Editor’s Note: This article is part of the PDF Color Learning Center
For any document that may be professionally printed, from your creation application you should embed all fonts and either create a PostScript file and process it through Acrobat Distiller, or use "Export to PDF" properly configured, which performs virtually the same function. You should always use the Distiller PPD for this process.
Does the PostScript level make any difference when preparing a file for distilling? Yes, it does. As you can see from the PDF & PostScript Color spaces table above, certain applications, such as duotones or spot color gradients can only be satisfied by PostScript 3 and PDF 1.3 or higher. In general, for maximum application capabilities, you want to create PDFs in the latest version. Naturally, you also need to have the latest version of Acrobat to go with it.
Much has been written elsewhere about various Distiller color parameters but it doesn’t hurt to review a few points. You should start with the "Press Quality" settings. With a few exceptions, Adobe has made pretty good decisions overall and you can make new settings to meet your needs.
Adobe Acrobat Distiller Images Settings Panel
First, a look at image compression. For color and grayscale images Adobe has specified "Automatic" compression at maximum quality, which usually equates to minimum compression and larger file size. The automatic compression chooses JPEG. JPEG is a "lossy" compression which means that a round trip gives you different data from that with which you started, but they look equivalent. If the data is 3 component, such as RGB, it also is transformed to separate luminance from chrominance. Adobe is trying to do the right thing, i.e., preserve the black channel. But there are no guarantees that transformation, compression, and decompression will be 100% successful. To avoid this transformation, change the selection to "ZIP". If greater compression is more important, select "JPEG" and a different quality level.
Adobe Acrobat Distiller Color Settings Panel
Press Quality specifies "Leave Color Unchanged" for Distiller’s PDF color creation. This is the clear choice if you are trying for a traditional CMYK workflow. The other options are variants of a color-managed workflow, and allow you to select ICC color profiles. Of these, I prefer "Tag Only Images for Color Management". With this setting text and graphics, which usually have precise color definitions anyway, will not be altered. Also, this selection allows black text to remain defined as CMYK black.
Adobe Acrobat Distiller Color Settings Panel
The default profile selection, "Dot Gain 20%", is Adobe’s worst idea on this panel. This choice adjusts only one type of data for a case that may or may not apply. The good news is that it is limited to grayscale images. The problem with this will become more obvious as we discuss transfer functions below.
The other color options address UCR/BG, transfer functions, and halftone functions. "Preserve under color removal and black generation" provides instructions for a PostScript RIP to convert RGB to CMYK with varying degrees of rich black. This is really a user preference.
In Acrobat 4 and 5 Adobe’s worst choice on this panel was selecting "preserve transfer functions" as the default. Reverting back to the good old days of Acrobat 3, the right choice, apply transfer functions, has reappeared as the default in Acrobat 6.
Why are transfer functions there anyway? The best I can determine is that they are often there to compensate for the fact that Photoshop CMYK JPEG files are the inverse of normal CMYK JPEG files. The simple workaround is to use TIFF format instead. Other cases seem to center around PageMaker trying to hide undesired elements. Those can simply be deleted.
There is no disagreement about "Preserve Halftone Information". Generally, this should be "Off" since it can be specified consistently for the entire job when going to the RIP. Having all the same halftoning will minimize moiré on the press. You might turn this option on if you want to preserve halftone information when reproducing special effects for single page output.
Distiller’s "Advanced" panel has one other parameter related to color, that is, "Preserve OPI Comments". In earlier versions of Distiller Adobe’s choice for this was "On" which has the side effect that images are created in PDF as Form XObjects rather than the simpler Image XObjects. Some Acrobat plug-ins have difficulties interpreting these more complicated objects. So if you don’t use an OPI workflow, leave this selection "Off". Conversely, if you use an OPI workflow, remember that only the low resolution preview is placed in the PDF. Any color operations performed in Acrobat or by plug-ins are not performed on the high-resolution file that is actually printed.
There is one other ambiguous aspect to OPI. With color management, you are "tagging" the images. But if you are also using OPI, is your tag for the preview only? Or will it also apply to the substitute high-resolution image? In PostScript, often the color space definition is within the "image object" but before the image stream. If your OPI server replaces just the image stream, then the profile will apply to the high-resolution data as well. But if the OPI server replaces the entire "image object", then the profile may be lost on substitution.
Colorized bit maps or "fake duotones" is another special case. Users will place a grayscale TIFF image into Quark XPress, apply a color to the placed image, and set the background of the picture box to a different color. On screen, the color background shows through the colored image. But for composite printing workflows, the image knocks out the background and spot colors are often converted to process. Agfa has developed a free Quark extension, "AgfaCTIFF", to "fix" colorizing images in this way. The extension is available at http://www.certitec.com/downloads.htm. After Distilling, the PDF contains DeviceN color space commands.
Occasionally, one wants to create a two-color document. Particularly with Microsoft applications on the PC, one selects from their color palette. Many people incorrectly assume that those are spot colors and are specifying a spot color and black. Not so! The palette colors are RGB. So they end up with data on black plus at least two other plates. If available, use Enfocus PitStop Professional (see below) to globally convert the second color to spot. Another approach is to use colors in the creation application that are close to ink primaries, or as Microsoft calls them, pink or turquoise for magenta or cyan, respectively. You can use any ink you like for the second color when you run it on a press.