Real World Adobe® Photoshop® CS2 [Electronic resources] : Industrial-Strength Production Techniques نسخه متنی

The Conceptual Framework

Photoshop's color management has some unique concepts, and places a different emphasis on some mainstream color management concepts than do other applications. So before we dive into dialog boxes, let's look at an overview of the controls and the kinds of things for which you'll use them.

Working Spaces and Device Spaces

In the Bad Old Days (and in many other applications today) you could use only one RGB space and only one CMYK space at a time. But Photoshop's "per-document color" means you can have multiple RGB and CMYK documents open at the same time, each one using a different color space. This is great for service bureaus and others who work with images from many different sources, but it also opens several cans o' worms. We now have to draw a distinction between the working space and the document space(s), and between working spaces and device spaces.

Most of us have become used to switching CMYK spaces as our output needs dictate (each output device or method requires a different CMYK space), but we've generally followed the practice of sticking with a single RGB space, mainly because Photoshop forced us to do so. Now that we're freed from the tyranny of the RGB working space, it's important to look at the pros and cons of leaving RGB images in the space in which they were opened or converting them to an RGB working space.

With per-document color, your chosen working space is just a fall-back position for untagged images (images that don't have embedded profiles). Photoshop offers several different RGB working spaces, and enterprising third parties have created still more. It may seem sensible to edit in the space in which the image was captureda scanner or digital camera spaceor in the final output space, such as an RGB inkjet. But working spaces have important properties that make them better suited to image editing than the vast majority of device spaces.

Most device color spaces are not perceptually uniform. That means that when you edit your file, the same editing increment in Levels, Curves, Hue/Saturation, or whatever, may have a much larger effect on some parts of the tonal range and color gamut than on others.

Most device color spaces aren't gray-balanced. One of the key features of the abstract RGB working spaces built into Photoshop (as well as most third-party working spaces) is that when R=G=B, you know you have a neutral gray. This isn't true for most scanner RGB spaces, many digital camera RGB spaces, and pretty much all printer RGB spaces.

Output device spaces typically clip some colors in the image because their gamut is almost always smaller than the original capture. For instance, if you simply apply a monitor or inkjet printer profile to an image you just scanned, you may not get all the colors you deserve.

Working spaces, in contrast, tend to be uniform, and are invariably gray-balanced. They do, however, differ widely in their gamuts, so gamut size is one of the key considerations when choosing an RGB working space for a particular job (see "Choosing an RGB Working Space," later in this chapter). So while you're no longer forced to use abstract RGB working spaces, you should do so for any serious image editing. Picking an RGB working space and sticking to it will also make your life easier.

Tagged and Untagged Images

Whenever you open or create an image, Photoshop treats it as a tagged or an untagged image from the moment of opening or creation, depending on how you set the Color Management Policies in Color Settings. Tagged and untagged images behave differently as follows:

Tagged images

Tagged images are those with an embedded profile, which may be different from the current working space profile. A tagged image keeps its original profile and stays in the "document space" rather than the working space, unless you explicitly assign a new profile, convert to a new profile, or untag the image, discarding the profile. The Color Management Policies let you automatically keep documents in their own space, convert them to the working space, or discard the profile.

Untagged images

Untagged images have no embedded profile. They exist as a bunch of numbers whose actual color meaning is open to interpretation. If you change the working space while an untagged image is open, the image gets reinterpreted to be in the new working space, and the appearance changes. If you move pixels (by either copy and paste or drag-and-drop) to another image in the same color mode, the numerical values are moved to the new document. For operations where Photoshop needs to make an assumption about the actual colors the numbers represent, such as mode changes or displaying on the monitor, Photoshop treats untagged images as being in the current working space for that mode. It also does so when you move pixels to a document in a different color mode, such as pasting from an RGB document to a CMYK one.

You can always convert a tagged image to an untagged one, or vice versa, by using the Assign Profile command (on the Mode submenu, under the Image menu), or the Embed Profile checkbox in the Save As dialog box, to embed a profile.

Tip: Document Profiles at a Glance

You can tell at a glance whether a document is tagged or untagged by choosing Document Profile from the popup menu at the lower left of the document window (see Figure 5-2). For tagged images, it shows the profile name. For untagged images, it displays "Untagged RGB" (or CMYK, or Grayscale, depending on the document's mode).

A subtler clue can be found in the document window's title bar (see Figure 5-3). The pound sign (#) at the end of the title bar display indicates an untagged document. An asterisk (*) at the end of the title bar display indicates a document that's tagged with a profile different from the current working space. If neither character appears, the document is tagged with the working space profile.

To tag or not to tag?

In the vast majority of cases, untagged documents are a Bad Thing because they force us to guess the intended appearance portrayed by the numbers in the file, and hence create extra work for everyone. The only situations that justify untagged documents are those where the numbers are unambiguous because of the context, or the appearance generated by the numbers is irrelevant.

For example, we didn't embed profiles in any of the CMYK images we used in this book, because they're all going to the same printing condition, and we set InDesign's CMYK working space to the profile that describes that printing condition. Our CMYK profile is about 2.8 MB, so by not embedding it in each and every image, we saved a huge amount of disk space and FTP transmission time. The CMYK numbers are unambiguous, because they're governed by the working space profile for the InDesign document.

By the same token, we don't embed profiles in images destined for the Web. The very few Web browsers that pay any attention to embedded profiles assume that untagged RGB is sRGB, so we convert our web images to sRGB, then export the image without the profile.

When we work with profiling targets, the whole point of the exercise is to find out what colors the device in question produces when we feed it the numbers in the target, so there's no point in making any assumptions about the appearance represented by the numbers.

Last but not least, if you're working in a closed-loop CMYK workflow, where you just don't want the CMYK numbers to change when sent to your printing process, there's no point in embedding a profile.

In all other situations, we strongly recommend embedding profiles in your images. Doing so lets you convey your color intentions clearly to all the devices and all the people in your workflow. Failure to do so forces the people to guess your intentions, causing extra work and frustration for all concerned.

Proofing Simulations

One of the hardestand most importanttasks in Photoshop is proofing what your final output will look like on your screen or on a color printer. Photoshop gives you very fine control over both, which we'll talk about in great depth later in this chapter and in Chapter 13, Image Storage and Output. Here's the quick version, though:

The Proof Setup command (in the View menu) gives you full control over on-screen proofing simulations. Proof Setup's simulations are window-specific, so you can simultaneously view the same file in different simulations.

You can view how different rendering intents will convert an image to a destination space before actually making the conversion.

You can see how an image prepared for one output process will behave when sent to another without adjustment: This is particularly useful when you're faced with the prospect of repurposing CMYK files made for one printing condition to work with another.

You can work inside an accurate output simulation to optimize your image for a particular output process.

But to make this magic work, you must calibrate and profile your monitor, and it's highly recommended that you take steps to control your viewing environment (see the sidebar "Creating a Consistent Environment").