Example of CRT gamma correction

Gamma correction demonstration: Each panel shows the display gamma that the pixel values have been adjusted for; for example, the pixels in the second panel are proportional to intensity to the 1/2 power, so the image looks approximately correct on a typical PC monitor. The cathode ray tube (CRT is a Vacuum tube containing an Electron gun (a source of electrons and a Fluorescent screen with internal or

Gamma correction, gamma nonlinearity, gamma encoding, or often simply gamma, is the name of a nonlinear operation used to code and decode luminance or tristimulus values in video or still image systems. Relative luminance follows the photometric definition of luminance, but with the values normalized to 1 or 100 for a reference white In the study of the perception of Color, one of the first mathematically defined Color spaces was the CIE 1931 XYZ color space (also known as CIE 1931 color space Video is the technology of electronically capturing, Recording, processing storing transmitting and reconstructing a sequence of Still images ^[1] Gamma correction is, in the simplest cases, defined by the following power-law expression:

where the input and output values are non-negative real values, typically in a predetermined range such as 0 to 1. A power law is any Polynomial relationship that exhibits the property of Scale invariance. A gamma value is sometimes called an encoding gamma, and the process of encoding with this compressive power-law nonlinearity is called gamma compression; conversely a gamma value is called a decoding gamma and the application of the expansive power-law nonlinearity is called gamma expansion.

Explanation

Gamma compression, also known as gamma encoding, is used to encode linear luminance or RGB values into video signals or digital video file values; gamma expansion is the inverse, or decoding, process, and occurs largely in the nonlinearity of the electron-gun current–voltage curve in cathode ray tube (CRT) monitor systems, which acts as a kind of spontaneous decoder. Digital video is a type of Video recording system that works by using a Digital rather than an analog video signal The cathode ray tube (CRT is a Vacuum tube containing an Electron gun (a source of electrons and a Fluorescent screen with internal or Gamma encoding helps to map data (both analog and digital) into a more perceptually uniform domain.

The following figure shows the behavior of a typical display when image signals are sent linearly (γ = 1. 0) and gamma-encoded (standard NTSC γ = 2. NTSC ( National Television System Committee) is the Analog television system used in the United States, Canada, Japan, Mexico 2). In the first case, the resulting image over the CRT is notably darker than the original, while it is shown with high fidelity in the second case. Digital cameras produce, and TV stations broadcast, signals in gamma-encoded form, anticipating the standardized gamma of the reproducing device, so that the overall system will be linear, as shown on the bottom; if cameras were linear, as on the top, the overall system would be nonlinear. Similarly, image files are almost always stored on computers and communicated across the Internet with gamma encoding.

Systems with linear and gamma-corrected cameras. The dashes in the middle represent the storage and transmission of image signals or data files. The three curves represent input–output functions of the camera, the display, and the overall system, respectively.

Generalized gamma

A gamma value is used to quantify contrast, for example of photographic film. This article is mainly concerned with Still photography film For Motion picture film please see Film stock. It is the slope of an input–output curve in log–log space, that is:

which is consistent with the power-law relation above, but applicable to more general nonlinearities. In the case of film, such nonlinearities are called Hurter–Driffield curves. Sensitometry is the scientific study of light-sensitive materials especially Photographic film.

Gamma values less than 1 are typical of negative film, and values greater than 1 are typical of slide (reversal) film.

Windows, Mac, sRGB and TV/video standard gammas

Plot of the sRGB standard gamma-expansion nonlinearity (red), and its local gamma value, slope in log–log space (blue). sRGB is a standard RGB (Red Green Blue color space created cooperatively by HP and Microsoft for use on monitors printers and the Internet

In most computer systems, images are encoded with a gamma of about 0. 45 and decoded with a gamma of 2. 2; in Macintosh systems, the corresponding typical values are 0. Macintosh, commonly nicknamed Mac is a Brand name which covers several lines of Personal computers designed developed and marketed by Apple Inc 55 and 1. 8. In any case, binary data in still image files (as JPEG) are explicitly encoded (that is, they carry gamma-encoded values, not linear intensities), as are motion picture files (such as MPEG). The Moving Picture Experts Group, commonly referred to as simply MPEG, is a Working group of ISO / IEC charged with the development of video and The system can optionally further manage both cases, through color management, if a better match to the output device gamma is required. In digital imaging systems color management is the controlled conversion between the Color representations of various devices such as Image scanners Digital

The sRGB color space standard used with most cameras, PCs, and printers does not use a simple power-law nonlinearity as above, but has a decoding gamma value near 2. sRGB is a standard RGB (Red Green Blue color space created cooperatively by HP and Microsoft for use on monitors printers and the Internet 2 over much of its range, as shown in the plot to the right. Below a compressed value of 0. 04045 or a linear intensity of 0. 00313, the curve is linear (encoded value proportional to intensity), so the gamma is 1. The dashed black curve behind the red curve is a standard gamma = 2. 2 power-law curve, for comparison.

