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Color Management
Color Fundamental: Color Image Processing
In 1666, Isaac Newton Discovered that when a beam of light of sunlight passes through a glass Prism, the emerging beam of light is split into a spectrum of colors ranging from violet at one end to red at the other.
Three basic quantities are used to describe the quality of chromatic light source.
1. Radiance: radiance is the total amount of energy that flows from light source measured in watts (W).
2. Luminance: Luminance gives a measure of the amount of energy an observer perceives from a light source – measured in lumens (LM).
3. Brightness: Brightness is a subjective descriptor that is practically unmeasurable.
Hue: Hue and saturation taken together are called chromaticity.
A color may be characterized by its brightness and chromaticity.
The amount of red, green, and blue needed to form any color are called the tristimulus values and are denoted, X, Y, and Z.
A color is then specified by its trichromatic coefficients which means
X + Y +Z = 1
The use of color is important in image processing because:
Color image processing is divided into two major areas:
1. LIGHT AND COLOR
To understand the process of color reproduction, it is first necessary to gain an appreciation of the phenomenon of color. To do this, we must examine the nature of light, without which color would not exist.
Light is radiant energy that is visible to the average human eye. For the purposes of this discussion it can be assumed that light travels in wave motion, with the color of light varying according to the length of the wave. The wavelengths can be measured and classified along with other forms of energy on the electromagnetic or energy spectrum. Light can either be a wave as was first proposed by Christian Huygens, or as a series of discrete particles as was first proposed by Sir Isaac Newton. Eventually it was decided that light could be both a wave and a series of particles.
The intensity (or luminosity) of a light source is measured in candles. The intensity of light reflected from a surface (or luminance) is measured in candles per square meter, or foot candles or foot lamberts.
Color is an optical phenomenon, a sensory impression conveyed by the eye and the brain. Light reflected or transmitted by an object is received by our eyes and transformed into nervous impulses, which trigger the colour sensation in our brain. Color is not a physical variable, accordingly it has no physical unit. An object is not colored, but the sensation of color is produced as a result of irradiation by light, Sunlight, which appears to be white, radiates on to an object and is partially reflected. Consequently an object that reflects the red area of the spectrum appears colored. An object that reflects completely in the entire visible spectrum usually appears to be white and a completely absorbent body appears to be black.
When perceiving and describing colors, physical and physiological effects are always involved. The physical components are measurable, where as the physiological components are not measurable.
The mixing of certain basic colors produces all of the colors we can perceive. There are three categories of colors: primary colors, secondary colors, and tertiary colors. Primary colors are those that are not formed by mixing of any other colors and can be said to be “pure” colors. Secondary colors are those formed by the mixing of two or more primary colors. Tertiary colors are those produced by mixing of two or more secondary colors. What constitutes a primary color differs depending on whether one is talking about light or pigments.
Interestingly, according to Hope and Walch in The Color Compendium, polls have consistently found that in Western Europe and North America over half of the adults surveyed name “blue” as their favorite color, while children under eight consistently name “red” as their favorite. (In Japan, however, over half of the people surveyed named either white or black as their favorite color).
Color preferences tend to vary by culture, not unexpectedly. This may seem like a trivial matter, but it is an important consideration in planning multinational advertising campaigns, designing products such as clothing for other markets, and other such endeavors. It also manifests itself in appropriate dress when visiting other cultures; white is not universally accepted as the bride’s dress color at a wedding, for example, nor is black universally appropriate for funerals or other mourning rites. In other words, color is a cultural specific concept; various colors are symbolic of different things, and these symbols are not universally consistent.
Here, the biological vision of human beings is contrasted with the process of physical measurement, as performed by a measuring system. Light falls on a sample. The sample absorbs part of the light, while the rest is reflected or re-emitted as diffused radiation.
We perceive this re-emitted light with our, eyes. In the process of seeing, cones in the retinas of our eyes are stimulated. Different cones are sensitive to blue, green, and red. The stimuli are transformed into excited states, in turn, causing signals to be sent along the optic nerve to the brain, which interprets them as colour.
This same process can be emulated in a measuring instrument. One such measuring instrument is the spectrophotometer. Of course, a measuring instrument cannot actually perceive anything, but it is able to perform calculations on predefined and measured values.
Thus, during the measuring process light also falls on the printed sample. The reflected light, also known as spectral reflectance, passes through a series of lenses to strike a detector. This then relays the values it registers to the computer. There, digital filters that simulate the visual sensitivity of our eyes are used to calculate values, referred to as standard stimuli, or tristimulus values.
The standard stimuli are equivalent to the excitation of cones in our eyes. These tristimulus values are then converted and mapped onto a colorimetric system. With the aid of the figures thus determined, a colour can be precisely described and compared with other colors. This, in very simplistic terms, is the measurement principle underlying a colorimetric instrument.
Colour is a very complex issue and there are many factors which need to be considered in order to understand how we perceive and reproduce it.
We can see the visible wavelengths between 380 and 760nm (one nanometre equals one millionth of a millimeter). If one particular wavelength dominates or, more specifically, the spectral power distribution is unequal we see a particular colour - if there is a balanced distribution of all wavelengths we see white or gray - i.e. - neutral. Light with a wavelength of 380nm appears as violet, 760nm ‘as red and 570nm as green. Colour, as we know it, can be in the form of a ‘physical’ solid, such as printing ink or colored toner; or in the form of an energy light source, such as with a TV or computer colour monitor.
The sensation of colour is the effect of light upon the eye interpreted by the brain. White light is composed of a mixture of all colours of the rainbow or spectrum, and most objects are visible by the light reflected or transmitted from them, depending upon whether the object is opaque or transparent. The colors of the visible spectrum include (in order of increasing wavelength) violet, indigo, blue, green, yellow, orange and red.
White light appears to have no color because all the wavelengths are present in equal amounts, effectively “cancelling” each other out. Objects appear colored because they reflect or transmit some parts of the spectrum and absorb the others. For example, a red object appears red as it reflects the red light and absorbs most of the violet, blue, green and yellow lights. White objects reflect or transmit almost all parts of the spectrum, while tones of gray absorb equal proportions of all its constituents and black absorbs almost the whole of it.
The perception or sensation of color, despite attempts to objectively quantity it, is a highly subjective phenomenon. We speak of, for example, a “red apple,” but the redness of the apple is more dependent on our own peculiar visual systems than any inherent “redness” in the apple. (To organisms with different types of photoreceptors, it could appear to possess a much different color.) Even among different humans, the redness perceived is not absolute, varying according to minute physiological differences in visual acuity or according to the illumination used.
