Week 1 Lightness Perception Flashcards Preview

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Flashcards in Week 1 Lightness Perception Deck (30):

What is lightness, what is its relationship to reflectance and how is it measured?

Lightness is a surface’s perceived shade of gray. It is measured on a Munsell scale that (theoretically) ranges from 0-10 from black to white. Essentially, it is a surface’s perceived reflectance.


What is the central problem in lightness perception?

The central problem in lightness perception is that it is impossible to know a surface’s reflectance by measuring its luminance, unless you also know the illumination (which you rarely know).


How does the Anchoring Theory of Lightness Perception propose that observers (implicitly) estimate the reflectance of surfaces?

According to the Anchoring Theory of Lightness Perception, when implicitly estimating the reflectance of a surface, humans assume that the surface with the highest luminance is white (i.e. has a reflectance of approximately 90%) and estimate the reflectance of other surfaces by comparing the luminance of these other surfaces to the luminance of the reference surface using the following equation:

Reflectance_1 = Lumiance_1/Luminance_HL x 90%.


What is a framework?

A framework is a group of surfaces that are assumed to be under the same illumination.


How do humans determine what the frameworks are?

According to Gilchrist et al. (1999) frameworks are determined by 5 factors:
1) Depth – surfaces that appear to lie in different depth planes are typically treated as belonging to different frameworks.
2) Orientation – surfaces of different orientations tend to be segmented into different frameworks.
3) Fuzzy boundaries (e.g. penumbrae) - tend to segregate surfaces into different frameworks.
4) Junctions – can be also be used to determine which surfaces may lie in a shadow, so need to be analyzed separately.
5) Grouping - surfaces that group together tend to be treated as a single framework.


What are the problems with the Anchoring Theory of Lightness Perception?

1. Does not apply to surfaces that are textured and glossy.
2. Does not handle transparency well.
3. Points within the same region do not always have the same lightness. Cannot explain grating induction.
4. Grouping do not always determine lightness.


What is the central tenet of scission theory?

When two aligned contours undergo a discontinuous change in the magnitude of contrast, but preserve contrast polarity, the lower contrast region is decomposed into two causal layers and this can result in the percept of transparency or a lightness transformation.


What are the problems with scission theory?

1. T-junction don’t always determine lightness – e.g. the Howe illusion (2001).
2. Transparency can occur even when there are no T-junctions.
3. Grating induction
4. Cannot explain Anderson & Winawer (2005)


What is a framework? What five factors do human use to determine the frameworks in a scene? In the figure below explain whether or not the two surfaces indicated below belong to the same framework and explain your answer.

A framework is a group of surfaces that are assumed to be under the same illumination.
The 5 factors humans use to determine the frameworks are depth, fuzzy boundaries, junctions, orientation, grouping.
The two indicated surfaces belong to different frameworks because they lie at different depths from the observer.


How do we measure lightness?

Using Munsell Values


If you can use a Munsell chart to relate lightness to reflectance then isn’t everything solved? If you want to know the lightness of an object, can’t you just calculate that from its reflectance?

Often this method does work. But sometimes it doesn’t. The problem is that humans sometimes misperceive the reflectance of an object. Instead of saying that lightness = physical reflectance, we should say that lightness = perceived reflectance.


What solution is proposed by the Anchoring Theory of Lightness?

If we could figure out the reflectance of any one surface (which we will call the reference surface REF), we could deduce the reflectances of all the other surfaces (under the same illumination). Humans assume that the surface with the highest luminance (HL) is white (i.e. has a reflectance of approx. 90%).
This is known as Anchoring to White.


What is the Gelb Effect?

A matte black surface which is illuminated in isolation will appear to be white regardless of the real colour of the object. This is called the Gelb Effect. In this absence of any other information, humans assume the surface with the highest luminance is white. This is the justification for the previous anchoring rule.


What is co-determination?

