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CarlosE. Mora's two-part Spanish translation: Losmodelos y espacios de color (II) Gestión del color y laciencia del color:Introducción | LasBases de la gestión del color.
The series begins with anIntroductionto color management and color science (this page).Implementationpart 1 describes how to set up color management andinterpret thecontents of(filesthat describe the color response of a device or a). It featuresPictureWindow Pro, but includes information on Photoshop.Implementationpart 2 discusses monitor profiling and workflowdetails. The seriescontinues withObtainingICC profilesand building them with MonacoEZcolor andEvaluatingprinters and ICC profiles.
Backgroundreading:
| RealWorld Color Management: Industrial-strengthproduction techniques,by Bruce Fraser, Fred Bunting, and Chris Murphy. Paperback, 560 pages.The closest thing to a Color Management Bible. Buy it if you want to gointo real depth. But as its subtitle "Industrial-strength productiontechniques"indicates, much of the material covers prepress applications (CMYK,etc.),which are of only indirect interest to photographers. | ||
| MasteringDigital Printing: The Photographer's and Artist'sGuide to High-QualityDigital Output, by Harald Johnson. Paperback, 400 pages. Perhaps moreforartists than photographers, it has a good introduction to colormanagement. | ||
| ColorScience: Concepts and Methods, Quantitative Dataand Formulae,by Günther Wyszecki and W. S. Stiles. Paperback, 968 pages. Acollectionof scientific review papers-- not for the casual reader. This is thereferenceI use to verify questionable statements on color and color managementinpopular literature. It's thesource. |
ColorHQ.com | ColorManaged.com
Theretina of the human eye has two categories of light receptor:,which are active in dim light and have no color sensitivity, and,which are active in bright light and provide us with our ability todiscriminatecolor. You probably learned that the three types of cone are sensitiveto red, green, and blue (R, G, and B). The relative sensitivityof the three receptorsfor the "normal" human eye, designated by Greek letters beta, gamma andrho (β,γ,andρ), is illustrated by the blue, green,and red curves on the right.Althoughthe beta and gamma sensors correspond closely to blue and green, therhosensor (the red curve) isn't even close to red. An ink with the samereflectivityspectrum would appear yellow-orange.The eye/brain discriminates color by processing therelativestimuli in the three sensors. R, G, and B are used as additive primarycolors because their distribution across the visible spectrum producesa wide-gamut color image,because theymatch the eye's response. Fewer than three colors is insufficient.Additionalcolors offers some advantage-- that's why recent inkjet photo printershave 6 to 8 colors. Combining three colors-- even monochromatic(spectrallypure) colors produced by lasers-- can produce most,thoughnot,of the colors the eye can see.
How is this known? A set of experiments is run usinga split screen.Half is illuminated by a monochromatic light source with variablewavelength.The other half is illuminated by an adjustable combination of red,greenand blue, which can be produced by lasers or by filters, which have abroaderspectrum than lasers. If the two halves of the screen can be matchedwithsome combination of the R, G, and B lights, then the color of the puremonochromatic light is within theof colors defined by the three light sources. If no match is possible--if white light must be added to the monochromatic source to provide amatch--then the color of the monochromatic source is. This experiment shows thatnocombinationof three real light sources can duplicate the full gamut of humanvision.
Monitors with three phosphor colors (RGB) havelimited color gamuts;printers with four ink colors (CMYK = cyan, magenta, yellow, black)haveeven smaller gamuts. Printers with additional colors-- 6 to 8 are notuncommon--have larger gamuts. The eye's peculiar response has consequences forthediscipline called,which has arisen to quantify human vision. A detailed exposition wouldoverwhelming, but a few aspects, which are widely discussed but poorlyunderstood, are important for color management.
