Successful color management in PDF workflows

In the PDF workflow, successful color management was performed. Today we will introduce the use of Distiller to generate PDF files.

Distiller is the most reliable way to convert files from different applications into PDF format. For some situations, such as Quark Xpress 4.1 format files, only Distiller can convert to PDF files.

Distiller's Job Option dialog box controls some variables, such as font embedding, image resampling, and the preservation of some job features, such as overprints. The ability to read device-related or device-independent color PDF files generated from Postscript files is provided under the Color tab of the Job Options dialog box.

There are four main options under this tab:

1. Leave Color Unchanged. In the prepress environment, colors are written into Postscript files as device-specific values, that is, colors have been optimized for specific devices. This option instructs Distiller to create PDF output using device-dependent color spaces DeviceCMYK, DeviceRGB, and DeviceGray.

2. Tag Everything for Color Management (mark all content for color management). This option is a way to convert from application-generated Postscript to device-independent PDF files. The ICC profile selected from the "Assumed Profiles" pop-up menu will be used to mark Postscript Device colors. The only restriction is that a specific type of color (such as CMYK color) will be marked with the same profile. If the images from several generated CMYK color software (perhaps optimized for different printing conditions) are combined on one page, it is likely that different related profiles will be required. In this case, special tools such as Enfocus PitStop Inspector are required to post-process PDF files. We will introduce such third-party plug-ins later in this article.

3.Tag Only Images for Color Management (marks images only for color management). This option applies only those properties files selected for the image sample data, but otherwise has the same advantages and disadvantages as the Tag Everything option. The reason for the difference between images and other objects is the special handling of fonts and vector images in PDF files.

4. Convert All Color to sRGB (converts all colors to sRGB space). The fourth option is designed specifically for web applications of PDF files, but sometimes it is also a viable option for printing applications. As mentioned above, the device-dependent color values ​​in Postscript are marked by a hypothetical profile, and then the profile is used in color management conversion to achieve the conversion to the sRGB color space. The output file contains the PDF's ICCbased color space color value, which is marked by the sRGB profile. These colors are device-independent (possibly slightly bounded by the color gamut) and can therefore be used primarily in networks and printouts.

The following is an example of using Distiller to generate a PDF file from a common desktop package.

Quark Xpress. Suppose the user has converted the scan file into a CMYK file. This process may use a scanner-specific color computer, but it is more likely to use Photoshop, and a profile has been created for the printer used by the user (this example is called " Press1.icc”). When using the PDF filter to generate Postscript, a device-related Postscript file is generated and the optimal color can only be reproduced on the printer owned by the user. If you want to extract a PDF file from this type of Postscript file, the software will specify the color management tag and use "Press1.icc" as the default CMYK profile.

What will happen? Distiller extracts the DeviceCMYK value and converts it to the ICCbased color space. Distiller then assigns the "Press1.icc" profile to define these values. The current PDF file is device-independent. When using Acrobat 5 to print a PDF file, users with different printers (printers) must specify the output profile that matches the device. These processes can all be achieved by clicking on the "Advanced" button in the drop-down menu in the AdobePS Print dialog box in Acrobat 5.0.
The next step depends on the selected profile. There are three possibilities:

1. If you have selected a profile for an output device such as a local inkjet printer, Acrobat will perform color management of all ICCbased colors in the PDF and convert it to the color space of the selected profile. This is very useful in the proofing workflow.

2. If the "Printer/Postscript Color Management" profile is selected, ICCbased colors are sent to the printer as Postscript device-independent colors, and the ICCbased color space in the PDF is defined using the CSA generated by the profile. This approach is very useful in a defined Postscript color management workflow.

3. If the "Same as source" setting is selected, the ICCbased value is sent to the printer as a device-specific color. Note that if you want to send the PDF file back to the original press, the original color values ​​will be copied without any color management.

There is an alternative solution for setting up workflows for pre-editing page objects: Use the XPress software's color management feature to convert images, transform images from a variety of different RGB and CMYK spaces into a printer profile for printing. Then, all DeviceCMYK colors in the output Postscript file are again converted into the color space defined by the current group printer profile, and marked in the PDF by ICCbased, thereby producing a device-independent PDF manager.

FreeHand. Like Xpress, you can place CMYK charts that have been optimally converted by the press in FreeHand, and then use appropriate property files for tagging when extracting device-independent PDF files. In addition, FreeHand's color management can also convert objects to a single CMYK color space when storing Postscript files. In this case, the characteristic file marked in Distiller will undoubtedly select the characteristic file for printing in FreeHand.

Office application software. In the software of Excel, PowerPoint, and Word, the colors of the students have common characteristics. First, regardless of the color space used in the color picker, these colors are written to the Postscript data stream in the DeviceRGB format. Therefore, using RGB percentages to define colors in these applications is obviously the best choice. Second, almost all three applications use a common color coding method; if all objects are assumed to use the correct profile, a series of tones created in each application will exhibit the same color appearance in the PDF. .

What is the correct profile for assigning RGB values ​​in office applications? Setting up Distiller with the correct profile and performance intent usually requires some experience. Experiments have shown that marking these colors with sRGB can cause color shifts; it is as if it were not defined in sRGB within the application software. The configuration that minimizes the color shift is flagged in the Adobe RGB (1998) profile.

Use Distiller as a reference. If you do not take into account Distiller's enormous effect on the Postscript interpreter, then any discussion of Distiller is incomplete. Postscript files that do not print the desired color effect can be reliably processed by using the "Leave Colors Unchanged" setting. Which device color should be used in the PDF file? If these color spaces or values ​​produce false or poor results, how do you combine the selection of these tag profiles to achieve a better look? And if different page objects need different profiles to get the best results, how will they be handled?

In order to solve the problem, first carefully observe Distiller's processing results, confirm that the problematic document has been diagnosed, and confirm that special PDF printing needs, such as color separation, are beyond the scope of Acrobat or Distiller. Therefore, they need a third-party solution.

Source: Henan Graphic Arts