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Tuesday, June 19, 2007

Mysteries Of (Color) Space

We've covered it before and we'll cover it again because the ins and outs of color management begin with a firm grasp of how devices handle color space

Working Space Vs. Device Space

Working space is the color space you're using when you process a file with Photoshop or another color-managed imaging application. The most commonly used working spaces are Adobe RGB and sRGB. Other editing spaces include ProPhoto RGB, ColorMatch RGB and Apple RGB, to name a few. All of the working spaces are device-independent, meaning they're free from a device that has its own interpretation of what a particular color should be.

Device space is the space used by a particular device, such as a digital camera, scanner, monitor or printer. These devices have their own description of color based on what the manufacturer of the device thinks it should be, or on the technical limitations of the device, and rarely, if ever, do they match each other in terms of numerical values. For example, scan a swatch of Pantone Reflex Blue, which we know from Pantone is R=0, G=32, B=159. Scanner A may read the swatch as R=2, G=32 and B=159, and Scanner B may read it as R=0, G=33, B=158, and so forth. In other words, the two devices don't “see” the color in the same way, and neither sees it precisely according to Pantone's specifications. This phenomenon is known as device-dependent color.

So, if a scanner, digital camera, monitor or printer all collect, display and print color differently, how can one maintain consistent color from device to device, from capture to output? The answer is with a color-managed workflow incorporating and implementing ICC profiles.

Color Profiles And Gamut

Profiles are nothing more than files that describe (numerically) a characteristic of a certain device or color space and are used when converting from one space to another. When doing so, two profiles are needed—a source profile and a destination profile. Don't confuse destination profile with output profile. In certain instances, an output profile also may be a destination profile, but not always. For example, when converting from an input profile from a scanner to a working space such as Adobe RGB, the source profile is the scanner profile, and the destination profile is Adobe RGB (and, as we learned before, Adobe RGB is a working space, not a device space).

During conversion, color doesn't transform magically from one space to another. It goes through a profile connection space (PCS). This space encompasses all color and is the medium through which transformation takes place. This is a crucial step because color must be rendered properly so that any color within or outside the destination space's color gamut isn't compressed beyond desired results.

Each working space and device space has its own parameters of color, known as a color gamut.

This illustration represents all of the color in the visible light spectrum, or those colors to which the human eye is sensitive. Within the visible light spectrum resides the parameters, or gamut, of different color spaces. Note how much larger the gamut of ProPhoto RGB is compared to that of Adobe RGB and sRGB. Because Adobe RGB is within the boundaries of ProPhoto RGB, Adobe RGB is said to be within ProPhoto RGB's gamut. This isn't true of Adobe RGB's relationship with sRGB, as parts of the Adobe RGB space are outside the gamut of sRGB.

The implicit challenge lies in converting an image from a working space with a wide gamut to an output space with a smaller gamut.

If the editing space is Adobe RGB and our destination space is a printing press (which uses CMYK), then a tremendous amount of color and tonal compression will occur upon the color conversion. There are a lot of colors in the Adobe RGB gamut that can't be reproduced on the printing press. To help control that, specific rendering intents are used. We'll come back to rendering intents later in the context of output. First, let's take an overview of a color-managed workflow.


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