Color Output

By Gary Armstrong, Planet PDF Contributing Editor
Vice President - Sales, ARTS PDF

Back to PDF Color Learning Center

Some might say that PostScript 3 RIPs can consume PDFs directly. This ignores the fact that many PostScript 3 RIPs cannot, and that there is still a large installed base of PostScript Level 2 RIPs. Additionally, even though it is technically feasible to interpret and RIP PDFs directly, it is rarely done. The RIPs must also support PostScript for all the traditional workflows. After all, they are still called PostScript RIPs. So most often, even in a PostScript 3 RIP, PDFs are converted to PostScript and then interpreted. In reality, it is not important if the PDF is consumed natively or not , as long as you get the desired output and any file conversion is rapid.

Overprint is another color output issue. The issue arises because the implementations have varied widely among the various PostScript levels. Level 1 did not support in-RIP separations, so all overprints were initiated in a pre-separated workflow. Level 2 was the first to support in-RIP separations. In this implementation you could overprint spot on spot, spot on process, process on spot, but not process color on process color. Postscript 3 supports all of the above mentioned modes plus overprinting on composite devices with separate colorants, enabling some interesting proofing possibilities. Adobe cautions that overprint implementations are device dependent. How this behaves in your workflow with your devices, you will have to determine for yourself.

Adobe realized that PostScript will be used to print PDFs rich in graphic arts content. PostScript already has a mechanism for taking common device independent color coordinates and converting them to device color coordinates, that is, into the CMYK color gamut. It is called a Color Rendering Dictionary (CRD) and its inclusion in PostScript pre-dates ICC. Each PostScript RIP has default CRDs for its output device. So, to a first approximation, colors on a proof will be consistent with final output.

Ideally, you should have different CRDs for different combinations of paper, ink, and printing technology (ink jet vs. lithography vs. flexo, etc.). Theoretically, this is possible with PostScript and would give you the most consistent color reproduction regardless of final printing conditions. Adobe provides some help here. But not all applications take advantage of it. The printer drivers (see table below) can convert ICC printer profiles to CRDs by rendering intent, include them in the PostScript file and download them to the printer to be used for the current job. So, indirectly, you could adjust CRDs by adjusting ICC profiles with a tool designed for editing/creating profiles.

ICC Profile to CRD Initial Support

ICC System
Adobe Driver
Non-Adobe Driver
AdobePS 8
Apple LaserWriter 8
Windows 95/98
Microsoft PScript
Windows NT4
Windows 2000
Microsoft PScript

As stated previously, ICC and PostScript recognize four rendering intents. In principle, there should be a CRD for each, but not all RIPs have all four CRDs. Some map the rendering intents into as few CRDs as one. You will want to pay attention to this if you are using device independent color and render intents are important to you.

Below is a diagram that shows how data defined in different color spaces traverse different paths as it is being processed in a PostScript RIP.

Copyright, Adobe Systems Incorporated

When is a color space not THE color space? Answer: when there is a "Default" color space defined in the RIP and UseCIEColor = true (see diagram). So, for example, the file calls for DeviceRGB, but a DefaultRGB is defined in the RIP that could be a CIEBasedABC. This approach might also exist for DefaultGray and DefaultCMYK. This set of possibilities is included in the PDF/X-1 specification - a standard for using PDF for digital distribution of ads for magazines. Yes, this default color space can affect your output because it alters the path through which color data is processed in the RIP. However, this slight of hand does not alter any of the other basic statements made in this Learning Center. Even though a "Default color space" is a technique for treating virtually all data in a ICC-based fashion, it has the disadvantage that it imposes an additional conversion and applies CRD processing to both the device independent and device dependent color coordinates, which may or may not be appropriate. Also, it can’t be counted on as an approach that will exist consistently at all sites or on all printing devices.

From the chart you will notice that CIE-based data still passes through several device color processing steps before marks are placed on media. Indeed, all data is processed through these last few steps. This is the place where maximum control can be exerted in a consistent fashion. This diagram should also make us reconsider what it means to calibrate a printing system. There are three main data paths for color data through the RIP - CMYK, RGB, and CIE (ICC). A color management system that claims to have an ICC output profile for your print conditions may not be addressing all three data paths. And typical "press fingerprinting" exercises only address the CMYK path. If you work exclusively in CMYK, then you only need to calibrate that path. But it is not uncommon to have two or all three of the basic color spaces together in the same document.

CMYK is the common data path for all PDF color data destined for print. Therefore, in all cases, this path should be calibrated first. Through the use of transfer functions and a test PDF document with known CMYK gradient percentages, this step should compensate for dot gain for the target paper, ink, and printing technology for the selected line screen frequency, spot function, and resolution. But also include 2 color overprints and 3 component gray scales to test color balance.

Independently, and assuming that they will be used, both the RGB and CIE data paths should be calibrated. The same PDF test files may be used, but specified in DeviceRGB and CalRGB or Lab, respectively. The RGB path can be used to set or verify the in-RIP UCR/black generation path to adjust how gray values are distributed between K and CMY. The effect on RGB text and images will be of greatest interest. The objective is to make sure your conversion mechanisms give you expected CMYK values. If interested, one could also generate a test file in CalRGB or Lab with the entire range of color values, which could be used to verify gamut mapping for each rendering intent.

The primary decision for you to make is whether to institute a CMYK or a color managed workflow. To the extent possible, life will be a lot easier if you can stick to one or the other 100% of the time. This is much easier if the original files are created internally. But if, you have multiple external sources for input files, then I recommend that you preflight them and convert them to your internal standards immediately. A color managed ICC workflow holds the promise of more reliable reproducible color, even at remote sites. A CMYK workflow is easier to manage and for your employees to understand and diagnose problems.

You can get excellent color out of the PDF workflows described in this Learning Center. You can have everything from duotones to spot colors. You can repurpose documents from print to web. The good news is that you have lots of choices.

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