2. THE ELECTROMAGNETIC SPECTRUM
Of the overall spectrum of electromagnetic waves, the human eye is only able to perceive a narrow band between 380 and 780 nanometers (nm). This visible spectrum is situated between, ultraviolet and infrared light. If the light of this visible range is passed through a prism, then the individual spectral colors can be seen.
However, light is not absolute. For example, if a printed image is compared with a proof under artificial light, the two may seem identical, but regarded in daylight, differences may suddenly appear.
Light is a small portion of the much larger electromagnetic spectrum, a broad range of different types of generated energy, ranging from radio waves and electrical oscillations, through microwaves, infrared, the visible spectrum, ultraviolet radiation, gamma rays, and high-energy cosmic rays. All of these sources of electromagnetic radiation exist as waves, and it is the variations in wavelength and frequency that determine the precise nature of the energy. These wavelengths range in size from many meters (such as radio waves) to many billionths of a meter (gamma and cosmic rays). Visible light is technically defined as electromagnetic radiation having a wavelength between approximately 400 and 780 nanometers (one nanometer is equal to one billionth of a meter).
The electromagnetic spectrum ranges from the extremely short waves of gamma rays emitted by certain radioactive materials to the radio waves, the longest of which can be miles in length. Light, the visible spectrum, ranges from about 400 to 700 nm (nanometers, or billionths of a meter) in length. Some sources suggest that the visible spectrum could range from about 380 to 770 nm, but the exact limits will depend on the visual system of a given observer. Below 400 nm are the ultraviolet rays, which are important when dealing with fluorescent materials. Above 700 nm are the infrared rays, which have significance in certain kinds of photography or image capture.
The visible spectrum occurs in nature as a rainbow. It can be duplicated in a laboratory by passing a narrow beam of white light through a glass prism. The spectrum appears to be divided into three broad bands of color-blue, green, and red-but in fact is made up of a large number of colors with infinitesimal variations between 400 and 700 nm. The colors in the spectrum are physically the purest colors possible. The splitting of white light into the visible spectrum, and the recombining of the spectrum to form white light, was first demonstrated and reported by the English scientist Sir Isaac Newton in 1704.
The reason that a spectrum can be formed by passing white light through a prism has to do with the refraction of light as it passes from one medium (air) to another(glass).The prism bends light of the shorter wavelengths more than light of the longer wavwlengths, thus speading the light out into the visible spectrum. In nature, drops of rain act in a manner similar to that of a prism: when a beam of sunlight breaks through the clouds it is refracted by by moisture in the air and a rainbow is formed.
Color reproduction is the process of making color images of an original scene or object. Generally speaking, it involves the use of an optical system, a light-sensitive material, an image processing method, and an electronic or colorant-based rendition system.
In the case of the printing industry, the process typically involves making reproductions from existing photographs or artists’ originals. Electronic camera images also are commonly used as the starting point for the printed color reproduction process Originals in full colour, such as transparencies and colour photographs, are mainly reproduced by four-colour process, using yellow, magenta, cyan and black printing inks. A separate screened negative/positive, printing ‘plate, cylinder or stencil is required for each colour, so that the printing combination of colours reproduce the full effect of the original. For the most faithful reproduction possible, special colours may be necessary, particularly in packaging and labels, where they may be used for overall solids or house colours. These are often specified as a PANTONE Matching System (PMS) reference.
There are two types of colour reproduction - 1. Additive Color Theory. 2. Subtractive Color Theory.
Photomechanical color reproduction is the traditional term that describes the printing industry’s color reproduction production process. This process may include the production of intermediate film, plate, or cylinder images prior to the stage when the colorants are physically transferred to a substrate. Some of the processes used by the industry form the image directly from digital data without the need for intermediate film or plates.
The yellow, magenta, and cyan subtractive primaries, plus black, that are used for making printed color reproductions are known as process colors. The term process color printing is often used to mean photomechanical color reproduction, but it also means the production of flat color tones by combined process colors.
The term color printing is a broad one that includes flat solid color (nonpictorial) package printing and fine art printmaking, as well as the photomechanical color reproduction process. Color printing may also be used to describe the production of photographic color prints or the generation of output from computer-driven desktop color imaging systems.
As previously mentioned it is possible to divide the spectrum of white light into three broad bands - blue/violet, green/yellow and orange/red - which appear essentially blue, green. and red to the eye: these are in effect the additive primary colours. If these colours, in the form of beams of coloured light, are in similar proportions upon a white screen then white light is created. With the overlapping primary colours of blue, green and red, the secondary colours of yellow, magenta and cyan are produced.
An additive mixture of colours is a superimposition of light composed of different colours. If all colours of the spectrum are added together, the colour white results. Red, green and blue are the additive primary colours. They are called one-third colors because each represents one third of the visible spectrum. The additive system starts with darkness (for example, a blank TV screen) and adds red, green and blue to achieve white.
The principle of additive color mixture is used in color TV and in the theaters to produce all the colors of the visible spectrum.
When wavelengths of light are combined or added in unequal proportions, we perceive new colors. This is the foundation of the additive color reproduction process. The primary colors of the process are red, green, and blue light.
Secondary additive colors are created by adding any two primaries:
Varying the intensity of any or all of the three primaries will produce a continuous shading of color between the limits.
Two methods for adding colors may be used: (i) red, green, and blue-light image records either overlap each other, or (ii) are placed side by side within a mosaic structure. The overlapping-primaries method of additive color reproduction has certain practical limitations that restrict its use. The side-by-side red, green, blue image element approach to additive color reproduction has, however, proved to be quite successful for certain applications.
Color television works on this basis: a magnifying glass will reveal the red, green, and blue mosaic structure of the screen (figure below). Many early color photography processes were also based upon the mosaic-structure type of additive color reproduction.
Additive color photography processes, however, have certain disadvantages when compared to subtractive methods. The drawbacks of the additive color reproduction photographic process are due to the fact that the red, green, and blue-filter mosaic absorbstwo thirds of the light in the whitest areas. Additive-process transparency photographs appear to have low contrast and saturation unless they are viewed using a relatively intense light within a darkened room.
Satisfactory reflection color photographs and color printing cannot be produced by the additive process. Red, green, and blue rotating reflection disks are often used to demonstrate the principles of additive color reproduction, but it is necessary to illuminate the disk with an extremely intense light to achieve satisfactory results.