Output to CRT-based television receivers and monitors does not usually require further gamma correction, since the standard video signals that are transmitted or stored in image files incorporate gamma compression that matches close enough the gamma expansion of the CRT. For television signals, the actual gamma values are defined by the video standards (NTSC, PAL or SECAM), and are always fixed and well known values. NTSC ( National Television System Committee) is the Analog television system used in the United States, Canada, Japan, Mexico PAL, short for Phase Alternating Line, is a colour -encoding system used in Broadcast television systems in large parts of the world SECAM, also written SÉCAM ( Séquentiel couleur à mémoire, French for "Sequential Color with Memory" is an analog color television system

Power law for video display

A gamma characteristic is a power-law relationship that approximates the relationship between the encoded luma in a television system and the actual desired image luminance. A power law is any Polynomial relationship that exhibits the property of Scale invariance. As applied to video signals luma represents the brightness in an image (the "black and white" or achromatic portion of the image Television ( TV) is a widely used Telecommunication medium for sending ( Broadcasting) and receiving moving Images, either monochromatic

With this nonlinear relationship, equal steps in encoded luminance correspond roughly to subjectively equal steps in brightness. Ebner and Fairchild ^[2] used an exponent of 0. 43 to convert linear intensity into lightness for neutrals; the reciprocal, approximately 2. 33 (quite close to the 2. 2 figure cited for a typical display susbsytem), would provide optimal perceptual encoding of grays. The following illustration shows the difference between a scale with linearly-increasing encoded luminance signal (linear input) and a scale with linearly-increasing intensity (i. e. , gamma-corrected) scale (linear output).

On most displays (those with gamma of about 2. 2), one can observe that the linear-intensity scale has a large jump in perceived brightness between the intensity values 0. 0 and 0. 1, while the steps at the higher end of the scale are hardly perceptible. The linearly-encoded scale, which has a nonlinearly-increasing intensity, will show much more even steps in perceived brightness.

A cathode ray tube (CRT), for example, converts a video signal to light in a nonlinear way, because the phosphorescent screen it contains is a nonlinear device. The cathode ray tube (CRT is a Vacuum tube containing an Electron gun (a source of electrons and a Fluorescent screen with internal or Phosphorescence is a specific type of Photoluminescence related to fluorescence. The light intensity I is related to the source voltage V_S according to

where γ is the Greek letter gamma. Electrical tension (or voltage after its SI unit, the Volt) is the difference of electrical potential between two points of an electrical The Greek alphabet (Ελληνικό αλφάβητο is a set of twenty-four letters that has been used to write the Greek language since the late 9th or early Gamma (uppercase &Gamma, lowercase γ Γάμμα is the third letter of the Greek alphabet. For a computer CRT, γ is about 2. 2. By coincidence, this results in the perceptually homogeneous scale as shown in the diagram on the top of this section.

For simplicity, consider the example of a monochrome CRT. In this case, when a video signal of 0. 5 (representing mid-gray) is fed to the display, the intensity or brightness is about 0. 22 (resulting in a dark gray). Pure black (0. 0) and pure white (1. 0) are the only shades that are unaffected by gamma.

To compensate for this effect, the inverse transfer function (gamma correction) is sometimes applied to the video signal so that the end-to-end response is linear. In other words, the transmitted signal is deliberately distorted so that, after it has been distorted again by the display device, the viewer sees the correct brightness. The inverse of the function above is:

where V_C is the corrected voltage and V_S is the source voltage, for example from an image sensor that converts photocharge linearly to a voltage. An image sensor is a device that converts an optical image to an electric signal In our CRT example 1/γ is 1/2. 2 or 0. 45.

A color CRT receives three video signals (red, green and blue) and in general each color has its own value of gamma, denoted γ_R, γ_G or γ_B. However, in simple display systems, a single value of γ is used for all three colors.

Other display devices have different values of gamma: for example, a Game Boy Advance display has a gamma between 3 and 4 depending on lighting conditions. The Game Boy Advance (often shortened to GBA) is a 32-bit handheld video game console developed manufactured and marketed by Nintendo. In LCDs such as those on laptop computers, the relation between the signal voltage V_S and the intensity I is very nonlinear and cannot be described with gamma value. However, such displays apply a correction onto the signal voltage in order to approximately get a standard γ=2. 5 behaviour. In NTSC television recording, γ is 2. NTSC ( National Television System Committee) is the Analog television system used in the United States, Canada, Japan, Mexico Television ( TV) is a widely used Telecommunication medium for sending ( Broadcasting) and receiving moving Images, either monochromatic 2.