Codetermination is a way for the visual system to hedge its bets. In this case, the actual lightness of the grey patch was mid grey, so while the prediction from the local framework was way off, the net prediction was quite close. While the local framework would predict its lightness to be white, the global frame (containing patches of higher luminance) predicts its lightness to be mid-grey. Combining these two predictions results in its perceived lightness of light grey. Lightness is codetermined by these two frameworks.


What is an X-junction?

It’s an X-junction because 4 surfaces comes together to form an X (the boundaries between the 4 surfaces look like an X). For each of the 4 boundary, a contrast polarity and line boundary can be determined. And for each set of a line boundary, I can look at the contrast polarity and notice that it reverses on traversing a set of a line boundary.


What is polarity?

The polarity of an edge is the direction in which luminance increases on crossing the edge.


Name the two types of x-junctions. Elaborate on each.

1. Double polarity-reversing X-junction: Traversing any two aligned arms of the X-junction causes the polarity to switch.

2. Non polarity-reversing X-junction: when we traverse any two aligned arms of the X, the polarity does not change


Which type of x-junction is consistent with shadow boundaries?

Non polarity-reversing X-junctions`


What are t-junctions and what does it signify?

The ATLP, assumes that T-junctions signify occlusion, thereby indicating that the two surfaces that share a stem should form a framework.


When does scission theory account for the Münker White illusion?

The scission account predicts the Münker-White illusion occurs only when contrast between grey bar and neighbouring stripe is less than contrast between stripes. This only occurs when luminance of grey bar is between luminance of stripes. Contrast polarity needs to be preserved.


Scission theory is dependent on what?

Scission Theory is dependent on the T-junctions, so if the T-junctions has not been altered, an illusion should not be altered.


Elaborate on ATLP:

For 2 surfaces under the same illumination, the ratio of their reflectances and luminances are the same. Thus, if we could figure out the reflectance of any one surface (which we will call the reference surface REF), we could deduce the reflectances of all the other surfaces. links to anchoring to white --


Why is the lotto illusion such a strong illusion?

Because the two surfaces (A&B) are segregated by 2 factors: penumbra and orientation. Combining factors which cause strong segregation can create a stronger illusion.


What are junctions?

Shadows with a sharp edge.


Elaborate on the first problem of ATLP: "Does not apply to surfaces that are textured and glossy."

ATLP is based on the Gelb Effect, and gelb effect only works if the surfaces are not textured and glossy. If we illuminate surfaces that are textured and glossy, it will look like a glossy black-painted object, and it doesn't appear to be white.


Elaborate on the second problem of ATLP: "Does not handle transparency well."

ATLP has no concept of transparency, and no mechanism to handle transparency. It cannot explain why the lightness of the background doesn't appear to change as it goes underneath the disc. ATLP has no concept of two different layers or seeing one surface through another.


Elaborate on the third problem of ATLP: "Points within the same region do not always have the same lightness."

ATLP cannot explain the grating induction, because it predicts the horizontal grey bar should have uniform lightness in colour because it is of the same luminance.


Elaborate on the core tenets of scission theory and how does this theory explain the münker white illusion (white's effect)?

Scission theory uses junctions (X, T), and there's a T-junction between the grey bar on top of the white stripes beside the black stripes. The lower contrast (grey surface) decomposes and forms a semi transparent grey film above a white background. The grey patches are perceived to be floating above the white stripes which means some of the lightness of the grey patches is attributed to the underlying white stripes. So the grey patches are perceived to be darker. Similarly, the grey patches on top of the black stripes are perceived to be lighter, because we attribute some of their darkness to the underlying black stripes.


ATLP's accounts of White's effect broke its down to simultaneous lightness contrast (SLC) and suggests that its illusion cannot be stronger than the latter. How does Scission theory argue against this?

Using the T-junctions, scission theory can explain why white's effect is an even stronger illusion than SLC through transparency. It actually predicts no illusion for SLC even.


Scission vs. Transparency:

Scission may reveal itself as either a transparency or a lightness transformation.

Scission as transparency: e.g. a grey film
Scission as lightness transformation: e.g. place a light source and occluder (produces same grey film but with a cause)