The RGB Working Color Space Debate

THE GOODS COLOR MANAGEMENT BY ANDREW RODNEY The primarily difference between one RGB working space and another is the color gamut, the range of color and tone The RGB working space debate it can reproduce. “Color gamut” is a fine term for describing output devices, but not necessarily for input devices such as scanners and digital cameras. Scanners and digital cameras do not have a fixed A recent article on the ProPhoto RGB raw. It’s essentially a grayscale data file, color gamut boundary, per se, but what working space published on a respected proprietary in nature (a huge issue in can be described as a color mixing photo-centric Web site was clear and itself) that needs to be rendered and function. The filters used in both devices concise about the RGB working space encoded into a color space (see my do have limits on what they can capture, (http://www.luminous-landscape.com) column of February 2005). The important but the scene itself and the film or print Dozens of posts to other photo-centric point here is that a RAW data file can be image of the scene do have a fixed color Web forums, however, revealed confusion encoded into any RGB color space you wish. gamut and dynamic range. Say you photo- about this working space, bringing to light In the Web article, the author presented graph a gray card and then go photograph some persistent misconceptions. I’ll see if some good reasons for using ProPhoto a field of highly color-saturated flowers I can clarify a few of the important points. RGB, which I’ll go into later. What’s key with the same digital camera. The One common misconception among is that you, the photographer, have the camera’s sensor is capturing differing photographers is that their digital cameras option to let the camera produce RGB ranges of color data. The color gamut of produce either sRGB or Adobe RGB data from the RAW file in either sRGB the scene itself is different. (1998). These shooters do not recognize the or Adobe RGB (1998), or to handle it In considering the most appropriate role of the RAW data file their camera yourself using a RAW converter and a RGB working space to use when systems produce. A RAW file is just that— different RGB working space. converting RAW data, the scene will play a role in what you can first capture, then contain in the encoded file. Many Figure 1: Two-dimensional gamut plots of three Figure 2: The gamut plot of an Epson Photo Printer users say they see no difference when popular RGB working spaces. The entire 2400 on glossy paper in reference to Adobe RGB horseshoe-shaped plot represents all visible (1998). Notice how some of the yellows and a they shoot and automatically convert colors. Note that ProPhoto RGB describes good deal of the greens and blues fall outside the into sRGB versus Adobe RGB (1998), colors outside of human vision. plotted gamut of this working space. yet the scene does play a role. The color gamut of the output device plays a role as well. The color gamut of a display is pretty small—very close to sRGB (unless you can afford the few Adobe RGB (1998) size gamut displays hitting the market). Your printer may have a gamut that cannot take advantage of the gamut you captured from the scene. There’s a great deal of interplay between scene, working space and output space gamut. One of the best tools for examining the gamut of a scene and selecting a working space for all Photoshop users is xx • www.ppmag.com Figure 3: Adobe Camera Raw reflects a series of histograms of the GretagMacbeth ColorChecker as shot with a Canon EOS 300D. In sRGB, the first histogram, the saturated colors are clipped at both ends of the scale. Fewer colors are clipped in Adobe RGB (1998), but none in ProPhoto RGB, due to the size of its color gamut. A single color seen butted on either end indicates saturation clipping (white indicates clipping in all three channels). In the sRGB histogram, the magenta clip on the far left indicates that neither red nor blue colors are clipped, but green is, and on the other end, red is clipped. Adobe Camera Raw software. The hist- triangular shapes represent the gamut ogram in Camera RAW updates on the of the three RGB working spaces we’ve fly as you select different RGB working discussed. Because of how the pure spaces. You’re given only four to chose from, primary colors (red, green, blue) are three of which we’ve already discussed, plotted in RGB working spaces, they all sRGB, Adobe RGB (1998) and ProPhoto have this familiar simple shape. RGB. Figure 1 shows a two-dimensional Note that most output devices have gamut map of these color spaces. vastly different and dissimilar shapes, You can view gamut maps all over which presents a slight problem when the Web, so let’s take a quick look at what you want to fit them entirely within the they represent. The horseshoe-shaped simpler shapes of a working space. Figure outline represents the gamut of human 2 shows the two-dimensional shape of vision. If you plot the colors in this the plotted gamut of of my Epson Photo outline, they represent the limit of the Printer 2400 as compared to the Adobe colors we can see: the pure spectral RGB (1998) working space. Some of the colors of visible light. We cannot see any plotted gamut of this output device fall of the colors represented by the points outside the color space, which tells me outside of the horseshoe. The three that this working space can’t fully contain September 2005 • Professional Photographer • xx THE GOODS all the colors I might be able to print. all the colors of this scene for use later, I with 8-bit-per-channel image files. Eight- It’s not something to lie awake worrying should pick ProPhoto RGB. That’s not bit files have only 256 steps from black about, but to simply be aware of. to say that picking even sRGB will ruin to white in each color channel. The same Now let’s look at Adobe Camera Raw. the image, but it wouldn’t give me all of file in sRGB would still have only has 8 I took a photograph of a GretagMacbeth the colors I could have at my disposal. bits per color, but in the ProPhoto RGB ColorChecker with my Canon EOS-300D Yes, there’s a downside of using ProPhoto file, the bits have to be further apart. Look Digital Rebel. It’s hardly the most satu- RGB, and for that matter, all wide gamut at Figure 1 and imagine you have to rated image I could find and the color spaces. First, its gamut exceeds the gamut describe all the colors in the largest and gamut isn’t enormous, but it is colorful. of your display. You might have captured smallest triangles in just 256 steps. As In Figure 3, compare the histograms of all the colors in the scene, but you won’t you edit the file, the wide-gamut data can the image in sRGB, then Adobe RGB be able to see the most saturated colors exhibit posterization, or more accurately, (1998) and finally ProPhoto RGB. Neither on your display. This could cause problems aliasing. So, when working in wide-gamut the sRGB nor the Adobe RGB (1998) when you edit the image in Photoshop, working spaces and using a product like working space can fully contain all the because you couldn’t see the actual results Camera Raw, be sure to bring the data colors of this scene, captured with this of your corrections until you output the into Photoshop with 16 bits of data. camera, in this lighting. The sRGB file. The output device would have to Once the file is edited and ready for histogram shows clipping of colors on have a wider gamut than your display, or output, you can send the print driver an either side, but that’s not the case with else you wouldn’t see anything anyway. 8-bit file, which Photoshop can do for ProPhoto RGB. So with this set of Another drawback of editing in wide you automatically when you use the circumstances, if I want to fully contain color spaces is the limitations of working Print with Preview command. I xx • www.ppmag.com September 2005 • Professional Photographer • xx.

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