The power-law function, or its inverse, has a slope of infinity at zero. This leads to problems in converting from and to a gamma colorspace. For this reason most formally defined colorspaces such as sRGB will define a straight-line segment near zero and add raising x+K (where K is a constant) to a power so the curve has continuous slope. sRGB is a standard RGB (Red Green Blue color space created cooperatively by HP and Microsoft for use on monitors printers and the Internet This straight line does not represent what the CRT does, but does make the rest of the curve more closely match the effect of ambient light on the CRT. In such expressions the exponent is not the gamma; for instance, the sRGB function uses a power of 2. 4 in it, but more closely resembles a power-law function with an exponent of 2. 2, without a linear portion.

Methods to perform display gamma correction in computing

Up to four elements can be manipulated in order to achieve gamma encoding to correct the image to be shown on a typical 2. 2- or 1. 8-gamma computer display:

• The pixel's intensity values in a given image file; that is, the binary pixel values are stored in the file in such way that they represent the light intensity via gamma-compressed values instead a linear encoding. This is done systematically with digital video files (as those in a DVD movie), in order to save a gamma-decoding step while playing. DVD (also known as " Digital Versatile Disc " or " Digital Video Disc " - see Etymology)is Similarly, pixel values in standard image file formats are usually gamma-compensated, either for sRGB gamma (or equivalent, an approximation of typical of legacy monitor gammas), or according to some gamma specified by metadata such as an ICC profile. In Color management, an ICC profile is a set of data that characterizes a color input or output device or a Color space, according to standards promulgated by the If the encoding gamma does not match the reproduction system's gamma, further correction may be done, either on display or to create a modified image file with a different profile.

• The rendering software writes gamma-encoded pixel binary values directly to the video memory (when highcolor/truecolor modes are used) or in the CLUT hardware registers (when indexed color modes are used) of the display adapter. Highcolor graphics (variously spelled Hicolor, Hicolour, and Highcolour, and known as Thousands of colors on a Macintosh) is a method See also True Colors (disambiguation. Truecolor is a method of representing and storing graphical image information (especially in computer A colour look-up table (CLUT is a device which converts the logical colour numbers stored in each Pixel of Video memory into physical colours normally represented In Digital electronics, especially Computing, a hardware register stores bits of information in a way that all the bits can be written to or read out simultaneously In computing indexed color is a technique to manage Digital images colors in a limited fashion in order to save computer's memory and file storage A video card, also known as a graphics accelerator card, display adapter, or graphics card, is a hardware component whose function is to They drive Digital-to-Analog Converters (DAC) which output the proportional voltages to the display. In Electronics, a digital-to-analog converter ( DAC or D-to-A) is a device for converting a digital (usually binary code to an Analog signal For example, when using 8-bit per channel, 24-bit RGB color, writting a value of 128 (rounded midpoint of the 0-255 byte range) in video memory it outputs the proportional ≈0. For a full listing of computer's color palettes see List of palettes This list of monochrome and RGB palettes includes A byte (pronounced "bite" baɪt is the basic unit of measurement of information storage in Computer science. 5 voltage to the display, which it is shown darker due to the monitor behavior. Alternatively, to achieve ≈50% intensity, a gamma-encoded look-up table can be applied to write a value near to 187 instead of 128 by the rendering software. In Computer science, a lookup table is a Data structure, usually an Array or Associative array, often used to replace a runtime computation with

• Modern display adapters have dedicated calibrating CLUTs, which can be loaded once with the appropriate gamma-correction look-up table in order to modify the encoded signals digitally before the DACs that output voltages to the monitor. In Computer science, a lookup table is a Data structure, usually an Array or Associative array, often used to replace a runtime computation with ^[3] Setting up these tables to be correct is called hardware calibration. ^[4]

• Some modern monitors allow to the user to manipulate their gamma behavior (as if it were merely another brightness/contrast-like setting), encoding the input signals by themselves before they are displayed on screen. This is also a calibration by hardware technique, but it is performed in an analogic way with the electric signals, instead of by digitally remap of binary values as in the previous cases.

