Visualizing Color Spaces

Color spaces are actually three-dimensional. The profile shown in figure 3-7 is sRGB, illustrated using the Color-Sync utility in Mac OS X. Windows XP has a similar utility, called MS Color Control Panel, that is available for free download [97]. In figure 3-7, the gray shape shows the Adobe RGB (1998) gamut as a comparison.

The printing industry also uses 2D charts to display color spaces. The color space plot shown in figure 3-8 was generated using the X-Rite ProfileMaker Pro Profile Editor.

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Prophoto Gamut

Figure 3-8:

A gamut display plotted by ProfileMaker Pro using a 2D Yxy diagram.

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> Since the eciRGB profile is not part of Adobe's Photoshop distribution, you have to download this profile from the ECI Web site (www.eci.org).

Profilemaker Creating Monitor Profile
Figure 3-7: A 3D Yxy diagram of sRGB (in color) and Adobe RGB (1998) (in gray).

Figure 3-8:

A gamut display plotted by ProfileMaker Pro using a 2D Yxy diagram.

In figure 3-8, you can see that the ProPhoto RGB gamut is extremely wide, while Adobe RGB (1998) is narrower, and sRGB is the narrowest of the three.

We often use Hahnemuehle German Etching watercolor paper with our inkjet printers. This paper's gamut (when used with Epson UltraChrome inks) exceeds the sRGB gamut but fits well with Adobe RGB. The color space of Epson Luster paper has a much wider range than Hahnemuehle Water-color paper and exceeds both sRGB and Adobe RGB (1998) in some blue and green tones. If you have ICC profiles describing different printing sets

Gamut Epson
Figure 3-9: The red line shows the simplified gamut, while the multicolored line shows the gamut in more detail. Both plots show the gamut of Epson Premium Semigloss paper on an Epson R2400 using K3 inks.

(i.e., combinations of printer, inks, and paper), diagrams like the ones in figure 3-7 and figure 3-8 allow you to compare the gamut and thus the color richness you can achieve using different papers.

Figure 3-7 and figure 3-8 were plotted using a Yxy plot where the Y axis represents luminance. In figure 3-8, the Y axis is perpendicular to the plotted plane that shows the gamut at a lightness value of 50% of maximum brightness (50% white). The gamut outlines in figure 3-8 are simplified. Figure 3-9 shows the gamut of Epson Premium Semigloss paper using a simplified (red) and a more detailed (multicolored) outline.

Although Yxy diagrams are a common way to plot the gamut of color spaces, there are a number of other 2D and 3D plots in general use. A brightness value of 50% is standard for 2D plots. Another popular plot uses the a* and b* axes from the L*a*b* model, with the L* axis perpendicular to the plotted plane. Figure 3-10 shows such a plot, comparing the gamut of Adobe RGB (1998) (orange outline) with that of Epson Premium Semigloss paper on an Epson R2400 printer using Epson K3 inks and specific driver settings. L*a*b* plots are quite good for comparing different gamuts, and ColorThink Pro [61] (used to create this plot) is a great tool for producing color space plots. You can also use ColorThink to inspect and correct ICC profiles.

b* axis) ProPhoto RGB Adobe RGB (1998) sRGB

Epson Premium Semigloss (using K3 inks on an Epson R2400)

Figure 3-10:

A 2D L*a*b* plot showing the gamut of several working spaces and that of Epson Premium Semigloss paper on an Epson R2400 printer using Epson K3 inks and with print quality set to RPM. This plot was created using ColorThink Pro 3.0.

b* axis) ProPhoto RGB Adobe RGB (1998) sRGB

Epson Premium Semigloss (using K3 inks on an Epson R2400)

Figure 3-11 shows a 2D plot that uses CIE-L*a*b* coordinates (Lightness + chroma a and b ). The vertical (invisible) axis is the lightness axis.

The plot in figure 3-10 also shows that there are a few colors (such as saturated yellows and some saturated cyans) that this printer + paper combination could print that lie outside of the Adobe RGB (1998) color space. If you shoot a digital image in RAW and convert it to Adobe RGB (1998), these colors will be clipped (i.e., they will be mapped to colors that are available in the gamut of Adobe RGB; section 3.3 explains how this is done). If you shoot your images in JPEG format, the conversion and color mapping from the camera's gamut to the color space of the JPEG image will be performed automatically by the camera, either using Adobe RGB (1998) (which we consider to be the better solution) or sRGB as the target space. This depends on your in-camera settings. For JPEG and TIFF images, advanced digital cameras allow you to select either sRGB or Adobe RGB (1998) as your target color space. We recommend using Adobe RGB (1998).

If you want to retain as much gamut as possible for editing, you should shoot in RAW format and use ProPhoto RGB when converting the image to an external RGB color space. However, as the gamut your monitor can reproduce is probably even smaller than Adobe RGB (1998), you won't be able to see these colors on your monitor. Photoshop (or whatever color-managed application you are using) will map those colors to colors the monitor can reproduce using the relative colorimetric mapping algorithm described in the next section. This sounds bad, but it is in fact a practical (though not ideal) solution.

At some point along the way from the camera or scanner to the print, the colors of your image have to be mapped to the colors that your printer, your inks, and your paper can reproduce. The same is true when reproducing colors on a monitor for viewing or editing. Quality applications, such as Photoshop, Lightroom, and many other color-managed viewers and image editors, allow you to define how colors are mapped for printing (see also section 3.3).

If you can't reproduce the rich gamut that your camera or scanner can capture on your monitor, why bother preserving the full gamut using a large color space like ProPhoto RGB or Adobe RGB (1998)? There are several reasons why this can be useful:

1. Devices are always going to improve. In the past, most monitors could only reproduce (roughly) the gamut of sRGB, which is why sRGB was defined in the first place. Today, monitors are available that cover almost the entire Adobe RGB (1998) color space. This development is sure to continue. Printer technology is improving too, and new ink sets that use 10 or more different colors further extend the reproducible color space.

2. While editing, you can selectively reduce or shift colors and also gain new colors by using various editing operations. It is therefore practical to start with a rich set of colors and a large color space and use 16-bit color depth for as long as possible.

ProPhoto RGB

Epson Premium Semigloss

ProPhoto RGB

Epson Premium Semigloss

Prophoto Rgb Ultrachrome Inks

Figure 3-11: A plot using LUV coordinates (the L axis is perpendicular to the plotted plane). The CIELuv color space is a perceptually uniform derivation of the standard CIE Yxy space. "Perceptually uniform" means that two colors that are equally distant in the color space (specified by the standard Cartesian distance function of the square root of color offsets) are also equally distant perceptually.

Figure 3-11: A plot using LUV coordinates (the L axis is perpendicular to the plotted plane). The CIELuv color space is a perceptually uniform derivation of the standard CIE Yxy space. "Perceptually uniform" means that two colors that are equally distant in the color space (specified by the standard Cartesian distance function of the square root of color offsets) are also equally distant perceptually.

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