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The Critically Important Color and Gamma Calibration Article


This article will explain how to understand and work with Photoshop's Color Management System (CMS), including how to create a custom profile, how to create an ICC profile for a monitor, how to set Photoshop's CMS defaults, how to use the Color Settings dialog box, and much more. It is excerpted from Inside Photoshop CS, by Gary D. Bouton. (Sams, 2004, ISBN: 0672326442).

Author Info:
By: Sams Publishing
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December 15, 2004
TABLE OF CONTENTS:
  1. · The Critically Important Color and Gamma Calibration Article
  2. · Understanding Photoshop's Color Management System (CMS)
  3. · International Color Consortium (ICC)
  4. · Translation, Please
  5. · Preparing to Create a Custom Profile
  6. · Install the Latest Drivers for Your Equipment
  7. · Setting Photoshop's Color Management Defaults
  8. · The Settings Drop-Down List
  9. · Choosing from the RGB Working Spaces Drop-Down List
  10. · Color Management Policies
  11. · Conversion Options: Which Engine to Use
  12. · Black Point Compensation
  13. · Assigned Profile
  14. · Color Management Policies in Action
  15. · Converting a Profile Means Changing the Data
  16. · Soft-Proofing, or Seeing Onscreen What an Image Will Look Like When It's Printed

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The Critically Important Color and Gamma Calibration Article - Assigned Profile
(Page 13 of 16 )

Photoshop uses ICC profiles in two ways. It looks to an assigned profile for instructions on how to interpret or change the numbers in the file. Assigning a profile to an image tells Photoshop how to interpret the numbers in an image file. When you use the Image, Mode, Assign Profile command to assign to an image a profile other than the default one you designated in the Color Settings dialog box, you are telling Photoshop to look at the numbers as though they had been changed to fit the requirements of some other color space. But the numbers, the data in the file, have not really changed.

An assigned profile instructs Photoshop how to interpret the numbers. The assigned profile is similar to a statement that might be associated with or assigned to this paragraph, a statement that says, "The groups of letters that form this paragraph are to be thought of as being English words that are arranged in a way that makes sense to English language readers. Additionally, if any modification (editing) takes place, interpret that in the same way you interpret the original paragraph."

But a new profile could be assigned to take the place of the first one; the new one could say to interpret the groups of letters in the paragraph according to the rules and grammar of the German language. This new interpretation might not be very pleasing or be the best way to interpret the current order and grouping of the letters, but it would let you know how much or little of the intended communication would be understood if "output" to a German language speaker. In both cases, the only things that change are the interpretation instructions (assigned profile), and not the actual letters, their grouping, or their sequence.

Changing an assigned profile is a game of "what if...." What would this data, these colors, look like if they were transferred to another color space? The color space could be anything for which you have an ICC profile—another monitor, a television, an Epson inkjet print, an HP inkjet print, a press using newspaper, or a Matchprint.

This is the important point to remember: Assigned profiles tell Photoshop and other color management–aware programs how to interpret the data.

But sometimes you want to change the data, not just its interpretation. To use the English-German analogy, sometimes we want to change the letters and their order, translate the data so that it is useful in another context, so that the paragraph makes sense to a German reader. In color management terms, that means that the data within the file, the numbers themselves, must be changed. The way that is done is covered in the next section.


Insider - Changing Your View

Dr. Alvy Smith, who founded PIXAR and is partially responsible for inventing the HSB color model, has a profundity that would seem to fit right in here:

Change your view of the data before you change the data.

In other words, when you change your view, and you've been shown plenty of examples of how to change image view, you are not disrupting original data. Nor are you making the potentially false presumption that your monitor is calibrated perfectly and the system of the person who did the artwork was off.

When you make physical changes to image data, you can almost never get back to the original's content. It's kinda like a turnstile in a subway station: Try exiting from a turnstile that is used for entrance. Manipulating the colors in an image, similarly, is a one-way trip. Choose to change your view as a first, second, and third measure for viewing a file accurately.


Using Profiles to Change the Color Mode of an Image

As stated before, profiles are sets of rules, definitions of color spaces, which programs use to interpret color data. But profiles are also used to provide some of the rules on how to actually change the data in the file to make all the colors fit within a particular color space. The intent—Perceptual, Saturation, Absolute Colorimetric, or Relative Colorimetric—chosen for the conversion also provides rules to guide the conversion process.

To continue with the English-German language analogy, a bilingual person who acts as the translator (the CMS) would look at the letters (the numerical color data), consult the rules of the German language (the destination profile) and the German language dialect that is desired (the intent), and then change the letters (the data) so that the data would actually be transformed from something that could be translated into German—to something already written in German.

If you want to permanently change the data in an image file, use the Convert to Profile command on the Image, Mode menu. Read the previous sentence out loud once or twice; it is an important concept.

This is not the only way to change the data in an image file. Photoshop users have been changing file data for years whenever they changed an image's color mode, from RGB to CMYK, for example. You still can go the traditional route, using the Color commands at the top of the Image, Mode menu, but you will give up the ability to fine-tune the process. When you use the Convert to Profile command, you choose which ICC RGB profile or which ICC CMYK profile you want to use. The traditional color mode commands use the default ICC profile settings you set in the Color Settings dialog box.


Note - Precise conversion

Internally, Photoshop uses the LAB color model as the heart of the conversion engine when going from one Image, Mode to another. So, the conversions are still fairly good, but not as precise as choosing Convert to Profile.


Putting Theory into Practice

If your head hurts from trying to assimilate all this data, you're in good company (refer to Figure 2.1 again). Color management is not a topic to digest the first or even the second time around. But when the light bulb goes on in your head and you shout, "Eureka! I really understand how this works!" you will have moved a long way from hoping your print jobs go okay, to knowing what they will look like before you see the finished results. So let's put into action the concepts of color management we've discussed. We'll start by creating a custom Color Settings preset file and then practice assigning and converting profiles.

Creating a Custom Color Settings File

  1. Launch Photoshop, if it is not already open.

  2. Press Ctrl(Command)+Shift+K to display the Color Settings dialog box. Make sure the Advanced Mode and Preview options are checked. Make the changes specified in the following four steps, if the options are not already set that way.

  3. Set the RGB working space to Adobe RGB (1998); the CMYK working space to U.S. Web Coated (SWOP); the Gray and Spot working spaces to Dot Gain 20%.

  4. Set all three Color Management Policies to Preserve Embedded Profiles. Check both Ask When Opening options and the Ask When Pasting options.

  5. In the Conversion Options section, set the Engine to Adobe (ACE). Set the Intent to Relative Colorimetric. Check both the Use Black Point Compensation and the Use Dither options.

  6. Make certain that the options in the Advanced Options section are not checked.

  7. Click the Save button. In the Save dialog box that appears, use the Save In drop-down and other controls to navigate your way to the folder in which the other color setting files are saved, if you are not already there. Then type IPCS in the File name field, and click Save.

  8. In the Color Settings Comment dialog box that appears, type something that describes these settings, such as Set used for exercise in Inside Adobe Photoshop CS book, that definitive guide—um, you get the picture. Click OK.

    IPCS now appears as the selected setting in the Settings drop-down list.

  9. Click OK to put these settings into use.

You can now choose the IPCS set of custom settings just as you would the ones that Adobe provided.

Now that you have the IPCS defaults set, you will be sure to get the same results as we do in the exercises that follow. As promised, the next set of steps gives you hands-on experience with assigned profiles.

This chapter is from Inside Photoshop CS,  by Gary D. Bouton. (Sams, 2004, ISBN:  0672326442). Check it out at your favorite bookstore today. Buy this book now.


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