The additive color reproduction process works for television and computer monitor imaging processes because the intensity of the self-luminous display screen is sufficient to overcome the room lighting effects. For best results, however, television and monitor displays should be viewed under dim ambient lighting conditions, and the viewing distance must be sufficiently great so that the eye cannot resolve the mosaic structure of the screen.
The limitations of the additive process for reflective light viewing can be overcome with the subtractive color reproduction process. The subtractive system starts with white (white paper illuminated by white light, for example) and subtracts red, green, and blue to achieve black.
The majority of commercial work is printed in four, rather than three colours, adding black to the process set. Black Color is included to compensate for deficiencies in the yellow, magenta and cyan pigments, and to allow type to print in only one dense, high contrast colour. Although the way in which the black separation is made can radically affect the final result, the theory of subtractive reproduction relates to the three primary colours of yellow, magenta and cyan. Subtractive color mixing operates by “subtracting” out one or more colors of light.
In ideal subtractive colour behavior, each of the primary colours would subtract one third of the spectrum. The yellow ink would absorb the blue portion and reflect a mixture of red and green light appearing yellow to the eye, which cannot analyse it into its component parts; the magenta ink would absorb the green portion and reflect blue and red; with the cyan ink absorbing the red portion and reflecting blue and green.
The subtraction of red, green, and blue is achieved by using colorants that are their opposites.
Colors are achieved by subtracting light away from the white paper (which reflects red, green, and blue). A combination of yellow (minus blue) and cyan (minus red) will, for example, result in green. Table below shows the possible combinations.
A continuous blend of colors between the gamut limits is obtained by varying the quantity of any or all of the primary colorants deposited within the image. In color photography, this is achieved in a purely subtractive manner, by varying the density of the cyan, magenta, and yellow dye layers. Most color printing, however, relies upon a combination of a fixed density (ink film thickness) and a variable area coverage to adjust the quantity of ink deposited. The “halftone” structure that results from the combination of inked dot areas printed upon a white paper base is optically fused by the eye to produce a continuous-tone appearance.
To produce a set of four colour separations the original is scanned/input on an, electronic colour scanner using RGB (red, green, blue) light sources and output for printing purposes as CMYK (cyan, magenta, yellow, black) separations.
The figure below, illustrates the use of BGR colour lights/ separation filters to produce YMC separations or printing plates; K (black) is reproduced from a yellow / orange combination-type filter.
The principle of colour separation is probably best considered from the traditional method, where the blue filter is dense in the areas of the image representing the parts of the original reflecting or transmitting blue, less dense where there is less blue light and transparent where there is none; the printing plate therefore produced from the blue filter is the yellow plate. On the same basis the green filter produces the magenta plate and the red filter the cyan plate.
The key objective in the photomechanical color reproduction process is to produce cyan, magenta, and yellow images that are negative records of the amount of red, green, and blue in the original. This is achieved by initially photographing the original, in turn, through red, green, and blue filters. The subsequent image records or signals are adjusted as required prior to generating a halftone image that suits the chosen printing process. The images are then used to generate image carriers, which may be plates, cylinders, or stencils. Each plate is inked with its appropriate color which is sequentially transferred, in register, to a white substrate. The more direct electronic (“digital”) printing systems eliminate films, or even plates, from the production process.
There are practical considerations that limit the thicknesses of cyan, magenta, and yellow inks that may be printed by most processes; consequently, a black printer is normally employed to compensate for the resulting loss of image contrast. The black printer is made by photographing the original sequentially through red, green, and blue filters, and then following procedures similar to the other colors. Below figure shows the complete process in schematic form. The exact nature of the printed image will depend upon the process used to form and transfer the image.
Reproduction: Color reproduction is the science of creating colors for the human eye that faithfully represent the desired color. It focuses on how to construct a spectrum of wavelengths that will best evoke a certain color in an observer. Most colors are not spectral colors, meaning they are mixtures of various wavelengths of light. However, these non-spectral colors are often described by their dominant wavelength, which identifies the single wavelength of light that produces a sensation most similar to the non-spectral color. Dominant wavelength is roughly akin to hue.
There are many color perceptions that by definition cannot be pure spectral colors due to desaturation or because they are purples (mixtures of red and violet light, from opposite ends of the spectrum). Some examples of necessarily non-spectral colors are the achromatic colors (black, gray, and white) and colors such as pink, tan, and magenta.
Two different light spectra that have the same effect on the three color receptors in the human eye will be perceived as the same color. They are metamers of that color. This is exemplified by the white light emitted by fluorescent lamps, which typically has a spectrum of a few narrow bands, while daylight has a continuous spectrum. The human eye cannot tell the difference between such light spectra just by looking into the light source, although the color rendering index of each light source may affect the color of objects illuminated by these metameric light sources.
Similarly, most human color perceptions can be generated by a mixture of three colors called primaries. This is used to reproduce color scenes in photography, printing, television, and other media. There are a number of methods or color spaces for specifying a color in terms of three particular primary colors. Each method has its advantages and disadvantages depending on the particular application.
No mixture of colors, however, can produce a response truly identical to that of a spectral color, although one can get close, especially for the longer wavelengths, where the CIE 1931 color space chromaticity diagram has a nearly straight edge. For example, mixing green light (530 nm) and blue light (460 nm) produces cyan light that is slightly desaturated, because response of the red color receptor would be greater to the green and blue light in the mixture than it would be to a pure cyan light at 485 nm that has the same intensity as the mixture of blue and green.
Because of this, and because the primaries in color printing systems generally are not pure themselves, the colors reproduced are never perfectly saturated spectral colors, and so spectral colors cannot be matched exactly. However, natural scenes rarely contain fully saturated colors, thus such scenes can usually be approximated well by these systems. The range of colors that can be reproduced with a given color reproduction system is called the gamut. The CIE chromaticity diagram can be used to describe the gamut.
Another problem with color reproduction systems is connected with the initial measurement of color, or colorimetry. The characteristics of the color sensors in measurement devices (e.g. cameras, scanners) are often very far from the characteristics of the receptors in the human eye.
A color reproduction system "tuned" to a human with normal color vision may give very inaccurate results for other observers, according to color vision deviations to the standard observer.
The different color response of different devices can be problematic if not properly managed. For color information stored and transferred in digital form, color management techniques, such as those based on ICC profiles, can help to avoid distortions of the reproduced colors. Color management does not circumvent the gamut limitations of particular output devices, but can assist in finding good mapping of input colors into the gamut that can be reproduced.
Colour management is a process that ensures consistent and predictable color reproduction across different devices such as monitors, printers, and presses.
