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ICC Color Workflow

What is an ICC color workflow? First lets remove ICC from the question. The purpose of a managed color workflow or color management system is to provide color consistency and predictability throughout the entire workflow. Ideally we would like to maintain color accuracy from start to finish, regardless of the hardware and software we use and we would like the process to be standardized and repeatable.

ICC is simply an abbreviation for International Color Consortium, a fancy name for a color standards organization that included big names like Kodak, Agfa and Fuji among others back in 1993. This group developed new standards and rules for color profiles, the data file that characterizes a devices color. This standardization made it possible for different software and hardware devices to communicate color in the same consistent and repeatable way. It paved the way for a workable color managed system or workflow utilizing any device or software, something once only possible with a closed loop system.

A closed loop system is just that, a color managed system that utilizes one input device, a single computer for data editing and one output device. Additional devices or software cannot be added to the mix. Photographers and lab managers have managed closed loop systems for years by painstakingly calibrating and monitoring their systems and by utilizing well established and proven process control procedures. For example Eastman Kodak developed strict process control procedures for color processes like E-6 ,C-41 and Kodachrome to help lab owners and photographers manage and control these processes. Professional photo processing labs, even today are highly revered for their process control. Their customer's livelihood (photographers) is dependent on the accuracy and repeatability of their equipment, processes and workflow.

With the advent of digital, however, the traditional closed loop controlled process gave way to an open and difficult to control process or workflow. The variables in the process increased dramatically. Traditional silver based processes were well known and understood, digital files and the digital process was not. With the advent of ICC standardization, however, digital files could now be quantified and labeled. Critical color information could be attached to digital files via a standard that other devices could translate and interpret.

Traditional Photography Analogy... Minilab process control: Maintaining a traditional photographic minilab, a chemical based closed loop workflow system, is the same in concept as the electronic one shown in the diagram below.

Color Gamut

The key to a color managed workflow is the color conversion process, how do we convert and maintain color consistency from one device to another. First we should take a look at color limitations and color gamut. Color gamut is the range of color reproducible by a particular device, for example computer monitors generally have a larger color gamut than inkjet printers, and can therefore reproduce and display a greater range of color than an inkjet printer. This is a very important concept to understand. Photographers and generally anyone that works with digital imagery tend to analyze and make corrections to their images on a computer monitor. Photographic images will never be an exact match in print to the screen because a conversion and re mapping of color values occurs during the printing process. Optimally the printed image will closely match the original on screen, but it will never match it exactly because each has a different color gamut.

These two images clearly demonstrate color gamut differences and limitations between devices. The original image on the left is a screen capture from Photoshop as displayed in Adobe 98 color space on a monitor. The image on the right is another screen capture from the monitor but print emulation (soft proofing) has been activated and out of gamut colors are displayed as black. What you see in this second image is an approximation (utilizing ICC standards, profiles and a color conversion engine) of what this image would look like when printed to an ink jet printer. Even though we have optimized and controlled our process the ink jet printer has color limitations, it cannot reproduce all of the vivid red colors displayed on the monitor as noted by the muted colors and "black" out of gamut warning.

Calibration vs. Characterization

Many of you are probably familiar with the word calibration but maybe not as familiar with characterization. These two words and their definitions are often confused when we discuss color management.

Calibration is the act of tuning or bringing a device to a known standard. If for example we have an adjustable thermometer (some of you may remember these from the darkroom days) you can place that thermometer into a bucket of water along with a more expensive, highly accurate and certified mercury thermometer. You can adjust the adjustable thermometer so that it matches the temperature read on the mercury thermometer, in effect calibrating it. In the digital color world, monitors, printers, scanners, etc. are all devices that can be adjusted or calibrated to a known standard.

Characterization is the act of characterizing or describing how a device performs; its capabilities, limitations, etc. We can for example characterize an automobiles performance? Does the vehicle perform well under adverse weather conditions, does it have front wheel, rear wheel or four wheel drive? How does it handle sharp turns and bumps in the road? Is it a luxury sedan (smooth ride) or an off road truck (rough ride)?

Digital cameras, scanners, printers, color management hardware, etc. can all be described in mathematical terms that computers can understand and utilize. Software enables us to characterize these devices. This characterization describes how the device interprets or creates color, what are its tendencies limitations or faults. A particular device may have a color bias or lack sensitivity to a certain color. Software lets us create files that describe our devices, this descriptive file, called a PROFILE can then be interpreted by hardware and software within an ICC color workflow. It enables us to convert colors from one device to another and manage color in a predictable and repeatable fashion.

Remember....Calibrate to a known standard, characterize to describe. It is important to remember these definitions and what role they play in managing color.

The Fuji 3500 printer on the left has a built in calibrator and calibration routine. The Epson printer on the right does not and thus cannot be calibrated. Once a profile is created for the Fuji printer the calibration routine will keep it in control (consistent color) even when paper is changed. If however ink or paper are changed on the Epson printer the original profile may not provide consistent or accurate color because it cannot be calibrated (brought to a known standard). Which printer would be easiest to manage, as far as color is concerned?


