Adobe Photoshop CS5 for Photographers - Martin Evening [271]
Think of a profile as being like a postcode (or ZIP code) for images. For example, the address label shown in Figure 12.4 was rather optimistically sent to me at ‘Flat 14, London’, but thanks to the postcode it arrived safely! Some labs and printers have been known to argue that profiles cause color management problems. This I am afraid is like a courier company explaining that the late arrival of your package was due to you including a ZIP code in the delivery address. A profile can be read or it can be ignored. What is harmful in these circumstances is an operator who refuses to use an ICC workflow. If you feel you are getting the runaround treatment, it may be time to change labs.
Figure 12.4 Even if you have never been to London before, you know it's a fairly big place and ‘Flat 14, London’ was not going to help the postman locate my proper address. However, the all-important ZIP code or postcode was able to help identify exactly where the letter should have been delivered. An image profile is just like a ZIP code – it can tell Photoshop everything it needs to know about a file's provenance.
Translating the color data
One way to comprehend the importance of giving meaning to the numbers in a digital file is to make a comparison with what happens when language loses its meaning. There is an excellent book by Lynne Truss called Eats, Shoots & Leaves: The Zero Tolerance Approach to Punctuation. It is partly a rant against poor punctuation, but also stresses the importance of using punctuation to assign exact meaning to the words we read. Remove the punctuation and words can soon lose their true intended meaning. Another good example is the way words can have different meanings in other languages. So a word viewed out of context can be meaningless unless you know the language that it belongs to. For example, the word ‘cane’ in English means ‘dog’ in Italian.
Color Management Modules
The International Color Consortium (ICC) is an industry body that represents the leading manufacturers of imaging hardware and software. The ICC grew out of the original Color Consortium that was established in 1993 and has been responsible for extending and developing the original ColorSync architecture to produce the standardized ICC format, which enables profiles created by different vendors to work together. All ICC systems are basically able to translate the color gamut of a source space via a reference space (the Profile Connection Space) and accurately convert these colors to the gamut of the destination space. At the heart of any ICC system is the Color Management Module, or CMM, which carries out the profile conversion processing. Although the ICC format specification is standardized, this is one area where there are some subtle differences in the way each CMM handles the data. In Photoshop you have a choice of three CMMs: Adobe Color Engine (ACE), Apple ColorSync or Apple CMM. There are other brands of CMM that you can use as well, but this really need not concern most Photoshop users, as I recommend you use the default Adobe (ACE) CMM in Photoshop.
The Profile Connection Space
If the CMM is the engine that drives color management, then the Profile Connection Space (PCS) is at the hub of any color management system. The PCS is the translator that can interpret the colors from a profiled space and define them using either a CIE XYZ or CIE LAB color space. The Profile Connection Space is an interim space. It uses unambiguous numerical values to describe color values using a color model that matches the way we humans observe color (see Figure 12.5). You can think of the PCS as being like the color management equivalent of a multilingual dictionary that can translate one language into any other language.
Figure 12.5 A color management system is able to read the profile information from an incoming RGB file and behind the scenes it builds a table that correlates the source RGB information with the Profile Connecting Space values.
If an ICC profile is embedded in the file, Photoshop