The Three Cs are essential principles for effective color management:
1. Characterization
2. Calibration
3. Conversion
A CMS is software and procedures that manage color throughout the workflow:
Typical workflow in a printing/packaging environment:
1. Source Creation – Design in Photoshop, Illustrator, or CorelDraw (RGB or CMYK)
2. Device Calibration – Monitor and printer calibration
3. Profiling – Creating ICC profiles for devices (printer, press, monitor)
4. Color Conversion – Transforming colors to a device-independent space (Lab) and then to target device
5. Proofing – Soft proofing (on screen) or hard proofing (printed sample)
6. Final Printing – Color-managed printing ensuring WYSIWYG output
Software
Function
Fiery
Color management and RIP software for digital presses
Curve
Profiling and calibration software
Star Pro
Color quality control, measurement, and profiling
iProfiler
Monitor profiling and color calibration
ICC Profiles
Device characterization and color conversion
Colour measuring software typically allows:
1. Profile Creation: Generate ICC profiles for devices
2. Calibration: Adjust devices to standard settings
3. Measurement: Measure density, Lab* values, and color deviation
4. Proof Verification: Compare target vs actual color
5. Reporting: Export data for quality control and documentation
6. Curve Adjustments: Modify tonal curves and gamut mapping
Color Standard: Certified Color Standards are a combination of a visual reference and a master electronic standard in the form of spectrophotographic refletance data. The measurement conditions are determined by the brand or retail program account and are typically stored in a qtx-formatted file. Certified color standards are customized for the brand or retailer with their individual color names or IDs, their logo, and their own layout.
COLOR MEASURING INSTRUMENTS
What is spectrophotometry?
Spectrophotometry is the measurement of the reflectance or transmittance of a sample at discrete wavelengths. Spectrophotometers usually provide illumination of the sample by white light and then contain a diffraction grating to refract the reflected light and enable measurement of the amount of light reflected at discrete wavelengths.
What is a colorimeter?
The word colorimeter is normally used for a device which uses three or more filters to produce a response similar to that of the eye, as opposed to a spectrophotometer which measures the amount of light reflected or transmitted at each wavelength. Both colorimeters and spectrophotometers can give the same tristimulus values though the spectral method is usually more accurate.
What is the CIE system of colorimetry?
A. Colour is the sensation achieved when light falls on the retina of the eye. In the retina colour sensitive receptors are ‘triggered’ to produce electro-chemical signals, which are sent to the brain to produce the sensation of colour. The light reaching the eye is the product of the light reflected at each wavelength by the sample and that of the illumination source shining on it.
The three types of receptor each peak in sensitivity at different wavelengths - one at short wavelengths, one medium wavelengths and one at slightly longer wavelengths. This means that any colour can be reproduced by just 3 coloured dyes, pigments or coloured luminous stimuli - so long as their peak absorption or emission wavelengths are also separated. It also means that colours can be seen to match despite having different spectral composition - a phenomenon known as metamerism. Such a match will generally fail when the light source shining on the sample is changed.
Colour (whether coloured light or print) is traditionally measured by specifying the amounts of Red, Green and Blue lights which would be needed to match it. Based on experiments in which observers were asked to match various colours by mixing three coloured lights, the international colour standards body International Commission on Illumination (CIE) defined a ?standard observer? as the average of these observers for a specific set of ?lights?. They then defined a system of measurement units and measurement procedures which enable any colour to be specified in terms of the amount of the three standard lights that would be needed to match it. These are the CIE XYZ values, and other quantities such as CIELAB are calculated from them.
Implementing color management
Color Management is based mainly on international standardization
Commission Internationaled’Éclairage(CIE)
- Specification of the Lab color space 1976
- Device-independent,
- Based on human color perception
International Color Consortium (ICC)
- Specification of the ICC profile format 1994
- Computer-independent,
- Manufacturer-independent
PROFILES FOR MONITOR, SCANNER AND PRINTER
ICC profile:
ICC profiles help you to get the correct colour reproduction when you input images from a scanner or camera and display them on a monitor or print them. They define the relationship between the digital counts your device receives or transmits and a standard colour space defined by ICC and based on a measurement system defined internationally by CIE. Thus, if you have a profile for each of your scanner, camera, display and printer, the fact that they refer to a standard colour space lets you combine them so that you obtain the correct colour as you get images from the scanner or camera and print or display them.
An ICC profile is one that conforms to the ICC specification. By conforming to this specification profiles may be exchanged and correctly interpreted by other users. The two main types of profiles are source (input) and destination (output) profiles and essentially consist of tables of data that relate the device co-ordinates to those of the standard colour space defined by ICC.
How do I make ICC profiles?
The main requirement is a software application that will generate profiles from measurement data. For output profiles, you also need a measurement instrument to measure your prints or display.
What is a rendering intent?
A rendering intent defines how the gamut of colours which can be achieved on one media is modified when reproduced on a media with a different colour gamut. Each profile contains three of these rendering intents and which should be used depends on the colour gamuts of the original and reproduction media.
Rendering intents
Scanned natural photographic images reproduced on prints or displays will usually use a perceptual rendering. This takes account of the fact that the range (gamut) of colours on a print or display is often lower than the original ? although for high gamut printing a colorimetric rendering (which attempts to produce an exact colour match) may be appropriate.
However, many other cases (such as proofing - simulating one device on another such as a print on a display) require a colorimetric intent when there are no colour gamut mis-matches. The saturation rendering intent is often used for business graphics and produces a maximum colourfulness on the print.
Integration into the workflow
Every device used in an open, digital color management workflow portrays color in its own specificmanner. Monitors for instance use different light sources and CCDs; and proof printers use different inks, laminates and papers. Monitors and scanners work in the RGB color space, proof printers work in the CMYK color space.
In order to integrate all these devices into a properly functioning color management system, it is necessary to ‘fingerprint’ each of them so the system can know how each device ‘sees’ or portrays color. This ‘fingerprint’ is also called the individual device profile – this is a table, which shows the actual values of the device that differ to the theoretical nominal values. With each portrayal or color space transformation, the fingerprint of each device is needed for print simulation whilst taking the
print standard (output profile) into account. A variety of manufactures can provide measuring devices and ICC compatible software for generating fingerprints.
Monitors
There are several different approaches for generating monitor profiles.
