Starting with last question first, the rendering intent is what you choose in Photoshop > Edit > Color Settings. At the top of the dialog box there in Photoshop are your choices for what color model your CMYK and RGB should be expressed in. These become the working color spaces for CMYK and for RGB. Working spaces imply that whatever picture you open and edit in Photoshop, it takes on this ICC profile of color model. Having a same choice from Photoshop for ICC color model means consistency in interpreting color, file after file. Near the bottom of the expanded Color Settings dialog box is the Rendering Intent, which is usually set to Relative Colorimetric.
What is Rendering Intent? ACE is the translator. His job is to translate visual color from one math number world to another math number world, with the intention of holding the visual color the same. When he translates numbers (representing colors) from one color space to another, inevitably some color will be lost in translation. Rendering Intent is a conscious decision for how to compromise colors that might be outside the gamut of the receiving color space. Bear in mind that many pictures, some color lost in translation is so slight so as to be scarcely perceptible.
Make your house rules choices here in Photoshop. Save as named .csf file. Then, in Bridge, click on Edit > Creative Suite color settings. Choose to apply this new custom-named house rules for your color workflow. In applying it, Bridge sends out instructions to Illustrator, InDesign, and Acrobat to follow the lead of Photoshop in adopting the same "house rules" for translating color.
If the Photoshop file being placed into InDesign already has the house-rules color space profile, it passes straight into InDesign without being re-interpreted/re-translated. (Like having a Smart Tag that allows you to drive through the toll-booth of the interstate. Like having a passport and express check-in at the airport. No need to re-examine you; you are already cleared. These are illustrations, by the way!)
Yes (your second question), ACE, in a behind-the-scenes support operation, checks all ICC color profile "passports" to see whether you need translating prior to entry, or merely waved through into InDesign. If the picture file already has the same working space as defined in Photoshop, it is allowed in unchanged. If the picture file doesn't have the same house color space, it is translated to the working color space, and allowed in afterwards. You might liken it to being issued a temporary passport. Do numbers get rounded off? Yes and no. Visual colors that exist in both color realms can be translated with great precision. Visual colors outside of the gamut of the receiving color space will change a bit, which is to say their numbers also change a bit.
As to your first question, is LAB at the center of things? Yes, in Photoshop. Lab color mode is the closest thing to a number system that consistently matches human visual color perception. Photoshop natively thinks in Lab, and translates out to all other color models, especially RGB and CMYK. Make sure to always run your pictures through Photoshop first (I like it's color management policies to force a permanent change to the house color space by choosing Conver to Working RGB/CMYK). After that, you place them into InDesign. In this way, all pictures already have a SpeedPass, so to speak, and InDesign does not re-translate the color. InDesign does not handle the Lab numbers, but is rather given the resulting conversion numbers by the translator, ACE, (if it needs to) working behind the suite of four softwares.
You might enjoy reading the book: Real World Color Management by Fraser, Murphy, and Bunting, and really geek out on this sort of thing.
Hope this helps,
Mike Witherell in Maryland
As I understand it, when I make a new CMYK swatch in InDesign and type my recipe in, Adobe converts it to LAB and that is the number that is used behind the scenes to convert to RGB or to different CMYK profiles.
When you make a CMYK color it has to get converted into your monitor RGB space for display, and Lab is the device independent color space that is used to make an accurate translation from the source CMYK space into monitor RGB. So for the CMYK preview to be accurate, the document CMYK profile has to match the press conditions and your system's monitor profile has to correctly represent your display.
Other conversions RGB>CMYK, CMYK>RGB, CMYK>CMYK, RGB>RGB work the same way, where Lab acts like a Rosetta stone for making the conversion.
is it known what rendering intent is used?
If you want to convert native CMYK colors to another CMYK space you have to use Edit>Convert to Profile and you have the option to choose a rendering intent in that dialog.
By default the Color Setting's Intent setting is used for conversions when you Export or use the Color Picker. You can override the Color Setting's intent for a document via Edit>Assign Profiles.
You can also convert a color from the Color panel, but it looks to me like Color Setting's Intent is ignored and perceptual is used. The Color Picker is more reliable.
Message was edited by: Rob Day