Why does Structure from motion (psychophysics) matter?

Because it connects several physics ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Structure from motion (psychophysics)?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Structure from motion (psychophysics).