Scanners
Manufacturers of high-end and mid-price scanners offer the possibility to scan in an IT8 reflective test chart for the calibration, and the scanner profile is calculated by comparing the ascertained colorimetric actual values with the theoretical nominal data of the test chart. If the scanner is also suitable for transparent copy, the same procedure can be followed with an IT8 transparent testchart. External scanning programs or plug-ins that permit calibration via IT8 are also available for many scanners. Prior to generating the scan profile, the operating sequence and the scan parameters must be determined (e.g. linear scanning, highlight and shadow settings). The purpose of a profiled scan is always to reproduce the scanned copy as identically as possible.
Proof printers
In principle, device profiles of proof printers are generated in exactly the same manner as output profiles for print standards. However, fingerprinting proofers is very complicated since digital proof printers seldom work in a linear fashion. It helps when the proof software for the basic calibration permits linearization of the proofer via gradation curves and also allows maximum inking to be produce better profiling results. If there are any last-minute changes to the whiteness of the substrate to be printed, these changes to the existing ICC profiles can be optimized to a certain extent by profile editors. This does not require a test chart to be printed or measured.
IMAGE REPRODUCTION PROCESS
How to implement ICC colour management?
To apply colour management, you need a profile for each of your scanner and/or digital camera and another for your monitor and/or printing device. Each of these relates the device colour data to the standard colour space which allows them to be combined to produce an overall transformation.
To combine profiles you need a Colour Management Module (CMM). At its most basic this is nothing more than an interpolation engine for combining LUTs. ICC do not specifically recommend a single CMM as some CMMs attempt to ‘add value’ for specific applications by picking up private tag information in the profile.
Many colour management-aware applications such as high-end RIPs and Adobe Photoshop contain an internal CMM. CMMs are also built in to the OS on the Mac (ColorSync) and Windows (ICM and WCS).
THE “THREE Cs” OF COLOR MANAGEMENT
Many people use the term “calibration” to mean all steps necessary to achieve accurate color during the production process, perhaps implying that reproduced colors are “calibrated” to match the original. “Color management” is a more meaningful term for matching color on different input and output devices, since the calibration of each device is only the first of three steps necessary to achieve accurate and consistent color throughout the reproduction process.
Calibration ensures that all devices (scanner, monitor, and printer) perform to a known specification, be it RGB illuminance, CMYK density, or CMYK dot area.
Characterization is a way of measuring and quantifying the color space, color gamut, or color behavior of a particular device under known conditions. It is a way of determining how an input device captures color or an output device records color when it is calibrated.
Conversion (also known as color transformation or color correction) refers to translating a color image from the color space of one device to that of another under known conditions. Color conversion can be done by manually correcting the image or automatically by using color management software.
To achieve the goals of color management, calibration, characterization, and conversion must be done in this sequence. Calibrating a device to specification serves as a foundation for characterization and conversion, and a device must be characterized before color data can be converted for accurate rendering.
CALIBRATION
Color management is based on the assurance that all devices in a color reproduction system are performing to specification. Calibration alone does not guarantee color matching; it simply ensures that the scanner, monitor, and printer are performing to their respective specifications, and provides a way of ensuring they will be consistent over time.
Scanner calibration means that when a specific light level is measured from a film or paper target, the scanner consistently records a corresponding digital value in the image file for that spot on the original. Monitor calibration means that the display card consistently displays a pixel corresponding to the specific digital value received from the file. Other items that require calibration include the color printer/proofer and the platesetter.
CHARACTERIZATION
After devices are calibrated, they must be characterized. Characterization defines the color gamut, or set of reproducible colors, that an input device can capture or an output device can record. Device characterizations are stored as profiles, digital files of data describing the color gamut of a device. In page-layout software, color management systems keep track of the input, display, and output devices the user has specified using tags or data appended to color files.
A variety of models can be used to characterize input and output devices, including RGB color space, CMYK color space, and CIE color space, which includes two models based on the dimensions of hue, chroma, and value. These are the CIExyY and CIELAB color spaces. In both models, hues are arranged around the perimeter of the color space, saturation increases from center to edge, and value varies along the third color space axis.
Scanners are characterized by software that measures the values in a scanned ITS.7 target and compares them to corresponding values in a reference file. The ITS.7 target is the internationally standard input target developed by the ITS subcommittee of the Committee for Graphic Arts Technologies Standards. The basis of the ITS.7 target is the Q60, a series of photographic film and paper test images for characterizing the gamut of input devices developed by the Eastman Kodak Co.
The printer must also be characterized. Output targets are measured with a spectrophotometer in CIExyY and/ or CIELAB color space to characterize the color gamut of an output device. As with scanners, the characterizations are stored as device profiles. Profiles for commonly available monitors are offered by the developers of color management software, although they are valid only when the monitor is performing to manufacturer’s specifications. Some software allows users to characterize their own monitors; other systems have built-in calibration.
CONVERSION
Conversion refers to translating color-image data from the color space of one device to that of another under known conditions. Color conversion is necessary so that a scanned image reproduces as a believable representation of the original on both the screen and the printer. Since output devices typically have smaller color gamuts than originals, scanners, and monitors, colors in the original must be fit into the gamut of the device, a process known as gamut compression.
Color management software converts or translates color from one space to another: from scanner to monitor, from monitor to printer, and from scanner to printer.
Once color management profiling software has been used to characterize the scanner, monitor, and printer, it is necessary to apply the profiles to the image according to the desired “matching” objectives.
Three methods of color conversion are used-one for photographs, one for spot colors, and another for business graphics. Perceptual rendering, used for continuous-tone photographs, maintains the relative range of colors in a photograph. It causes the white portions of an image to have no ink on the paper, and the black portions to have the darkest color that the device can print.
Colorimetric rendering, most effective for spot colors, maintains an absolute color match. It renders colors that are within the device’s gamut identically, and brings colors outside the gamut to the closest color the device can print.
Saturation rendering is appropriate for bright saturated illustrations and graphs like those used in business presentations. This rendering style produces pure, saturated colors in print according to the printing device’s limitations. It does not try to precisely match printed colors to those on the monitor.
Electro magnetic radiation in the wavelength range from 380 nm to 730 nm is seen as light by our eyes. Low wavelengths show as blue light, then the spectrum continues from green to yellow, orange, and red. UV radiation is located in the range below 380 nm; the range above 730 nm is called infrared radiation. The visual impression of a colored body changes by the composition of the incoming light.
Each color has its typical spectral curve. The color pigments absorb specific wavelengths of the incoming light while other wavelengths are reflected. Perfect white reflects the entire incoming light (i.e.: no absorption) while perfect black absorbs all wavelengths at 100%. If you add saturated dyestuffs to a white base, specific sections of the spectral curve are lowered more and more. The spectral curve can never be raised by dyestuffs.
