In our last posting we introduced everyone to some basic color theory as it applies to print reproduction of digital files. Now you probably remember that CIE/LAB is the color value system that is utilized by all modern digital print devices. In computer parlance this feature is characterized as device independent. So, unless your printer was manufactured on Mars, CIE/LAB is the numerical value system which tells your printer what colors to spit out.
Now, let’s have a chat about output devices. The full range of color available to you often depends on your output device (ie…a desktop printer, a high-end inkjet printer or a 4-color offset press like a Hedelberg – you wish!). The range of your output device can be measured (or profiled) as a 3D model in LAB space (see InkjetVsSWOP png). Here we see the model of an 8-color solvent printer compared to the industry averages for a standard 4-color web operated press. The solvent inkjet printer resembles some kind of mutant meat-loaf while the web press looks more like a professional sized Frisbee. While the press looks like it has a greater range of pure hues, the solvent inkjet printer can print a much, much wider array of brights and darks. Of course there’s more to color then printers. Color space can also be affected by the selection of media. Obviously an uncoated bond paper which can not tolerate a heavy saturation of ink will limit the range of color you can image. On the other hand, a coated paper with a gloss finish will not only increase your color range but enhance your brights by having a high-contrast white point.
Now let’s jump back to your digital art. You like what you see (and for the sake of argument) you’re ready to print on a small desktop printer on your home system. Once you hit print, you’re likely presented with a series of options by either the program you’re printing from, or your operating system. Remember how we described LAB as an NBA basketball and your printer as a jelly-doughnut? In the print options, you’re being asked how you’d like that basketball compressed (converted) to fit in your mutant meatloaf or pro-sized Frisbee. These image conversion options are properly called “Render Intents” (see Render Intents png).
There are only 4 Render Intents: Absolute Colorimetric, Relative Colorimetric, Perceptual & Saturation. Each render intent performs the function of compression in a different manner.
If you select Absolute Colorimetry your printer will do it’s best to faithfully reproduce the colors in your digital file with no adjustment for differing gamuts. If all the colors in your file fit within your printer’s abilities your image might be a good match for what you see on your screen. However, that’s rarely the case. What will likely happen is colors that are outside the range of your printer will be clipped at the printers maximum range. In addition, color will not be adjusted to take into account the white point and color caste of your printer paper. If your paper is off-white with a yellow or reddish caste your image will not be adjusted. For the most part, Absolute is used for certain proofing devices to aid in the selection of stock and to check gamut compatibility.
Relative Colorimetry is similar to Absolute in that in-gamut colors are faithfully reproduced and out-of-gamut colors are clipped. However, the white point and color caste of stock are included in the calculations in order to better honor the colors selected in the digital art. Relative is the standard for offset print services and most large format providers because it provides a uniform and predictable working condition.
Perceptual intent is the intent most often utilized by photographic and fine-art imaging services. Under the Perceptual model of compression, the individual image is measured against the available gamut and reduced to fit the print environment. The white point and caste of the print stock is part of the image calculation too. Although in-gamut colors are shifted, the overall range of color is still represented. We recently used this method to reprint a vivid photo on a matte stock for a client. Even though the stock had a soft, light, yellow-gray caste to it; once the image was printed the light areas of the photo appeared white to the eye. No colors were clipped and the visual range of light to dark and bright to dull were well represented. End result, a happy client and repeat business.
Saturation is probably the least employed render intent. It is most useful when the digital art is made of solid color graphics like pie-charts and informational displays. Like Perceptual, the entire image is scaled to fit out-of-gamut colors within the available printer space. And since the whole image is scaled to fit the available range of color, in-gamut colors are shifted in a less than predictable manner.
And this is probably enough for you to think about for now. Congratulations on sticking it through to this point.
Next Installment: Those Pesky ICC Profiles!