5.4. Raster Image Processors (RIPs)
RIP stands for Raster Image Processor; raster refers to a continuous tone bitmapped image, made up of dots like a photograph. RIPs are expensive and require a lot of time to set up, but that can be more than offset by the productivity gains and more accurate control of color. For the amateur photographer on a tight budget, a RIP may not be necessary, but for a professional photographer who is looking to produce accurate color on different types of nonstandard media or is producing a large volume of work, a RIP may be required.
For many people, using the standard Windows or Macintosh print driver works perfectly well. So why would we want to use a RIP? The two main advantages are productivity and control. For those who need to print PostScript files, a PostScript RIP is absolutely necessary.
First, let's discuss the productivity advantage. Productivity can be increased by letting a separate RIP print to the printer, freeing up the printer driver from having to process the file. In most cases, this does not speed up printing if the software RIP is on the same computer that you are printing from. The speed advantage comes when you put the RIP software on another computer or use a separate hardware RIP. This spares the local computer from processing and printing the file.
5.4.1. Hardware or Software RIPs?
RIPs come in two flavors, software and hardware. Software RIPs are installed on your own computer, while hardware RIPs are installed either in the printer, as for some of the HP printers, or are standalone, such as the Fiery RIP. Hardware RIPs have the advantage of being Plug and Play and work relatively well. The disadvantage of hardware RIPs is that they cannot be updated. Software RIPs, such as Onyx, have the advantage of being less expensive, having more controls, and being upgradeable.
5.4.2. The Right RIP for You?
There are a wide variety of RIPs on the market with different feature sets and price ranges. Currently, the majority of RIPs are on the PC platform, but there are numerous RIPs for the Mac also. Listed in the table at left are some of the major features of a RIP. Note that not every RIP has all these features.
First, you need to ask yourself, "Do I need a RIP at all?" As just mentioned, using a RIP is about productivity and control. My recommendation is to start with the normal print driver and see if it works for you. You may find that the color and productivity are adequate for your needs. But if you work with nonstandard media, want better color, or need greater productivity, consider using a RIP.
Let's look at the color issue. The manufacturers of ink-jet printers have done a fairly good job of getting decent color from their printers using their standard media and inks. With a good RIP, you can control color even better, and one may be necessary when you're using custom ink or media.
5.4.3. RIP and Printer Calibration
To obtain optimum color, first you need to calibrate the printer. Calibration of an ink-jet printer involves three major steps: ink restriction, tone distribution, and overall ink limiting.
188.8.131.52. Ink restriction
The first calibration step is to restrict the amount of ink being applied to the paper, a process that is conveniently called ink restriction. In extreme cases in which there is too much ink on the substrate, ink may drip and run. To avoid this, ink restrictions can be applied for each primary ink color (CMYK) separately. Some printers use multiple inks for each color, such as cyan and light cyan, and the ink can be restricted for both the dark and light ink. A general rule of thumb is the higher the print resolution, the more the ink needs to be cut back because the print heads are moving more slowly. On the other hand, if you decide to print at a lower resolution, less ink needs to be cut back because the heads are moving much more quickly. Another reason for ink restriction is to control the amount of hue error that results from too much ink being applied to specific media. For some combinations of printers and media, if you put down too much ink, you'll actually get color shifts. Cyan and magenta inks tend to shift the most, while yellow is relatively pure and may only shift a little. Too much black will tend to fill up the shadows.
So how much do we restrict the ink? If we limit the ink too much, then we limit the gamut of the device. If we don't limit enough, we get hue errors that result in compressed tones. These hue errors can be mostly corrected for with ICC profiles, but that method will not achieve optimum results, and that's the point here. With practice and experience, we can manually determine the proper ink restriction for a specific combination of ink and media.
Ink restriction can be done visually, but a better and more efficient way to determine ink restriction is by using a combination of software and hardware, where you can measure the ink with hardware and then use this data in software to assist you. One excellent software tool that can help you to accomplish this is Colorshop X from X-Rite, which allows you to visually plot the hues (colors) of your individual colorants (CMYK) and helps determine the maximum amount of ink you can apply and still minimize hue errors.
To the left are two examples of plotting the hue angles, one bad (top) and one good (bottom). Both figures show the three primary printing colorscyan, magenta, and yellowin 10 percent increments, from 100 percent on the outside to 0 percent on the inside. In the first figure, with no ink restrictions at all, the cyan and magenta show major color (hue) changes as the amount of ink is increased. The yellow and black do not show these hue shifts. In the figure on the bottom, the ink was restricted considerably. Notice that both the cyan and magenta show much less of a hue shift than they did with no restriction. Some hue shift is tolerable because it can be corrected in the ICC profile. If the ink is restricted too much, you will restrict the gamut of the device too much, and you will be unable to achieve a wide color gamut. If you are working with Pantone colors, for instance, it is important to not restrict the ink too much. Restricting ink requires a compromise between gamut and hue shift.
Also, notice the boxes going down the black line in both figures. This line represents the neutral axis and determines the neutrality of the grays. When the hues angles are distorted, it is difficult to achieve a good gray balance. But with good ink restriction, you are able to control the hue shifts and also achieve better neutrals.