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 | In order to quantify humancolor vision, theCIE(CommissionInternationale de L'Éclairage)has established a set of imaginary "red," "blue," and "green" primarycolorsthat, when combined, cover the full gamut of human color vision, i.e.,a combination of the three can match any monochromatic light source.Thesecolor primaries (shown on the left) have a curious property-- they havenegative energy in portions of their spectra, i.e., they are notphysicallyrealizable. CIE Imaginary primaries |
If two objects with different spectral reflectivities have thesamecolor appearance (tristimulus values) under one light source, they aresaid to bematched.If they exhibit a marked difference under another light source (as withtwo pieces of cloth that look identical in a shop but differentoutdoors),they suffer from,frequently called.(This definition is misleading, but it has become so prevalent I won'ttry to fight it.) This phenomenon is most visible with neutral colors(grays).It is a problem with many color printers, particularly with Black&White prints; it was particularly severe with the Epson 2000P, butnewer printers (2005 and later) have largely solved the problem. Inactuality,metamerism, properly defined, is not a bad thing: it enables acombinationof just a few inks to take on the colors of a wide range of objects.
The colors in thediagram are not accurate, but the representation produced byGamutvision isabout as good ascan be obtained with the limited color gamut of a computer monitor.
The horseshoe line starting at 400 nm on the lowerleft and wrappingaround the top to 700 nm on the right is called the. It represents the pure spectralcolors-- the beautiful,intense colors produced by a prism in clear sunlight. The screen imageis but a pale approximation. The straight lineconnecting the endpoints of the horseshoe is called the. The full gamut of humanvision lies within thisfigure. The vertical axis gives an approximate indication of theproportionof green; the horizontal axis moves from blue on the left to red on theright. The location of white depends on the illuminant colortemperature.Some typical values (fromefg):
The 1931chromaticity diagram is not without itsflaws. The distance betweenjust noticeable color differences (called ΔE) ismuch greater in thegreen region on the top than at the bottom; it is not perceptuallyuniform.For this reason, CIE has defined additional color spaces, particularly(1976) and(1976), whichrepresent colors more uniformly. But the 1931 diagram persists becauseof historical inertia. All the CIE color spaces encompass the fullgamutof human vision and all are device-independent.BruceLindbloom presents theequationsfor converting between them. But unlike RGB color spaces, they aren'tintuitive.The precise meaning of their coordinates is difficult to visualize andthey contain values outside the gamut of human vision. Hence theyaren'tused as working color spaces for image editing. They play a vital butinvisibleroll in color management; you don't need to understand their details tomanage color effectively.
The gamut of sRGB, shown on the right inside the gamut ofAdobe RGB (1998), is quite limited. It maintains a distancefrom the line of purples and is weak in green and cyan, although thisweaknessis greatly exaggerated by the distortion of the 1931 diagram. Inmonitors,the gamut is limited by the phosphors, which are chosen for brightness,longevity, low cost, and low toxicity. Ideal phosphors-- with colorslocatednear 450, 520 and 650 nm on the spectrum locus-- don't exist. Mostmonitorswith P22 phosphors have similar gamuts.
Printers, whose colors are based on variants of CMYK(cyan, magenta,yellow, black) subtractive primaries, have gamuts whose shape is morecomplexthan a simple triangle-- often somewhat hexagonal with additionalvorticesat the Cyan, Magenta, and Yellow primaries. Because inks have imperfectspectraand are somewhat opaque, dark or light colors tend to have smallergamutsthan middle tones. Most four-color CMYK printers have smaller gamutsthanmonitors, but high quality inkjet printers with more than four colors(typicallywith the addition of light C and light M) may have larger gamuts. Colorslidefilms have considerably larger gamuts.
Now we approach one of the key issues in colormanagement: How is colorrepresented in computers? We focus on RGB color spaces because they areused for digital image files, and there are a good many of them. Othercolor models (CMYK, HSV and HSL) are discussedelsewhereon this site and inMicrosoft'sMSDN library.
In 24-bit RGB color spaces, color is described bythree 8-bit bytes,each of which can take on values 0 through 255. Pure red is (255,0,0),green is (0,255,0), blue is (0,0,255), black is (0,0,0), and white is(255,255,255).Some programs likePictureWindow Pro andPhotoshop CS can utilize 48-bit color that allows values of 0 through65,536for each primary color. 48-bit color eliminates banding that can occurwhen colors are adjusted in the 24-bit mode.
