Color Management in Adobe Photoshop® part 2:
Other Color Settings
Please note: this section covers technical aspects of Photoshop's color settings that most users should not change from the North American Prepress 2/ US Prepress defaults. It was written for those who want to understand all of the implications of the options available in color settings. Those who just want a general overview of color management in Photoshop can set their color settings to North American Prepress 2 and skip straight to section 3.
Further down on the color settings are the color management policies. The most important part of this section is the bottom, where all of the “ask” boxes should be checked. This forces Photoshop to display a dialog box every time that a color management decision is made and allows you to intercede in that decision. The top of the policies section, regarding whether to preserve embedded profiles or to convert to the working space, will only affect which default choice comes up in that dialog box when opening or pasting from a file using a color space other than your working space. The other choices in this section (“convert to working space” and “do not color manage”) will also show up in the dialog box, as long as the “ask” boxes are checked. If they are not checked, the default choice will be used, possibly causing a color conversion to take place as the file is opened or pasted. If this happens, some color information may be lost. You may not realize it however, because no dialog box shows up and the conversion takes place before you see the file on your screen. This topic will be covered in depth in Section 6: Saving embedding, and Opening Files. For now just make sure that all of the ask boxes until you are confident enough with color management to turn one of them off.
Next down the color settings box is the “Conversion Options” section. The engine or CMM (color management module) is the “calculator” that performs the translations between color spaces/profiles/modes. Early on, some color management software vendors adjusted their engines to tweak color conversions. As color management matured a proliferation of generic color management modules began to ship with imaging software (ACE- Adobe Color Engine) or operating systems (Apple Colorsync). At that point it became more realistic to build those tweaks into the profiles (and sometimes color spaces) themselves. According to the wonderful color management book “Real World Color Management”, Kodak is about the only company that still builds significant proprietary advantages into their color engines. So, as long as you don’t use profiles created with Kodak software (Which would be rare- you would probably know if you were using Kodak profiles), you should see little difference between color engines. While it might be a good idea to test all of your available engines, if you are using Photoshop you have Adobe ACE for “free”. ACE is a tried and true conversion engine that is used by many if not most color management experts, making it a wise choice. The only drawback to ACE has been that for years it was only available within Adobe applications. So RIPs and other third party software were unable to take advantage of it. In 2007 this changed, as Adobe decided to offer the ACE engine as a free download that could be configured for use by non-adobe Applications.
Rendering intent is your next choice. Very often there are colors in a color space or input profile that simply do not exist in a destination color space or output profile. When converting between them, the out-of-gamut colors must be “mapped” to some color that is reproducible in the destination space or profile. Rendering intent specifies exactly how this is done. There are several options, and a discussion of the various pros and cons would be considerably longer than this discussion of Photoshop's color settings. We will talk about rendering intent in more detail later in the "Convert to Profile" section of this series, but here in the color settings we are just setting the default rendering intent. Any time that you will be making a critical profile conversion in PhotoShop, you will have the option of choosing something other than the default intent. Because of what this default controls, which I will cover in the next two paragraphs, it is my opinion that for almost all users the relative colorimetric intent is the best choice. So to get started you can just select relative colorimetric and leave the rendering intent debate for later.
Whatever intent you choose, the most important part of the choice is understanding exactly what this default rendering intent in the color settings box actually controls. Most noticeably, it controls what default choice comes up in all "convert to profile" dialog boxes. However, the convert to profile dialog allows you to change from the default to any other rendering intent at any time via pull-down menu, so this really isn't a big issue. What is more significant is that the intent chosen in color settings is used for all mode changes that do not go through the convert to profile dialog. So for example, opening an RGB file and using Image>Mode>CMYK would cause the file to be converted to CMYK via the default rendering intent. Please note that you can force a convert to profile dialog by selecting Image>Mode>convert to profile and choosing a profile or color space from the mode that you want to switch to.
A more subtle effect of the rendering intent chosen in color settings is the control of the way that color modes other than the one used in the current file are represented in Photoshop's info palette. The info palette's secondary readout is set to CMYK by default, and experienced color management users will often set the it to display device independent Lab values. To display this information, Photoshop must convert the current color mode to calculate the second readout. In performing this conversion, some method of mapping must be used. So for users who count on this information to be correct, such as prepress operators who use the info palette to track final CMYK values in an RGB file, the default intent must match the intent used when the conversion to CMYK is performed or the info palette values will be inacurate. A similar dilemma exists for the small percentage of users who use Photoshop to try to hit specific Lab colors on output and use the info palette to track them. For these users absolute accuracy of this information, rather than a ballpark approximation, is very important and absolute colorimetric rendering intent must be chosen as the color setting default.
