Contrast



Contrast has several meanings:

Visual perception
In visual perception, contrast is the difference in visual properties that makes an object (or its representation in an image) distinguishable from other objects and the background.

$$(Lmax-Lmin)/(Lmax+Lmin)$$
 * In visual perception of the real world, contrast is determined by the difference in the color and brightness of the light reflected or emitted by an object and other objects within the same field of view.
 * Contrast is calculated using the following formula:

Where Lmax is the maximum luminance and Lmin is the mininmum luminance values


 * The human contrast sensitivity function shows a typical band-pass shape peaking at around 4cycles/degree with sensitivity dropping off either side of the peak (Campbell & Robson, 1968).
 * This tells us that the human visual system is able to detect gratings of 4cycles/degree at a lower contrast than at any other spatial frequency.
 * The high-frequency cut-off represents the optical limitations of the visual systems ability to resolve detail and is typically about 60 cycles/degree, another important factor in the high-frequncy cut-off is the packing density of the retinal photoreceptors, a finer matrix can resolve finer gratings. This is known as the Nyquist Theorem.
 * The low frequency drop-off is due to lateral inhibition within the retinal ganglion cells, a typical retinal ganglion cell presents a centre region with either excitation or inhibition and a surround region with the opposite sign. By using coarse gratings, the bright bands fall on the inhibitory as well as the excitatory region of the ganglion cell resulting in lateral inhibition and account for the low-frequency drop-off of the human contrast senstivity function.


 * In imaging, contrast depends additionally on the image source, the medium, and the ambient lighting.

For example, in the case of graphical computer displays, contrast depends on the properties of the picture source or file and the properties of the computer display, including its variable settings. For some screens the angle between the screen surface and the observer's line of sight is also important.

Telecommunication
In telecommunication, the term contrast has the following meanings:


 * In display systems, the relation between (a) the intensity of color, brightness, or shading of an area occupied by a display element, display group, or display image on the display surface of a display device and (b) the intensity of an area not occupied by a display element, a display group, or a display image. Deprecated synonym: brightness ratio.
 * In optical character recognition, the difference between the color or shading of the printed material on a document and the background on which it is printed.

Source: from Federal Standard 1037C

Music

 * In music and musical form, procedures of contrast include stratification, juxtaposition, and interpolation. Procedures of connection include gradation, amalgamation, and dissolution.

Linguistics and semantics
In linguistics and semantics contrast is a relationship between two discourse segments. Contrast is often overtly marked by contrastive markers like but or however, such as in the following examples:

(1) It's raining but I am taking an umbrella. (2) We will be giving a party for our new students. We won't, however, be serving drinks.

In (1) the first clause, It's raining implies that the speaker will get wet, while the second clause I am taking an umbrella implies that the speaker will not get wet. Both clauses (or discourse segments) refer to related situations, or themes, yet imply a contradiction. It is this relationship of comparing something similar, yet different, that is believed to be typical of contrastive relations. The same type of relationship is shown in (2), where the first sentence can be interpreted as implying that by giving a party for the new students, the hosts will serve drinks. This is of course a defeasible inference based on world knowledge, that is then contradicted in the following sentence.

The majority of the work on contrast and contrastive relations in semantics has concentrated on characterizing exactly what semantic relationships can give rise to contrast. Much early work in semantics also concentrated on identifying what distinguished clauses joined by and from clauses joined by but.

In discourse theory, and computational discourse, contrast is a major discourse relation, on par with relationship like explanation or narration, and work has concentrated on trying to identify contrast in naturally produced texts, especially in cases where the contrast is not explicitly marked.

Medicine
Radiocontrast is used in radiology exams (e.g. CT scans) to highlight the differences between internal structures.