Posts Tagged ‘ICC’

Converting raster images from an RGB colorspace into a print CMYK colorspace has two significant impacts:

1) Typically a compression and alteration of colors as the image is transformed from the original RGB gamut to the different gamut used for CMYK presswork.

2) The on-press printability of the imagery in terms of color stability, press performance/runnability, and ink usage (i.e. cost).

Converting images from one CMYK separation condition into a different CMYK separation condition by reseparating files is primarily intended to enhance the printability of the imagery while maintaining the appearance of the original CMYK
imagery. Put another way, reseparating CMYK files is effectively a way to optimize press forms.

Under Color Removal & Grey component Replacement (UCR & GCR)

The principle of RGB to CMYK separation:
In order to go to press, RGB color images must be converted to their process color counterparts; cyan, magenta, and yellow. An achromatic black channel is added because if the color black in presswork is just made from CMY it can often appear “muddy” or “patchy.” Also, making dark colors from the three chromatic process colors can lead to a higher than desirable volume of ink on the press sheet.

Neutral colors made up of three process colors are also more difficult to maintain consistent on press as solid ink densities normally vary through the run compared with a neutral made primarily of a single black ink. The net effect of introducing black ink in process printing is a reduction of ink usage/costs, stabilization of color (especially gray
tones), and and better printability.
The conversion process is done by taking the 3 channel RGB image, passing it through a 3 channel device independent CIEL*a*b* profile connection color space where the RGB is converted to CMY and the black channel added, and finally
outputting the result as a 4 channel CMYK image.

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sRGB v4 Preference:

The sRGB v4 ICC preference profile is a v4 replacement for commonly used v2 sRGB profiles. It gives better results in workflows that implement the ICC v4 specification. It is intended to be used in combination with other ICC v4 profiles.

The advantages of the new profile are:

  1. More pleasing results for most images when combined with any correctly-constructed v4 output profile using the perceptual rendering intent.
  2. More consistently correct results among different CMMs using the ICC-absolute colorimetric rendering intent.
  3. Higher color accuracy using the media-relative colorimetric intent.

A typical use case would be to print sRGB images captured with a digital still camera. In this case a user could open the image in Adobe® Photoshop®, assign the sRGB v4 profile as shown in Figure 1. Adobe® Photoshop® is either a registered trademark or trademark of Adobe Systems Incorporated in the United States and/or other countries

Assigning the sRGB v4 ICC preference profile.

The user would then convert the data to printer specific values with the sRGB v4 profile as source, the v4 printer profile as destination, and selecting the perceptual rendering intent, as shown below

Conversion from sRGB v4 ICC preference profile to v4 printer profile in Photoshop using the perceptual rendering intent.

The three available rendering intents of the sRGB v4 profile should normally be used as follows:

  • The ICC-absolute colorimetric rendering intent should be used when the goal is to maintain the colors of the original on the reproduction,
  • The media-relative colorimetric intent should be used when the goal is to map the source medium white to the destination medium,
  • The perceptual intent should be used when the goal is to re-optimize the source colors for the reproduction medium while maintaining the “look” of the source image

The re-optimization performed by the perceptual rendering intent of the profile is a result of using the Perceptual Medium Reference Gamut defined in the ICC v4 specification as an intermediate rendering target. No perceptual reference medium is defined for v2 profiles.

Advice for caution

Ideally the ICC v4 profile should not be combined with ICC v2 profiles. If that is unavoidable, see the intermediate-level ICC White Paper 26 ‘Using the sRGB_v4_ICC_preference.icc profile’ for additional information and recommendations.

The sRGB v4 profile will in some cases not produce the same results as would be obtained using an sRGB v2 profile. Differences will depend on the particular sRGB v2 profile and rendering intent used. See the intermediate level white paper for additional information and recommendations.