The CIELAB color space is the most common used color space in the industry. The vertical L* axis reflects the lightness of a color. Here L*=0 represents absolute black and L*=100 represents perfect white. The positive a* axis represents the red parts of a color and the negative a* axis represents the green shares. The positive b* axis is for the color yellow and negative b* values mean blue. Thus, in this three-dimensional structure you can "address" all real existing colors at one light type, measurement geometry, and standard observer.
Another way to display the a* and b*-axes is the representation in polar coordinates. C* is named chroma and shows the difference from the neutral gray axis to the sample. H is called the Hue angle. It is always measured counterclockwise starting from the positive a* axis. This set of definitions is mainly used for saturated colors, since values in numbers are easier to understand. For instance, the color orange is more saturated than the sample, not more red and more yellow. The difference from a sample to the standard is stated in delta values (Δ or d):
- ΔL* = 0.5 sample is 0.5 units brighter - Δa* = -1.5 sample is -1.5 units greener - Δb* = -3.6 sample is -3.6 units bluer - ΔC* = -3.9 sample is -3.9 units less colored - ΔH = 0.7 sample is 0.7 depending on the location of the color shades
Similar to the axes of the CIELAB system, several directions must also be observed for the tolerances. The difference from standard value to tolerance limits need not always be the same. Especially in saturated colors, a tolerable color difference along the chroma axis (=ΔC) can be much higher than a color difference in the hue (=ΔH) or in the lightness (=ΔL).
Optical brightening agents (OBAs) absorb invisible UV radiation and emit in the visible range. This can create reflectance values of more than 100%, i.e.: at specific wavelengths more light is reflected by the sample than had come in at these wavelengths. This effect is utilized in white papers or textiles, where blue light is excited. The CIE whiteness correlates better to the visual assessment than ISO whiteness, since the entire visual range is taken into consideration.
TONAL ADJUSTMENTS
Tonal adjustments can also be made in a scanned or to be-scanned image. This can take the form of adjusting the endpoints of an image (i.e., whitest white and blackest black, or highlight and shadow, respectively) or adjusting the midpoint of the image or the distribution of tones in the image. Similarly, color correction may be needed, depending on the quality of the scanner. Sometimes, a scanner will impart a color cast to an image, and at other times a few of the colors in the image will be off. Global correction is the correction of the color throughout the entirety of the image, which can consist of darkening all the reds, for example. Local correction is the changing of the color of one particular portion of an image, such as only the red of a fire hydrant present in the image.
Depending upon the nature of the image and the context in which it is ultimately to appear, further types of manipulations may be required, including forming collages, removing elements from the image, inserting elements in the image, etc.
There is no hard and fast rule to these adjustments, of course; most good software and scanning programs have “preview” functions that allow the user to see what the effects of a particular adjustment will be before they are actually made. The best judge of any image or color correction operation is the human eye.
Tone Reproduction: In printing, a tone reproduction curve is applied to a desired output-referred luminance value, for example to adjust for the dot gain of a particular printing method.[6] Dot-based printing methods have a finite native dot size. The dot is not square, nor any other shape that when stacked together perfectly fills an image area; rather, the dot will be larger than its target area and overlap its neighbors to some extent. If it were smaller than its target area, it would not be possible to saturate the substrate. A tone reproduction curve is applied to the electronic image prior to printing, so that the reflectance of the print closely approximates a proportionality to the luminance intent implied by the electronic image.
It is easier to demonstrate the need for a TRC using halftoned printing methods such as inkjet, or xerographic technologies. However, the need also applies to continuous-tone methods such as photographic paper printing.
As an example, suppose one wants to print an area at 50% reflectance, assuming no ink is 100% reflective and saturated black ink is 0% (which of course they aren't). The 50% could be approximated using digital halftoning by applying a dot of ink at every other dot target area, and staggering the lines in a brick-like fashion. In a perfect world, this would cover exactly half of the page with ink and make the page appear to have 50% reflectivity. However, because the ink will bleed into its neighboring target locations, greater than 50% of the page will be dark. To compensate for this darkening, a TRC is applied and the digital image's reflectance value is reduced to something less than 50% dot coverage. When digital halftoning is performed, we will no longer have the uniform on-off-on-off pattern, but we will have another pattern that will target less than 50% of the area with ink. If the correct TRC was chosen, the area will have an average 50% reflectance after the ink has bled.
A TRC can be applied when doing color space conversion. For example, by default, when transforming from L*A*B* to CMYK, Photoshop applies an ICC profile for SWOP standard inks and 20% dot gain for coated paper.
Color Balance: Take a digital camera and click a picture of a scene. This is the color reproduction of the original scene. The success of a color reproduction lies in how close the reproduced scene is to the original scene. Note that the term ‘close’ can be measured both quantitatively and qualitatively. There are two types of color reproduction system, additive and subtractive and we will discuss both in details.
In subtractive color mixture, the color of a surface depends on the capacity of the surface to reflect some wavelengths and absorb others. When a surface is painted with a pigment or dye, a new reflectance characteristic is developed based on the capacity of the pigment or dye to reflect and absorb the different wavelengths of light. Consider a surface painted with yellow pigment which reflects wavelengths 570 − 580nm and another surface painted with cyan pigment which reflects 440 − 540nm. If we mix both the pigments the resulting color will be green. This is because the yellow pigment absorbs the shorter wavelengths below 500nm and some of the middle band wavelength from 500−550nm. The cyan pigment absorbs all of the longer wavelengths 560nm and above. The energy distribution of all these are shown in Figure 1.1. Thus the yellow absorbs the wavelengths evoking the sensation of blue while the cyan absorbs the wavelengths evoking the sensation of yellow. Hence, what is left behind after this is a sensation of green. This is called subtractive color mixtures since bands of wavelengths are subtracted or cancelled by the combination of light absorbing materials. And the resulting color, as you have probably noticed, is given by the intersection of the two spectrums. The yellow, cyan and magenta is termed as the color primaries of the subtractive color mixtures, because these are the minimal number of pigments required to produce all other colors. Dyes and inks usually follow subtractive color theory and hence images generated using these mediums are the result of subtractive color reproduction.