The final choices under Conversion options are black point compensation and dither. Black point compensation maps the darkest point in the source to the darkest black in the destination (much like white point shifting in relative colorimetric). Not using black point compensation can cause problems in the way that shadows are mapped. For instance, a range of shadows could correspond to lower L values (Lightness from LAB) than a particular paper can reproduce. Without black point compensation, all of the range darker than the measured paper black would be mapped to paper black, causing shadow gradation to abruptly stop at a flat black blob. Unless there is a rare and specific reason to render the source black absolutely correct by measurement, “use black point compensation” should always be checked.
Dither introduces a small amount of digital “noise” to hide the transition banding that may occur when converting a tonal file between spaces and/or profiles. Introducing this negligible amount of noise during the conversion process is beneficial for most tonal photographic images because it prevents banding in smooth gradations. In contrast, this noise may be objectionable in flat spot colors. The general rule is to always use dither when converting tonal files (which includes all photographic images) but not to use dither for files containing only flat, spot, or solid colors.
At the end of our tour of Photoshop’s color settings is the “advanced controls” box. The first option is to desaturate monitor colors by a user defined percentage. After profiling your monitor, the last thing you want to do is alter it’s color response. This setting exists primarily for non- color managed workflows. If you are serious about color accuracy, use color management and leave the desaturate box unchecked.
Finally we have the “blend RGB colors using gamma” setting. Photoshop’s description states that this control is used to modify edge effects of blended colors, as would happen with a Gaussian blur. While this is true, the check box also controls the way that blended colors are calculated. The effects of using this control can be dramatic. In order to fully discuss this control option, a basic understanding of the concept of gamma is necessary.
Gamma is a correction designed to compensate for the non-linear nature of human vision. In other words, humans do not perceive equal steps in density as being equal, and gamma attempts to correct for this. (Think of a darkened room. Lighting one candle would seem to make a large increase in the amount of light in the room. The same candle giving off the same amount of light outdoors in bright sun would barely be noticeable.) The correct gamma number necessary to compensate for this phenomenon varies somewhat based on the situation. Most color spaces use gammas between 1.8 and 2.2, with a higher gamma number indicating a more dramatic correction curve and therefore more compensation. A gamma of 1.0 (often called linear gamma) has no correction and is the same as not using gamma at all.
Most applications blend colors using the native gamma of the images’ color space, and this is what Photoshop does by default. Checking the blend using gamma box allows you to enter a different gamma. Linear gamma (a setting of 1.0) is used as an alternate gamma most often because it is more colorimetrically correct, meaning that colors would blend in a logical way that closely follows how the colors would mix in real life. The drawback to linear gamma blending is that colors that are compliments of each other mathematically (colors whose coordinates add up to equal numbers, such as 30,15,0 and 0, 15, 30 whose sum is 30,30, 30 for example) do not blend to a neutral gray as they should. Blending in native gamma causes mathematically complimentary colors to blend to a perfect gray, but ranges in between may pick up hue shifts that do not seem logical.
Considering these factors with the original aim of the control, to minimize blending artifacts in blurs, there is not a single setting for this control that is optimal in all situations. Good arguments could be made to either leave the default setting of “off”, or to check the box and use a gamma of 1.0. In images where blends become critical, you could open the color settings dialog box on screen and move it so that you could see the blends. You could then use the settings box’s “preview” button (on the right side under the load and save buttons) to evaluate the effects of different settings. (Change the setting and turn preview on and off.) In conclusion, to configure Photoshop’s color settings: 1. Open the color settings dialog box. (Edit> color settings… (Windows) or Photoshop> color settings… (Macintosh). 2. Change the settings: pull-down to “U.S. Prepress Defaults” and check “advanced mode”. 3. Select an RGB working space. If you are unsure of what to use for RGB, Adobe 98 is a very widely used compromise between small spaces that restrict gamut and large spaces that are difficult to use because of the large size of each color “step”.
Your color settings are now configured. You can save the settings with the save button to use as your own default in the future. Remember to close out of Photoshop even if you save the settings, because Photoshop only records changes to its preference files when it shuts down. If your system were to crash, Photoshop would load your previous color settings from the last time Photoshop closed down properly, not your new ones. You are now ready to take advantage of Photoshop’s powerful color management potential.