In a correct calibrated system, ideally only one of the elements, the one the first creates image data, should have the gamma encoder role, in order to avoid successively cascaded gamma encodings. ^[4] Subsequent stages may change the gamma to correct for different requirments, and finally the output device will do gamma decoding back to a linear intensity domain. All the encoding and correction methods can be arbitrarily superimposed, without mutual knowledge of this fact among the different elements, which if done incorrectly can lead to highly distorted results but if done correctly as dictated by standards and conventions will lead to a properly functioning system.

In a typical system, for example from camera through JPEG file to display, the role of gamma correction will involve several cooperating parts. The camera encodes its rendered image into the JPEG file using one of the standard gamma values such as 2. 2, for storage and transmission. The display computer may use a color management engine to convert to a different color space (such as a Macintosh's gamma=1. In digital imaging systems color management is the controlled conversion between the Color representations of various devices such as Image scanners Digital 8 color space) before putting pixel values into its video memory. The monitor may do its own gamma correction to match the CRT gamma to that used by the video system. Coordinating the components via standard interfaces with default standard gamma values makes it possible to get such system properly configured.

Simple monitor tests

To see whether your computer monitor is properly hardware adjusted and can display shadow detail in sRGB images properly, you should see the left half of the circle in the large black square very faintly (or not at all), but the right half should be clearly visible. A visual display unit, often called simply a monitor or display, is a piece of Electrical equipment which displays images generated from the Video If not, you can adjust your monitor's contrast and/or brightness setting. Contrast is the difference in visual properties that makes an object (or its representation in an image distinguishable from other objects and the background Brightness is an attribute of Visual perception in which a source appears to emit or reflect a given amount of Light. This alters the monitor's perceived gamma.

This procedure is not suitable for calibrating or print-proofing a monitor. The aim of color calibration is to measure or adjust the color response of a device (input or output to establish a known relationship to a standard Color space. It can be useful for making your monitor display sRGB images approximately correctly, on systems in which profiles are not used (for example, the Firefox browser prior to version 3. 0 and many others) or in systems that assume untagged source images are in the sRGB colorspace.

On some operating systems running the X Window System you can change gamma-correction settings, by issuing the command xgamma -gamma 2. 1 for setting gamma value to 2. 1, and xgamma for querying current value. In Macintosh systems, the gamma and other related screen calibrations are made though their OS control panel. Macintosh, commonly nicknamed Mac is a Brand name which covers several lines of Personal computers designed developed and marketed by Apple Inc Microsoft Windows systems lack (up to the XP version, inclusive) a true native calibration tool. Microsoft Windows is a series of Software Operating systems and Graphical user interfaces produced by Microsoft. Windows XP is a family of 32-bit and 64-bit Operating systems produced by Microsoft for use on Personal computers including home and Third-party tools, as those provided with Nvidia graphic cards' drivers or the Adobe gamma loader tool for Adobe products play the same role under Windows. The multinational NVIDIA Corporation ( (ɪnˈvɪdiə specializes in the manufacture of graphics-processor technologies for Workstations A video card, also known as a graphics accelerator card, display adapter, or graphics card, is a hardware component whose function is to In computing a device driver or software driver is a Computer program allowing higher-level computer programs to interact with a Hardware device Adobe Systems Incorporated (pronounced a-DOE-bee əˈdoʊbiː ( is an American Computer software company headquartered in San Jose California

In the test pattern to the right, the linear intensity of each solid bar is the average of the linear intensities in the surrounding striped dither; therefore, ideally, the solid squares and the dithers should appear equally bright in a properly adjusted sRGB system. sRGB is a standard RGB (Red Green Blue color space created cooperatively by HP and Microsoft for use on monitors printers and the Internet

Photography

The same term (gamma) has long been used in photography to describe an analogous nonlinearity. Photography (fә'tɒgrәfi or fә'tɑːgrәfi (from Greek φωτο and γραφία is the process and Art of recording pictures by means of capturing In photography, gamma refers to the slope of the straight-line region of the sensitometry curve (Hurter–Driffield curve), which is a plot of density (or the logarithm of opacity) of the film image versus the logarithm of the film's exposure to light. Sensitometry is the scientific study of light-sensitive materials especially Photographic film. In Mathematics, the logarithm of a number to a given base is the power or Exponent to which the base must be raised in order to produce

Ansel Adams describes the gamma concept, but then dismisses it as "a term of interest and significance only to the research scientist and the manufacturer", and elaborates:^[5]

'7 minutes at 68°F in Ansco 47 for Isopan' represents 'normal' to me. I have no idea what the actual effective gamma is, nor do I care. I could consider this degree of development as yielding Gamma = 1. 0 or being Development No. 9 or Operation H, or any other symbol I choose. But why should I inject an unnecessary and confusing symbol for a perfectly simple statement of procedure? 'Isopan/Ansco 47/68°F/7minutes' is definite and easily expressed and understood as the means of obtaining my 'normal' negative.