If sheets are pulled freshly printed from the delivery and measured, the ink is still wet and has a shiny surface. While drying, the ink penetrates the paper (absorption) and loses its gloss. This not only changes the color tone, but also the density. It is only possible to a limited extent for the press operator to use densitometry to compare wet sheets with the reference values, which also refer to dry ink.

Why Polarizing filters?

To get round this problem, two linear polarizing filters at right angles to one another are placed in the path of the densitometer. Polarizing filters only permit light waves oscillating in a certain direction to pass. Part of the resultant aligned beam of light is reflected by the surface of the ink, but its direction of oscillation remains unchanged. The second polarizing filter is rotated 90° in relation to the first, which means that these reflected light waves are blocked.

Fig1. Polarizing filter

However, if the light is only reflected after it penetrates the film of ink, either by the ink or the paper, it loses its uniform direction of oscillation (polarization). Consequently, part of it passes through the second polarizing filter and can be measured.

Filtering out the light reflected by the glossy surface of the wet ink thus has the effect of making the densitometric measurement values for wet and dry ink roughly equivalent.

Brilliant colors true to the original on a premium surface – that ̓s what distinguishes a high-quality print product. Many elements in prepress and printing impact on a product ̓s color fidelity. The various input and output units in a print shop ̓s production process, consumables and other factors can cause deviations in color. A consistently employed color management system eliminates this problem.

In this series of blog tips we ̓ll show you the most common sources of error and give you tips on color management. We ̓re pleased to have caught your interest.

What is Color Management?

The color management process enables an identical color impression to be produced error-free on various output units, such as monitors, proofers and printing presses.

a. World of Color Management

With a consistently employed color management system, a template made on any input unit can be reproduced virtually identically at any output unit. Color management systems can also harmonize a range of devices, such as scanners, digital cameras, monitors, printers, filmsetters and platesetters. The color is then shown according to the print conditions, for example.

The print result is at the heart of the color management process, because the possibility of making corrections directly at the printing press is limited.

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.

ICC recommends the use of profiles made according to the v4 specification in place of the older v2 specification, as it provides better support for consistent results in open, vendor-neutral, cross-platform color management systems.

Benefits that users will see with v4 are:

  • More consistent and accurate colorimetric intent transforms resulting from a fully-defined media white point and more consistent handling of black points
  • More consistent and higher quality perceptual intent transforms because there is now a fully-defined perceptual reference medium dynamic range and gamut
  • Smaller and more accurate profiles that make use of new efficient transform structures
  • Generally improved consistency between different profiles and CMMs as a result of improvements to the specification throughout
  • Better support for re-purposing and proofing of images rendered with the perceptual and colorimetric intents through more accurate inverse transforms

V4 raison d’etre to enable intelligent CMMs and improve results for non-linear color spacescontains the V2 – V4 technical changes and benefits list.

More information on the v4 specification and the advantages of v4is available.

While ICC recommends the use of v4 profiles wherever possible, it recognizes that v2 profiles continue to be used in some workflows and can give good results. Some recommendations on making v2 profiles are provided for developers.

Version 4 of the ICC profile specification was published in 2001. The v4 profile format has introduced a number of changes when compared to the previous v2 specification. These changes provide a number of advantages, the most significant of which follow from the removal of ambiguities from the specification and a more precise definition of the PCS. More information about the advantages of the V4 specification can be found here.

These lead to an improved predictability of performance of a profile in use which will lead to a reduction of major differences of interpretation. Therefore, when pairs of profiles are used they should always produce the same result – regardless of which CMM is used. A summary of reasons to use the v4 specificationis available.

ICC – International Color Consortium

As color management requirements evolve, the profile specification undergoes a process of continuous review by members of the ICC. The current version is ICC.1:2004-10 (Version 4.2.0.0) and is available on the ICC Specification page, together with Amendments approved by ICC members.

The specification has been approved as an ISO standard (ISO 15076) and the version on the ICC Specification page is compatible with the ISO publication.

Most platforms and color management applications have now migrated to ICC V4, or are in the process of doing so. You can test whether your system is V4-compatible here