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COLOR MANAGEMENT: The Color Gamut, Gamut Mapping and Gamut Clipping and Your Rendering Intents

By understanding the relationships of color theory and the physical limits of your printers you can better apply color managment to achieve the desired results.

By Wasatch, Inc. Staff

For any printing device there will be physical limits to the range of color that can be reproduced.

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  • As the illustrations show, there is no way to mix the "dull" inks to produce the pure cyan or yellow, or even the black, of the "bright" inks. This principle applies as well to the red, green and blue of photographic film and television screens, and in fact applies to all color reproduction systems.

    On the other hand, it is almost always possible to mix the "bright" inks to produce any color that can be produced by the "dull" inks, that is essentially what is happening with colorants on your computer monitor right now, as you view the illustration. The full range of colors that can be produced by any color reproduction system is called the color "gamut" of that system.

    Curiously, this very real phenomenon is made a scapegoat for all kinds of other deficiencies in color management, for instance we've actually heard it blamed for an inability to match SWOP printing values with the "extreme" gamut inks sold by some large-format inkjet manufacturers. Nothing could be further from reality. With real color management of the sort discussed elsewhere at this web site, one always wants the most "extreme" gamut available. Simulation of other systems, such as SWOP, is thereafter simple because all of the desired colors are available.

    It is popular to draw a picture of available colors (a color "space") as a colored disk, and to then draw out the available gamut as a polygon on that illustration. In the next illustration, we've actually shown two polygons. Each of them has six points, corresponding to six "primary" colors: cyan, magenta, yellow, red, green, and blue. The area inside a polygon represents all the colors that can be achieved with that particular set of inks. It's a picture of the "gamut".

    In this illustration, the black polygon corresponds to the "bright" inks, and the white polygon, corresponds to the "dull" inks. The colored disk on which the polygons are displayed is typically a "plane" from within a "CIE color space". This is a handy way that color scientists display the relationship between the "gamuts" of two different color reproduction systems. The fact that all the colors achievable by the "dull" inks are also achievable by the "bright" inks is nicely illustrated. The fact that the bright inks can achieve colors that can't be achieved by the dull inks is likewise illustrated.

    Gamut Mapping
    When we are limited to printing with the dull inks, and we're asked to reproduce an image that is specified for the "bright" inks, we're forced to make some sort of compromise. This is called "gamut mapping".

    One simple solution is to move all the points outside the white polygon directly inward to the nearest point on that polygon, while matching all other points as accurately as possible. This provides the best possible match to all colors that can be accurately matched, and is great for hitting spot colors, but it tends to produce lousy reproductions of photographs. Consider a photograph of an apple in which the reds of a highlights have to all be moved, and that by these rules they're all moved to the same point on the white polygon. As we view the photograph, we'll see a terrible "fringe" surrounding the highlight as the area of out-of-gamut colors that have been run-together transitions to the area where more accurate color reproduction is possible.

    This is often called a "colorimetric" correction, and if you have a "colorimetric ICC profile", this is what you've got.

    A more satisfactory solution would be to somehow "deform" the entire surface of the above diagram so that all points are moved into the white polygon, while avoiding "clipping" colors so that colors that differed in the original are knocked down to be the same color in the reproduction. Colors that are within the reach of the dull inks (inside the white polygon) will be less accurately reproduced, but your reproductions will be free of the nasty "fringes" described above.

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    This is often called a "perceptual" or "photometric" correction and if you have a "perceptual ICC profile", this is what you've got.

    There is an infinite variety of ways to perform these "deformations" of the "color space", and this is the real art of color management. The beauty of standards for exchanging these things, such as the ICC profile standard, is that if you don't like one supplier's art, it is easy to substitute another's, or to substitute your own.

    When Gamut Mapping is Not the Issue
    Suppose that you actually have the bright inks, and you've been asked to reproduce a picture specified for the duller inks. This is the case when your printer is equipped with "extreme" inks, and you're being asked to reproduce color specified with good old SWOP printing standards. The entire white polygon is now contained within the polygon of the inks that you've got for printing, and you can hit all the colors. The "colorimetric" and "perceptual" corrections are the same, art is not called for, and science reigns!

    If you find that you can't do this, don't go back to dull inks - get better color management. Solving the problem by going back to dull inks is like buying an abacus to replace your computer. There is real color management available, and it's reasonable for you to insist on using it.

    Gamut Clipping Comparison
    The original on the left contains a range of brilliant greens which are outside the color gamut of the printer. The closest printable green is the same for all of them. In the image on the right, use of a colorimetric rendering intent has mapped all of the greens to a single color - the one best match. Using the best match may sound like a good idea, but by strictly mapping all of the greens to that one best match, color management has wiped out important detail in the image. This is the typical problem with "gamut clipping".

    The use of perceptual rendering intent will prevent this problem by preserving the relationships between similar colors. That involves a compromise to color matching, but it would produce a far better picture in this example. This is the reason perceptual rendering intent may be preferred in situations involving large changes to gamut.

    ICC Rendering Intents- Introduction to Rendering Intents
    Different devices have different ranges of possible colors (different "color gamuts"), and often have different paper colors or "white points". This creates special problems for color matching. The four rendering intents defined by the ICC are essentially "matching styles", that address these issues in different ways.

    The discussion below describes the four rendering intents defined by the ICC, along with some recommendations.

    Perceptual: This rendering intent maps color "smoothly", preserving relationships between similar colors. This prevents "gamut clipping" with its potential loss of detail and "tonal banding" problems. Gamut clipping occurs when colors that are different in the input image appear the same when printed. Perceptual rendering intent makes small compromises throughout the entire color space in order to preserve color relationships. It sacrifices some precision of in-gamut colors in order to ensure pleasing results.

    Perceptual intent will produce the most predictable results when printing from a wide range of image sources, for example, when printing RGB images on CMYK devices, or when trying to match CMYK devices that are radically different from each other. We consider this "foolproof" setting to be best for users who handle the wide variety of images that commonly enter large format printing facilities. It is usually not precise enough for processes where input images are well controlled, such as color proofing and "giclee".

    Absolute Colorimetric: When a color is not printable within the gamut of the output device, this rendering intent simply prints the closest match. It reproduces in-gamut colors without compromise, as faithfully as possible. This produces the most accurate matching of spot colors. Unfortunately, it can also result in "gamut clipping" where two colors that are different in the original are identical on the print. White points are similarly clipped, which tends to cause similar color relationship problems in the highlights of images. Such clipping, and the resultant problems, make this choice generally unsuitable for work involving anything but spot colors.

    Relative Colorimetric: When a color is not printable within the gamut of the output device, this rendering intent prints the closest match along with an adjustment that maps white to the paper of the output. This mapping of "white point" prevents the problems of "Absolute Colorimetric" when images (or anything other than spot colors) are involved. When producing color match proofs on inkjet printers, which typically have larger gamuts than the printing presses being simulated, this is a superior choice. When a pair of ICC profiles is loaded for runtime linking, one for the device to be simulated and one for the device being used, this rendering intent will provide good precision (minimal delta-E) for the match-proof process.

    Saturation: This preserves the saturation, or "brightness" of colors when transforming them for output. It maps fully saturated source colors to fully saturated target colors. This rendering intent is used where color matching and exact relationships between colors is less important than bright colors. This is a "pretty picture" intent that will produce brilliant spot colors.

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