In additive color mixture systems, colors are mixed in a fashion in which bands of wavelengths are added to each other. This is called additive mixture of colors. Thus, the spectrum of the color formed by superposition of multiple colors is given by the addition of their respective spectrums. This is similar to how the human eye visualizes color. Devices like camera, projectors follow additive color mixture. The basic principles of good quality color reproduction are the following. 1. Correct mapping of critical reference colors such as sky, foliage and skin tones. This may not mean an exact match but simply that the reproduced color is not grossly wrong. For example, almost any shade of blue will produce a satisfactory sky; even shades of purple would be fine, but green is clearly wrong. 2. Correct mapping of white and neutral colors that constitute the gray axis or the neutral axis which runs from black to white. These colors should look neutral, else the image will have an overall color cast, or an overall color tint. 3. Control of the tone reproduction involves mapping of the overall contrast and brightness. Image reproduction often involve tone compression. The goal is to reproduce, as best as possible, detail at all levels of brightness throughout the image while maintaining a correct overall appearance. 4. Control of the overall colorfullness so that the image does not look washed out or gaudy. 5. Control of sharpness, texture and other visual artifacts that contribute to image appearance.
Traditional color reproduction was a linear process. However, in digital color reproduction, this linear model is replaced by a star shaped model. In traditional color reproduction, it was sufficient to capture images in one medium and then tune it to be output to one specific media. This is no longer true. In digital color reproduction, images can be captured in different ways and then tuned to be output in different media. Hence, it is often essential to go through a device independent color description like the CIE XYZ color space.
1. DENSITOMETERS
1. Within a densitometer the light passes through the optical system bundled from a stabilized light source on the printed surface.
2. The amount light absorbed depends on the ink density and pigmenting of the ink.
3. The non-absorbed light penetrates the translucent (transparent) ink layer and is weakened The remainder is re-emitted by the surface of the material, i.e. diffusely reflected or scattered A part of this scattered light passes through the ink layer and is weakened again.
4. A lens system captures the light rays coming from the ink layer and sends them to a photodiode.
5. The light striking the photodiode is converted into electric energy.
6. The electronics compares this current with a reference value.
7. The difference between the measured current and the reference value forms the basis for calculating the absorption behavior of the measured ink layer.
1. Polarisation filters serve to prevent differences in the measured values obtained from a shining wet surface and from the surface of a dry ink.
During the drying process, the ink adapts to the irregular structure of the paper surface, and the reflection effect decreases. If a given ink is measured first in wet and then in dry condition, different readings will result In order to eliminate this effect, two crossed linear polarisation filters are inserted in the path of the rays. Polarisation filters allow the light of only one particular vibration direction to pass, while blocking all light waves which are vibrating in other directions.
2. Colour filters are inserted for measurements of colours. The colour filters in a densitometer are tuned to the absorption performance of cyan, magenta and yellow.
Printing inks Filter colour
cyan red
magenta green
yellow blue
2. SPECTROPHOTOMETERS
A spectrophotometer is a device for measuring light intensity by measuring the wavelength of light.
1. Place the spectro on the sample and click the button to measure.
2. The spectro projects a light onto the sample’s surface.
3. The sample absorbs some of the light and reflects the rest into a set of filters.
4. The spectrophotometer measures that reflected light at many different points to capture the fingerprint, or signature, of the sample and presents it as a reflectance curve.
5. The reflectance curves of samples can be compared to the reflectance curves of standards to determine even the most minute color differences.
6. This is how manufacturers determine whether the color they’re producing is on target.
1. Light source
i1Basic Pro 3
UNIT-3
Analysis of Print Attributes
1. Solid Ink Density (SID)
1. Dot gain
Z (%) = FD – FF
Here are some other factors that cause dot gain:
1. Ink and water balance.
2. Blanket construction or other properties.
3. Blanket height.
4. Roller settings.
5. Bearer pressure settings.
6. Plate wear.
7. Ink temperature.
8. Piling.
9. Print Contrast
K(%) = Ds – Dt x 100
Ds
4. Ink Trapping
4. Dot Area
In multicolor printing, maintaining consistent quality is critical. Printers use print control strips (also called color control strips or QC strips) on the edges of the sheet to monitor and control:
These strips contain various elements, each designed to test a specific printing characteristic.
Typical Sections in a Print Control Strip:
1. Solid patches – for each color (C, M, Y, K)
2. Tint patches – 25%, 50%, 75% for tone checks
3. Registration marks – to check color alignment
Key Points:
UNIT-4
Colour models are systems for representing colours numerically for different applications (e.g., design, printing, displays). They can be broadly classified into families.
1. Hue (H): Colour type (Red, Blue, Green, etc.)
2. Value (V): Lightness (0 = black, 10 = white)
3. Chroma (C): Colour saturation (from neutral gray outward)
CIE (Commission Internationale de l’Éclairage) models are device-independent, designed to standardize colour measurement.
ΔE=(L2∗−L1∗)2+(a2∗−a1∗)2+(b2∗−b1∗)2\Delta E = \sqrt{(L_2^*-L_1^*)^2 + (a_2^*-a_1^*)^2 + (b_2^*-b_1^*)^2}ΔE=(L2∗−L1∗)2+(a2∗−a1∗)2+(b2∗−b1∗)2
Example Calculation:
ΔE=(55−50)2+(18−20)2+(33−30)2=52+(−2)2+32=25+4+9=38≈6.16\Delta E = \sqrt{(55-50)^2 + (18-20)^2 + (33-30)^2} = \sqrt{5^2 + (-2)^2 + 3^2} = \sqrt{25+4+9} = \sqrt{38} \approx 6.16ΔE=(55−50)2+(18−20)2+(33−30)2=52+(−2)2+32=25+4+9=38≈6.16
Use: To evaluate colour difference tolerance in printing, textiles, and manufacturing.
C=1−R,M=1−G,Y=1−BC = 1 - R,\quad M = 1 - G, \quad Y = 1 - BC=1−R,M=1−G,Y=1−B K=min(C,M,Y)K = \min(C, M, Y)K=min(C,M,Y)
These are human-oriented models, derived from RGB.
