Contrast คือค่าความเข้มของภาพ

Contrast is the dissimilarity or difference between things:

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Contrast (linguistics), expressing distinctions between words
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.

Contrast (vision), the difference in color and light between parts of an image.
Contrast is the difference in visual properties that makes an object (or its representation in an image) distinguishable from other objects and the background. In visual perception of the real world, contrast is determined by the difference in the color and brightness of the object and other objects within the same field of view. Because the human visual system is more sensitive to contrast than absolute luminance, we can perceive the world similarly regardless of the huge changes in illumination over the day or from place to place.
The human contrast sensitivity function shows a typical band-pass shape peaking at around 4 cycles per degree with sensitivity dropping off either side of the peak.[1] This tells us that the human visual system is able to detect gratings of 4 cycles per degree at a lower contrast than at any other spatial frequency.
The high-frequency cut-off represents the optical limitations of the visual system's ability to resolve detail and is typically about 60 cycles per degree. The high-frequency cut-off is related to the packing density of the retinal photoreceptor cells: a finer matrix can resolve finer gratings.
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 sensitivity function.
One experimental phenomenon is the inhibition of blue in the periphery if blue light is displayed against white, leading to a yellow surrounding. The yellow is derived from the inhibition of blue on the surroundings by the center. Since white minus blue is red and green, this mixes to become yellow.[2]
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.
Contrast is also the difference between the color or shading of the printed material on a document and the background on which it is printed, for example in optical character recognition.

Contrast (statistics)
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In statistics, particularly analysis of variance, a contrast is a linear combination of two or more factor level means (averages) whose coefficients add up to zero.[1][2] A simple contrast is the difference between two means.
Contrasts are sometimes used to compare a mixed effect. A common example can be the difference between two test scores — one at the beginning of the semester and one at its end. Note that we are not interested in one of these scores by itself, but only in the contrast (in this case — the difference). Since this is a linear combination of independent variables, its variance will match accordingly, as the sum of the variances. This "blending" of two variables into one might be useful in many cases such as ANOVA, regression, or even as descriptive statistics in its own right.
Another example would be comparing 5 standard treatments to a new treatment, hence giving each old treatment a weight of 1/5, and the new sixth treatment a weight of −1. If this new linear combination has a mean zero, this will mean that the old treatments are not different from the new on average.
The usual results for linear combinations of independent random variables mean that the variance of a contrast is equal to the weighted the sum of the variances.[1] If two contrasts are orthogonal, estimates created by using such contrasts will be uncorrelated. This has implications in ANOVA and regression where in certain designed experiments the design is such as to ensure that estimates for a number of different parameters will be uncorrelated.

Contrast medium used to distinguish structures or fluids within a body, often shortened to just "contrast"
A medical contrast medium is a substance used to enhance the contrast of structures or fluids within the body in medical imaging. It is commonly used to enhance the visibility of blood vessels and the gastrointestinal tract.
Contents[hide]
1 Types
1.1 X-ray attenuation
1.1.1 Iodine
1.1.2 Barium
1.2 MR signal enhancing
1.3 Ultrasonography
2 Adverse effects
2.1 Allergy Reactions
2.2 Toxicity
2.3 Drug Interactions
3 See also
4 References

Contrast ratio, a measure of a display system
The contrast ratio is a measure of a display system, defined as the ratio of the luminance of the brightest color (white) to that of the darkest color (black) that the system is capable of producing. A high contrast ratio is a desired aspect of any display, but with the various methods of measurement for a system or its part, remarkably different measured values can sometimes produce similar results.
Contrast ratio ratings provided by different manufacturers of display devices are not necessarily comparable to each other due to differences in method of measurement, operation, and unstated variables.[1] Manufacturers have traditionally favored measurement methods that isolate the device from the system, whereas other designers have more often taken the effect of the room into account. An ideal room would absorb all the light reflecting from a projection screen or emitted by a CRT, and the only light seen in the room would come from the display device. With such a room, the contrast ratio of the image would be the same as the device. Real rooms reflect some of the light back to the displayed image, lowering the contrast ratio seen in the image.
Moving from a system that displays a static motionless image to a system that displays a dynamic, changing picture slightly complicates the definition of the contrast ratio, because of the need to take into account the extra temporal dimension to the measuring process. Thus the ratio of the luminosity of the brightest and the darkest color the system is capable of producing simultaneously at any instant of time is called static contrast ratio, while the ratio of the luminosity of the brightest and the darkest color the system is capable of producing over time is called dynamic contrast ratio.
Contents[hide]
1 Methods of measurement
2 Dynamic contrast ratio
3 Contrast ratio in a real room
4 Notes
5 External links

Display contrast, of electronic visual displays
Contrast in visual perception is the difference in appearance of two or more parts of a field seen simultaneously or successively (hence: brightness contrast, lightness contrast, color contrast, simultaneous contrast, successive contrast, etc.).
Contrast in physics is a quantity intended to correlate with the perceived brightness contrast, usually defined by one of a number of formulae (see below) which involve e.g. the luminances of the stimuli considered, for example: ΔL/L near the luminance threshold (known as Weber contrast [1]), or LH/LL for much higher luminances [2].
A contrast can also be due to differences of chromaticity specified by colorimetric characteristics (e.g. the color difference ΔE CIE 1976 UCS).
Visual information is always contained in some kind of visual contrast, thus contrast is an essential performance feature of electronic visual displays.
The contrast of electronic visual displays depends on the electrical driving (analog or digital input signal), on the ambient illumination and on the direction of observation (i.e. viewing direction).
In the field of electronic visual displays the following forms of contrast can be distinguished:
luminance contrast,
color contrast,
full-screen contrast (time sequential),
full-swing contrast (maximum and minimum luminance),
static contrast (optical response settled),
transient contrast (optical response not settled),
dynamic contrast (technique for improving the sequential contrast of LCD-screens by backlight modulation)
dark-room contrast (no ambient illumination),
"ambient contrast" (short for: contrast in the presence of ambient illumination),
concurrent contrast (dark and light areas presented at the same time, simultaneously),
successive contrast (contrast between visual stimuli presented in sequence).


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