Photographic film has a much greater ability to record fine differences in shade than can be reproduced on photographic paper. This article is mainly concerned with Still photography film For Motion picture film please see Film stock. Photographic paper is paper coated with Light -sensitive chemicals used for making Photographic prints Photographic paper differs from photo papers Similarly, a video screen is not as capable of displaying the range of brightness (dynamic range) which can be captured by electronic cameras. For this reason, considerable artistic effort is invested in choosing in which reduced form the original image should be presented. The gamma correction, or contrast selection, is part of the photographic repertoire used to adjust the reproduced image.

Terminology

The term intensity refers strictly to the amount of light that is emitted per unit of time and per unit of surface, in units of lux. In photometry, illuminance is the total Luminous flux incident on a surface per unit Area. LUX is the principal centre for the promotion and distribution of experimental Film and Video works in the UK. Note, however, that in many fields of science this quantity is called luminous emittance, as opposed to luminous intensity, which is a different quantity. In photometry, illuminance is the total Luminous flux incident on a surface per unit Area. In photometry, luminous intensity is a measure of the wavelength-weighted power emitted by a Light source in a particular direction per unit Solid These distinctions, however, are largely irrelevant to gamma compression, which is applicable to any sort of normalized linear intensity-like scale.

Luminance can mean several things even within the context of video and imaging:

• Luminance is the photometric brightness of an object, taking into account the wavelength-dependent sensitivity of the human eye (in units of cd/m²);
• Luminance (video) is the encoded video "luma" signal, i. Luminance is a photometric measure of the density of Luminous intensity in a given direction The candela (kanˈdɛlə /-ˈdiːlə/ symbol cd) is the SI base unit of Luminous intensity; that is power emitted by a light source in a particular The metre or meter is a unit of Length. It is the basic unit of Length in the Metric system and in the International As applied to video signals luma represents the brightness in an image (the "black and white" or achromatic portion of the image e. similar to the signal voltage V_S.
• Luminance (relative) is the luminance signal used in a color-space encoding, relative to a white level. Relative luminance follows the photometric definition of luminance, but with the values normalized to 1 or 100 for a reference white

Likewise, brightness is sometimes applied to various measures, including light levels, though it more properly applies to a subjective visual attribute. Brightness is an attribute of Visual perception in which a source appears to emit or reflect a given amount of Light.

Gamma correction is a type of power law function whose exponent is the Greek letter gamma (γ). A power law is any Polynomial relationship that exhibits the property of Scale invariance. The Greek alphabet (Ελληνικό αλφάβητο is a set of twenty-four letters that has been used to write the Greek language since the late 9th or early Gamma (uppercase &Gamma, lowercase γ Γάμμα is the third letter of the Greek alphabet. It should not be confused with the mathematical Gamma function. In Mathematics, the Gamma function (represented by the capitalized Greek letter '''&Gamma''') is an extension of the Factorial function Gamma (lower case) is a parameter of the former; gamma (upper case, Γ) is the name of the latter (as in Γ(x)). In Mathematics, Statistics, and the mathematical Sciences a parameter ( G auxiliary measure) is a quantity that defines certain characteristics To use the word "function" in conjunction with gamma correction, one may avoid confusion by saying "generalized power law function. "

In lack of context, a given gamma value can be either the encoding or the decoding value. Caution must be taken to correctly interpreting the value as that to be applied-to-compensate or to be compensated-by-applying its inverse. In common parlance, in many ocassions the decoding value (as 2. 2) is employed as if it were the encoding value, instead of its inverse (1/2. 2 in this case) which is the real value that must be applied to encode gamma.

See also

• Brightness
• Luminance
• Luminance (video)
• Luminance (relative)
• Contrast (vision)
• Color management

References

External links

General information

• Rehabilitation of Gamma by Poynton
• Gamma tutorial (from the PNG specification)
• Frequently Asked Questions about Gamma
• CGSD - Gamma Correction Home Page by Computer Graphics Systems Development Corporation
• Gamma error in picture scaling the gamma is not taken into account by graphic software

Monitor gamma tools

• Measuring Gamma for Monitors (Gernot Hoffman)
• Monitor test pattern for correct gamma correction (by Norman Koren)
• Links browser Gamma Calibration page
• Monitor Calibration Wizard
• QuickGamma
• Gamma Panel
• World Wide Gamma online tool to crowd-source the average gamma. Portable Network Graphics ( PNG) is a bitmapped image format that employs Lossless data compression.