But whatexactly is meant by"pure" R, G and B? To accommodate the widerange of gamuts indifferent devices-- digital cameras, film, scanners, monitors, andprinters,a variety of color spaces has been developed. The de facto standard forthe Internet and Windows, sRGB, has a limited gamut corresponding to atypical CRT monitor. Other color spaces have larger gamuts--correspondingroughly to high quality printers or to film. Some have extremely largegamuts, covering most of the colors the eye can see and some it can't.Maintaining consistent color appearance in the translation betweendifferentdevices and color spaces is no easy task; color management provides areasonablysane and practical solution. But it's is no panacea. Even the mostsophisticatedsystem can't make two devices with different gamuts displayexactlythe same colors; it can't make a monitor or CMYK printer displayVelvia-saturatedcolors.
In a color-managed workflow, the color response of eachdevice, eachimage file, and each image in the computer's active memory ischaracterizedby a file called an.ICC profiles have the extension ".icm" and are stored in specificlocationson Windows computers.
Youmay need to do somebookkeeping in these folders because problems canarise (especially with pre-XP versions of Windows) when more than about30 profiles are present. Since profiles are loaded by profile creationprograms such as MonacoEZcolor, Windows itself, image editors, anddevicedrivers, they can easily proliferate. I recommend creating a foldercalled"Unused profiles" for profiles you don't use. If nothing else, it willshorten the drop-down lists. You can obtain a printout of profiledescriptionsfrom Picture Window Pro by clicking on, to bring up the ColorManagement dialog box,then clicking ICC profiles can also be embedded aswithinimage files:TIFF, JPEG,PNG,and BMP are supported by most ICC-aware image editors.
ICCProfilesconsist primarily of tables that relate numeric data, for example, RGB(222,34,12), to colors expressed in a device-independent CIE colorspacecalled a CIE-XYZ or CIELAB. The colors may be the objectssensed bya scanner or produced by a printer or monitor. They can also refer tooneof the numerous.Monitor profiles have the same format as color space profiles. Profilesmay contain additional data, such as a preferred rendering intent andgamma,Monitor profiles often contain instructions for loading video cardlookuptables, i.e., forcalibrating the monitor.
The heart of color management is the translation orgamutmapping between devices with different color gamutsand files withdifferent color spaces. Mapping functions are shown in the yellow boxesin the illustration, above. They are performed by a or(),also called a, using data in the profiles.The CMM combines theinput and output profiles, both of which are referenced to a PCS, toperforma direct conversion between the devices or color spaces. Itinterpolatesdata in printer profile tables, which would be prohibitively large ifallpossible color values were included.
Picture Window Pro uses either the Windows defaultcolor engine,ICM2.0, or an alternative engine,LittleCMS. AdobePhotoshop has its own color engine, ACE. Colorengine mappingsmay be called from ICM-aware programs or device drivers. You must beawareof where the translation takes place in your environment. If you arecareless,mapping can take place twice (or not at all), with undesirable results.We will show examples.
For reference,ColorSyncis Apple's color engine.ColorMatchRGB is Apple's default color space, with gamma =1.8 andgamutbetween sRGB and Adobe RGB (1998).is an older color space with a narrower gamut. Easy to get confused.
is performed withone of the fourrenderingintents (gamut mapping algorithms) recognized bythe ICC standardand by Windows ICM 2.0. The rendering intent determines how colors arehandled that are present in the source but out of gamut in thedestination.Since there are several nomenclatures for gamut mapping, I use a colorcode to distinguish the sources:,,.I'll generally stick with thenomenclature.
,also calledor. Thisis PW Pro's default, and is generally recommended for photographicimages.The color gamut is expanded or compressed when moving between colorspacesto maintain consistent overall appearance. Low saturation colors arechangedvery little. More saturated colors within the gamuts of both spaces maybe altered to differentiate them from saturated colors outside thesmallergamut space. In the diagram on the right, the left and right of thecolorspace blocks represents saturated colors; the middle represents neutralgray. Perceptual rendering applies the sameBruceFraser points out that for an image with unsaturated colors,e.g.,| Gamut mapping is acomplextopic, particularly for Perceptual rendering intent. The details of theare contained within the profile. There are a great ways of performingPerceptual rendering. Here are some highly technical papers, guaranteedto generate more questions than they answer. Like what techniques areactuallyused in your profiles? This doesn't soundanythinglike the familiar"you push the button, we do the rest" marketing hype. |
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