Out of Gamut: Proofing with inkjet printers
In a previous column (Out of Gamut: Color Management Made Stupid) I described the various pitfalls associated with printing accurate color out of Photoshop on low-cost inkjet printers. What I explained was how to get the most accurate match possible to your original RGB image. But what if you use CMYK images? In fact, you can also use printers of this class as proofing devices for final CMYK images since they generally have a wider gamut – range of color -- than offset presses.
To make an inkjet printer simulate a press (or a proofing system such as Fuji ColorArtor Imation Matchprint) you need to jump through an entirely different set of hoops than you would if you were simply trying to get the printer to print a source image as accurately as possible. The workflow is a bit more complex, too. I don't suggest using an inkjet printer as a substitute for a conventional contract proof, even if you can find a printer who would agree to do so, but you can certainly use these printers to cut down on multiple proofing and editing cycles, and to get a pretty accurate idea of the appearance of the final printed piece.
The following instructions apply to printing from Photoshop to QuickDraw (Mac) or GDI (Windows) printers-those that expect RGB input. They don't apply to inkjet printers that use a PostScript RIP to process the data.
The basic theory is straightforward. You can make any device that has a wider gamut than the final printing conditions simulate those printing conditions by applying the right color conversions. Essentially, all you need is a profile for your final CMYK output (this could be a press profile created by your press operator, a profile for a proofing system such as Matchprint or ColorArt, or even a Photoshop Built-in CMYK Setup), and a profile for your inkjet printer. The generic profiles supplied by inkjet vendors such as Epson and Hewlett-Packard work quite well as long as you use the OEM inks and paper for which the profiles were made. But if you have a custom profile, so much the better. You create your final CMYK file, then convert from the final CMYK profile to your inkjet printer's profile, and print the result on the inkjet. If you press the right buttons in the right order, your inkjet print should simulate the final output quite closely.
In theory, page layout applications such as QuarkXPress 4.0x, Adobe PageMaker 6.5, and Adobe InDesign 1.0 will let you proof to a composite color printer, but in practice this often simply doesn't work. If you need to proof composed pages, I suggest printing to disk from your page layout application, then rasterizing the PostScript file to final CMYK in Photoshop and using one of the approaches covered here to make the proof.
The first step in making your proof is to create the final CMYK file. If you have a profile for your press, or, if you're want to use a proofing system like Matchprint as your aim point, you should load that profile in Photoshop's CMYK Setup (see Figure 1). In this dialog box, you also have to set Black Point Compensation and choose a Rendering Intent. Since understanding and controlling rendering intents is critical to making decent proofs, a little explanation is in order.
Loitering with intent to render
When you use ICC profiles to convert color from one device's profile to another's, the color engine always uses a "Rendering Intent," a set of instructions that tell it how to handle out-of-gamut colors-those colors present in the source color space that the destination device cannot reproduce. There's no "right" way to do this, because the output device simply can't make these colors, so you have to choose some other colors that it can make, or, "you can't get there from here, so you have to go somewhere else.
The ICC profile format specifies four different methods of handling out-of-gamut colors-four different "somewhere elses" to go-that in ICC-speak are called Rendering Intents. (Various applications call them different things, but that's a rant for another day.) The official names for these four Rendering Intents are: Perceptual, Absolute Colorimetric, Relative Colorimetric, and Saturation. Here's what they do.
Perceptual Rendering (sometimes called Image, or Photographic) tries to compress the gamut of the source profile's space into the gamut of the destination profile's space in such a way that overall color relationships are preserved, even though all the colors in the image may be changed in the process. Usually, preserving the color relationships preserves an image's overall appearance, so use perceptual rendering on photographic images when going from a large source gamut to a smaller destination gamut.
Absolute Colorimetric Rendering reproduces those colors the target device can reproduce exactly, and clips the non-reproducible colors to their nearest reproducible hue, sacrificing lightness and saturation. Typically this rendering intent is used for reproducing spot colors, and sometimes for proofing, when you want to simulate the color of the paper on the proof.
Relative Colorimetric Rendering works the same way as Absolute, except that it scales the white of the source to the white of the target. Use this rendering intent for proofing when simulating the paper color isn't critical.
Saturation Rendering maps fully saturated colors in the source to fully-saturated colors in the destination, even if the hues change. It's really only useful for business graphics, such as when we want slices of a pie chart to be bright red, blue, yellow and green, without being particularly concerned about the specific hues of red, blue, yellow, and green.
Black Point Compensation is a Photoshop-specific setting that seeks to address a hole in the ICC profile specification. When it's turned on, Photoshop makes sure that black in the source is mapped to black in the target. When it's off, Photoshop tries to map black in the source to its actual value in the target space. That means that if the source black is lighter than the darkest black the target device can reproduce, the resulting image will contain no true blacks, and may appear washed out. If you're using an inkjet printer to proof newsprint separations, for example, you'll probably want to leave black point compensation turned off so that you get an honest idea of the relatively weak blacks newsprint makes.
When creating CMYK separations from an RGB file, I generally want to use Perceptual Rendering to preserve the overall appearance of the image, and keep black point compensation on to make sure that we use the entire dynamic range of the target device. So to create your final CMYK separations, you want to set CMYK Setup to the profile for your final output, with Perceptual Rendering, and Black Point compensation on, as in Figure 2.
Producing the proof
But when it comes to proofing the CMYK file, different rules apply. To view the CMYK separation accurately on screen, you need to set CMYK Setup to Relative or Absolute Colorimetric (Relative if you don't want to simulate paper white on screen, Absolute if you do), with Black Point Compensation turned off, as in Figure 3. (You can make a Photoshop Action to switch quickly between the two settings.)
To make the inkjet printer simulate the CMYK file, you need to create a new image that will be sent to the printer. I do this by duplicating the final CMYK file, then using Photoshop's Profile-to-Profile command to convert it from CMYK Color to the inkjet printer's RGB profile. If the final paper stock is very different in color from our inkjet paper, we use Absolute Colorimetric rendering, which makes the inkjet lay down ink in the white areas to match the paper stock (see Figure 4).
If the paper stocks match fairly closely, we use Relative Colorimetric rendering instead-our eyes can adapt easily to small differences in white point (see Figure 5). In both cases, leave Black Point Compensation off, because you want to see the black you'll get on-press, not the black of the inkjet printer, which is often darker.
When you've done the profile-to-profile conversion, you end up with an RGB file that looks strange on screen, but when printed to the printer with no further color adjustments provides an accurate simulation of our final press output. You must make sure that all color adjustments are turned off in the printer driver. Set the "Space" pop-up menu to RGB color to prevent Photoshop from altering the color, and turn off any printer color matching features (see Figure 6.)
Be warned that the proof may look worse than you're used to seeing from your inkjet, particularly if you're proofing newsprint or another such low dynamic range, small-gamut output process. But it's almost certainly an honest reflection of the way the final manufactured piece will appear.
Bruce Fraser is a self-confessed color geek and co-author of Real World Photoshop 5 (Peachpit Press). He can be reached at firstname.lastname@example.org. "Out of Gamut" appears monthly on creativepro.com.
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