The Color Conversion Process

OK, how do we make this all work? We know that digitally captured images cannot be reproduced on the printed page so as to completely match the original scenes color. The color gamut of our original scene must be compressed and altered, but how can we retain as much of the color and color relationships of that scene as possible?

As shown in the illustration above the colors from the original scene had to be compressed throughput the process and the number of colors available from the original to the printed image is dramatically reduced. The color conversion process that takes place within an ICC workflow manages this compression by re mapping colors to retain the look of the original, even though the color gamut may often be compressed or reduced. The method used to remap colors from one device to another is critical to the success of a Color Management System or CMS.

There are four components that make up an ICC color management system, the PCS (Profile connection space, normally CIELAB), the device Profile, a CMM (color management module) and Rendering Intent. Lets take a look at all four of these components and the role they play in a color management system.

PCS: The profile connection space is the color space in which the color conversion or remapping takes place. We already know that CIELAB is an ideal color space for this conversion because it is not Device Dependent, for this reason CIELAB is usually the profile connection color space used.

Profile: A Profile is a file that describes the color characteristics of a particular device or color space. For example monitors, scanners, printers and cameras can all have profiles that describe their unique color properties. An image is often described as tagged with a profile, basically this is a very small file that is actually "tagged" or associated with your image file. A profile can be removed or changed, but doing so does not alter the original file.

CMM: A color management module is the engine that converts color data from one profile or color space to another. For example Apple's Colorsync engine can convert an images colors from a monitor color space to a printer color space and vice versa.

Rendering Intent: When a color conversion is applied from one profile to another four different rendering intents can be applied. A rendering intent is simply the method for mapping or re mapping color values. You can select the rendering intent that best suits your purpose or image characteristics.

Relative Colorimetric: Is the default standard for digital imagery. Any colors that fall outside of the devices color gamut are replaced within the target gamut, hue and lightness are preserved. For most images this intent best retains the overall color qualities of an image as it is converted from one profile space to another.

Perceptual: When the color conversion or remapping takes place colors may be shifted, but the relationships between colors is retained, perceptually retaining the look of the original. If the Relative Colorimetric intent does not produce satisfactory results, perceptual mapping is generally the second choice for images.

Absolute Colorimetric: Colors that reside outside of the target devices color gamut are simply clipped or removed. This is not an ideal choice for images.

Saturation: Tries to retain color saturation of original at the expense of hue and lightness. Often used for graphics where saturation is important, but again generally not a good choice for photographic images.

Click on the image below to see how different rendering intents effect color.


Color Conversion in Action...

OK, lets put this all together. As an example, lets look at the workflow we might use if we wanted to ICC color manage an image from scan to output (print).



The diagram above illustrates a typical ICC color managed workflow from scan to print. If the device can be calibrated then by all means do so. Calibrating to a known standard gives us a standard and consistent base on which to view and judge color.

As the digital image or file passes from one device to the next a color conversion takes place via these device profiles. Remember device profiles characterize how a device renders color. A color management module (CMM) like Apple computers ColorSync translates color information from one device to another by utilizing these profiles.

The CMM takes in data and converts it according to the specification (Rendering Intent) you select. A simple analogy would be baking cookies. You start with a recipe, put it all together and bake it in an oven (the conversion engine). If the recipe changes (profile/color lookup table) or the oven is set to a different temperature (rendering intent), the resulting cookies will be different. The important point to remember here again is that the goal is to produce predictable and consistent results. A properly adhered to cookie recipe and color managed ICC workflow can both do that for us.


Key Concepts and terminology....

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  • The purpose of a managed color workflow or color management system is to provide color consistency and predictability throughout the entire workflow.
  • Calibration is the act of tuning or bringing a device to a known standard.
  • Characterization is the act of characterizing or describing how a device performs.
  • The ICC (International Color Consortium) is a color standards organization.
  • Color gamut is the range of color reproducible by a particular device,
  • Profile: File that describes numerically the color characteristics of a device or color space.
  • PCS: The profile connection space is the color space in which the color conversion or color re mapping takes place.
  • CMM: A color management module is the engine that converts color data from one profile or color space to another.
  • Rendering Intent: The method used for remapping color values. There are 4 different rendering intents used in an ICC color managed workflow, Perceptual, Relative Colorimetric, Absolute Colorimetric and Saturation.
  • Relative colorimetric is the best rendering intent choice for most digital image files.
  • Color managed workflow goal is to produce consistent and predictable results.

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Sect. I| ColorIntro.| WhatsColor|ColorAccuracy| ColorPercept.| ColorTemp.| ColorModels| ICCWorkfl| ColorTools|

Sect. II| Monitors| Scanners| Printers| Photoshop|

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