Rendering Intents… ICC Profile Vol 5

Posted: September 27, 2012 in Color, Density, ICC, ICC profile, PCS
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Rendering intents

In general, actual device color gamuts (the range of all possible colors which can be represented or produced on the device) will not be large enough to reproduce the desired color appearances communicated by the PCS values. Four rendering intents (gamut mapping styles) are defined by the ICC in order to address this problem. Each one represents a different compromise. The colorimetric rendering intents enable within gamut colors to be reproduced accurately (though possibly with compensation for the whiteness of the media) at the expense of out-of?gamut colors. Compensation can be made for chromatic adaptation when the viewing condition assumed is different to the reference viewing environment. The other rendering intents modify the colorimetric values as-needed to account for any differences between devices, media, and viewing conditions.

a. Colorimetric Intents

As stated earlier, the colorimetric intents preserve the relationships between in-gamut colors at the expense of out-of-gamut colors. Mapping of out-of-gamut colors is not specified but should be consistent with the intended use of the transform.

It should be noted that in transforms for the media-relative and ICC-absolute colorimetric intents, the PCS values may represent a preferred color rendering of the actual original captured for input profiles rather than a faithful reproduction. Likewise for output profiles, the PCS values may be color rendered by the output device to the actual medium. However, wherever ICC profiles are used, the PCS values resulting from such transforms are interpreted as the colorimetry of the original and reproduction, regardless of whether such colorimetry is the actual colorimetry.

b. Media-Relative Colorimetric Intent

This intent rescales the in-gamut, chromatically adapted tristimulus values, such that the white point of the actual medium is mapped to the white point of the reference medium (for either input or output). It is useful for colors that have already been mapped to a medium with a smaller gamut than the reference medium (and therefore need no further compression). In transforms for the media-relative colorimetric intent the PCS values represent media-relative measurements of the captured original (for input profiles), or media-relative color reproductions produced by the output device (for output profiles).

c. ICC-Absolute Colorimetric Intent

For this intent, the chromatically adapted tristimulus values of the in-gamut colors are unchanged. It is useful for spot colors and when simulating one medium on another (proofing). In transforms for the ICC-absolute colorimetric intent the PCS values represent measurements of the captured original relative to a hypothetical perfectly reflecting diffuser (for input profiles), or color reproductions produced by the output device relative to a hypothetical perfectly reflecting diffuser (for output profiles).

Note that this definition of ICC-absolute colorimetry is actually called “relative colorimetry” in CIE terminology, since the data has been normalized relative to the illuminant.

d. Saturation Intent

The exact gamut mapping of the saturation intent is vendor specific and involves compromises such as trading off preservation of hue in order to perserve the vividness of pure colors. It is useful for images which contain objects such as charts or diagrams.

e.Perceptual Intent

The exact gamut mapping of the perceptual intent is vendor specific and involves compromises such as trading off preservation of contrast in order to preserve detail throughout the tonal range. It is useful for general reproduction of images, particularly pictorial or photographic-type images.

A profile that enables perceptual rendering and transcends the actual device needs to represent the desired appearance. It is difficult to know how to generate such a profile. It is helpful to conceptualize a “reference medium” which is a hypothetical medium on which the colors are being rendered. It has a large gamut and dynamic range which approximate the limits of current reflection-print technology. It is described using “realworld” specifications so that even though the medium is not real, it can be treated as if it were real. It is also necessary to define a “reference viewing environment” which is the environment in which the reference medium is to be viewed. This environment is used to determine the observer’s adaptation state and establishes the connection between color stimulus and color appearance.

The concept of a reference medium viewed in the reference viewing environment helps the profile designer to understand how to produce “desired appearance” in the PCS. At the same time, it preserves the goal of decoupling the characteristics of actual media through a virtual intermediate reproduction description.

So, in perceptual transforms the PCS values represent hypothetical measurements of a color reproduction on a reference medium. It is defined as a hypothetical print on a substrate having a neutral reflectance of 89%. The darkest printable color on this medium shall have a neutral reflectance of 0,30911%, which is 0,34731% of the substrate reflectance. These are the white point and black point of the reference medium. The reference medium therefore has a linear dynamic range of 287,9:1 and a density range of 2,4593. By extension, for the perceptual intent, the PCS represents the appearance of that reproduction when viewed in the reference viewing environment by a human observer adapted to that environment. If the actual viewing environment differs from the reference viewing environment perceptual transforms need to include a compensation for the differences in viewing environments. Note: It is important to remember that the reference viewing condition and reference medium only apply to the perceptual transform.

Because perceptual renderings are vendor specific it is unlikely that profiles produced by different vendors will produce the same result. Users need to be aware of this and ensure that their workflows enable consistency when it is required (such as in distributed printing) by, for example, transmitting output profiles along with their images. There is currently no agreed ICC specification for this procedure and the onus is on the user. However, ICC are reviewing the needs of such workflows in conjunction with various standards groups.

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