Model
Components
Range
Use
HSI
Hue, Saturation, Intensity
H:0–360°, S:0–1, I:0–1
Image processing, segmentation
HSV
Hue, Saturation, Value
H:0–360°, S:0–1, V:0–1
Computer graphics
HSL
Hue, Saturation, Lightness
H:0–360°, S:0–1, L:0–1
Web design, CSS
Conversion (RGB → HSL):
Cmax=max(R,G,B),Cmin=min(R,G,B)C_{max} = \max(R,G,B), \quad C_{min} = \min(R,G,B)Cmax=max(R,G,B),Cmin=min(R,G,B) L=Cmax+Cmin2L = \frac{C_{max}+C_{min}}{2}L=2Cmax+Cmin S={0L=0,1Cmax−Cmin1−∣2L−1∣otherwiseS = \begin{cases} 0 & L=0,1\\ \frac{C_{max}-C_{min}}{1-|2L-1|} & \text{otherwise} \end{cases}S={01−∣2L−1∣Cmax−CminL=0,1otherwise H=angle based on which channel is maxH = \text{angle based on which channel is max}H=angle based on which channel is max
YUV
Y (luma), U (Cb), V (Cr)
Analog TV, PAL/SECAM
YIQ
Y (luma), I (in-phase), Q (quadrature)
NTSC TV
YCbCr
Y (luma), Cb (blue-difference), Cr (red-difference)
Digital video, JPEG, MPEG
Conversion Example (RGB → YCbCr for JPEG):
Y=0.299R+0.587G+0.114BY = 0.299R + 0.587G + 0.114BY=0.299R+0.587G+0.114B Cb=128−0.168736R−0.331264G+0.5BCb = 128 - 0.168736R - 0.331264G + 0.5BCb=128−0.168736R−0.331264G+0.5B Cr=128+0.5R−0.418688G−0.081312BCr = 128 + 0.5R - 0.418688G - 0.081312BCr=128+0.5R−0.418688G−0.081312B
1. RGB ↔ CMYK (for printing)
2. RGB ↔ HSI/HSL/HSV (for visualization, segmentation)
3. RGB ↔ CIE XYZ ↔ CIELAB/LUV (device-independent, Delta E)
4. RGB ↔ YUV / YIQ / YCbCr (for video)
The purpose of a color model is to facilitate the specification of colors in some standard way. A color model is a specification of a coordinate system and a subspace within that system where each color is represented by a single point. Color models most commonly used in image processing are:
In this model, each color appears in its primary colors red, green, and blue. This model is based on a Cartesian coordinate system. The color subspace is the cube shown in the figure below. The different colors in this model are points on or inside the cube, and are defined by vectors extending from the origin.
Figure 15.3 RGB color model
All color values R, G, and B have been normalized in the range [0, 1]. However, we can represent each of R, G, and B from 0 to 255.
Each RGB color image consists of three component images, one for each primary color as shown in the figure below. These three images are combined on the screen to produce a color image.
Figure 15.4 Scheme of RGB color image
The total number of bits used to represent each pixel in RGB image is called pixel depth. For example, in an RGB image if each of the red, green, and blue images is an 8-bit image, the pixel depth of the RGB image is 24-bits. The figure below shows the component images of an RGB image.
Cyan, magenta, and yellow are the primary colors of pigments. Most printing devices such as color printers and copiers require CMY data input or perform an RGB to CMY conversion internally. This conversion is performed using the equation
𝐶 1 𝑅
[𝑀] = [1] − [𝐺]
𝑌 1 𝐵
where, all color values have been normalized to the range [0, 1].
In printing, combining equal amounts of cyan, magenta, and yellow produce muddy-looking black. In order to produce true black, a fourth color, black, is added, giving rise to the CMYK color model.
The figure below shows the CMYK component images of an RGB image.
The RGB and CMY color models are not suited for describing colors in terms of human interpretation. When we view a color object, we describe it by its hue, saturation, and brightness (intensity). Hence the HSI color model has been presented. The HSI model decouples the intensity component from the color-carrying information (hue and saturation) in a color image. As a result, this model is an ideal tool for developing color image processing algorithms.
The hue, saturation, and intensity values can be obtained from the RGB color cube. That is, we can convert any RGB point to a corresponding point is the HSI color model by working out the geometrical formulas.
The hue H is given by
𝐻 = {𝜃 if 𝐵 ≤ 𝐺}
Where
360 − 𝜃 if 𝐵 > 𝐺
1 [(𝑅 − 𝐺) + (𝑅 − 𝐵)]𝜃
= 𝑐o𝑠−1 { 2 }√(𝑅 − 𝐺)2 + (𝑅 − 𝐵)(𝐺 − 𝐵)
The saturation S is given by
𝑆 = 1 − 3 (𝑅 + 𝐺 + 𝐵)
[min (𝑅, 𝐺, 𝐵)]
The intensity I is given by
𝐼 = 3
All RGB values are normalized to the range [0,1].
Converting colors from HSI to RGB
The applicable equations depend on the value of H:
If 0° ≤ 𝐻 < 120° :
𝐵 = (1 − 𝑆)
𝑅 = 𝐼 [1 + 𝑆 cos 𝐻 ]
cos (60°− 𝐻)
𝐺 = 3𝐼 − (𝑅 + 𝐵)
If 120° ≤ 𝐻 < 240° :
𝐻 = 𝐻 − 120°
𝑅 = (1 − 𝑆)
𝐺 = 𝐼 [1 + 𝑆 cos 𝐻]
cos (60°−𝐻)
𝐵 = 3𝐼 − (𝑅 + 𝐺)
If 240° ≤ 𝐻 ≤ 360° :
𝐻 = 𝐻 − 240°
𝐺 = (1 − 𝑆)
𝐵 = 𝐼 [1 + 𝑆 cos 𝐻 ]
𝑅 = 3𝐼 − (𝐺 + 𝐵)
The next figure shows the HSI component images of an RGB image.
Basics of Full-Color Image Processing
Full-color image processing approaches fall into two major categories:
In full-color images, color pixels really are vectors. For example, in the RGB system, each color pixel can be expressed as
𝑐𝑅(𝑥, 𝑦)
𝑅(𝑥, 𝑦)
𝑐(𝑥, 𝑦) = [𝑐𝐺(𝑥, 𝑦)] = [𝐺(𝑥, 𝑦)]
𝑐𝐵(𝑥, 𝑦)
𝐵(𝑥, 𝑦)
For an image of size M×N, there are MN such vectors, c(x, y), for x = 0,1, 2,...,M-1; y = 0,1,2,...,N-1.
Color Transformation
As with the gray-level transformation, we model color transformations using the expression
(𝑥, 𝑦) = 𝑇[ƒ(𝑥, 𝑦)]
where f(x, y) is a color input image, g(x, y) is the transformed color output image, and T is the color transform.
This color transform can also be written
𝑠i = 𝑇i(𝑟1, 𝑟2, … , 𝑟𝑛) i = 1,2, … , 𝑛
For example, we wish to modify the intensity of the image shown in Figure 14.8(a) using
(𝑥, 𝑦) = 0.7ƒ(𝑥, 𝑦)
𝑠i = 0.7𝑟i i = 1,2,3
𝑠i = 0.7𝑟i + 0.3 i = 1,2,3
𝑠3 = 0.7𝑟3