184.108.40.206. Tone distribution
The second step of calibration is to distribute the tones so that you can see a full range of values from dark to light, which is called linearization or, as some call it, calibration. As we discussed in previous chapters, calibration is a term that is often misunderstood. Technically, calibration is bringing a device or output into a consistent and repeatable state, which often involves multiple steps. Calibration is done by printing a series of CMYK patches to the printer and then measuring the patches. The RIP software then distributes these tones so that you can see the whole tonal scale with even steps in-between for each color.
220.127.116.11. Ink limiting
The third and final calibration step is ink limiting, which is similar to ink restriction in that you are cutting back the ink. The difference with ink limiting is that you are cutting back combinations of inks. For instance, red is a secondary color, being a combination of magenta and yellow, and this combination can be limited separately. Green and blue are also secondary colors and can be limited independently of primary colors. In addition to these three secondary colors, you can also limit combinations of three and four inks. Some RIPs offer this feature, while other do not.
5.4.4. Creating an ICC Profile
Once you have achieved good calibration, creating the ICC profile is much easier. To create a profile of a device, use profiling software (such as ProfileMaker) to send the device a series of color patches. In Chapter 4, we covered the same thing, except now we're going to send a CMYK target instead of an RGB target. Remember that the standard print driver does the conversion from RGB to CMYK for the printer. Almost all RIPs allow you to control directly the CMYK inks of the printer. Read the patches with a spectrophotometer, and create a CMYK profile. In most profile software packages, you'll have a variety of different options to control how the profile is made.
Due to the nature of ink-jet printers, accurately controlling the generation of black is crucial to creating good profiles.If a printer has only one black ink and it is introduced too early, you'll get an effect called peppering, apparent in the light to middle tones. If you use GCR (Gray Component Replacement), black is introduced too early. UCR (Under Color Removal), on the other hand, introduces black later on. So using UCR is preferred when creating profiles for ink-jet printers with a single black. To reduce peppering, we don't want to introduce black until at least 50 percent. It may take some experimentation to find the optimum settings for creating a good profile.
As we covered in Chapter 4, the real test for checking the calibration and profile of a device is printing several standard test images. A variety of standard test images work well for this purpose. You can go to Eddie's website (www.eddietapp.com; select the E:Technique button) to download some different test images that are in the Adobe RGB (1998) color space. Be sure to use this color space as your embedded profile when printing one of these test images. If you have a good test image of your own, you can also use that. When printing a test image, be sure to print from the same application that you print from regularly, and be sure to use the proper calibration and profile you created previously. Because we normally print through the RIP, it would be the best place to apply the profile.
Evaluate the image after it is printed. Look for neutral grays, good shadow detail, natural flesh tones, smooth transitions in saturated colors, and so on. If you have created a good calibration and profile, the image should look very good. If the image does not look good, you may need to create a new calibration for your printer or create a new profile with different settings. If the image you printed looks good but just needs some minor adjustments, it would be best to go back and edit the profile slightly (see the earlier section in this chapter, "Profile Editing").
5.4.5. Productivity Controls with RIPs
Now that we've looked in detail at the color controls that are available in a good RIP, we're going to discuss some of the productivity advantages of using a RIP. One large advantage of using a RIP is speed, both for processing the files and printing. Offloading the printing to another computer frees up your own computer. Having the RIP on another computer allows you to share the printer over a network. When sharing a printer over a network, you can set up a variety of queues, each of which can be configured with different parameters, such as different media or sizes.
Another great feature of a RIP is nesting, which allows you to use paper more efficiently. Different RIPs may use different terminology for this feature. The RIP shown at left is the Onyx RIP. For example, say that you have a bunch of 8 x 10 images that you would like to print to a 36-inch printer. You can either manually place the images across the sheet to minimize waste, or you can set up nesting, which will automatically place the images for you, saving a lot of time and intervention.
Another feature that some RIPs have is the ability to simulate another device, which is useful if you're trying, for instance, to simulate another device, that has a smaller gamut than your printer. Using this feature requires a profile for both your printer and the device that you are trying to simulate.
RIPs also allow you to print Pantone and spot color accurately. The RIP offers a series of LAB numbers for each of the Pantone colors. These values are then converted using the printer profile so that the color is printed correctly. In order for the RIP to see these Pantone colors, they must be sent as a postcript file (PS, EPS, PDF); the Pantone colors come over as named colors, and the RIP picks up these names. The RIP will also allow you to create you own custom spot color, which is useful when you have a client who is trying to match a logo color.
In addition to automating the workflow, some RIPs allow you to edit images at the RIP workstation, including color correction of an image after it has been ripped and printed. This feature is used to fine-tune an image and saves time, but it should not be used to fix poor original images or bad profiles. "Step and repeat" is a great tool for printing multiple copies of the same image without having to place the same image over and over again. Image sizing and rotating are self-explanatory. Tiling is a great tool if you are trying to place an image that is larger than the media size and needs to be spread over multiple sheets or rolls. With tiling, you are also able to control things such as overlap and where the image will break.
As we've seen, whether you're using a software or hardware version, a RIP can assist you in better controlling your output and increasing your productivity. The marketplace offers many different RIPs. You can decide on one by assessing your needs, skill level, budget, and so on. At this point, you should be better able to determine if you need a RIP and, if so, what features you want.