Does painless RGB to CMYK conversion exist?

Jun 1, 2005 12:00 AM, By Julie Shaffer

Not without color management!

Prepress

I recently told a friend I was writing an article about painless RGB to CMYK conversion. "A whole article?" he frowned. "Can’t you cover that in one sentence?"

Now, my friend is a designer with years of prepress experience, so I was somewhat taken aback by his question. Nonetheless, I thought I knew what he meant. "Is ‘Use Photoshop to change the color mode from RGB to CMYK’ the sentence you had in mind?" I asked.

"Well, yes," he said. "I mean, that’s pretty easy." The key word here is "painless." For our purposes, let’s define painless RGB to CMYK conversion as "changing the color space of an image in a way that honors the original creator’s intent (within the limits of the printing process) and doing so in as automated a fashion as possible."

My friend confessed that his customers were sometimes so unhappy with the color resulting from his "easy" conversion method that he often had to correct the images. As for process automation, he admitted his company had none—other than allowing RGB images to convert to CMYK in its RIP. Because the RIP isn’t set up for color management, this method frequently results in a poor color conversion, and, in some instances, a free second press run. Many printers have a better method for dealing with supplied RGB color, but my friend’s technique is not unique and I would certainly not call it painless!

Digital cameras spawn more RGB files
Dealing with supplied RGB images is a relatively recent phenomenon for commercial printers, coming on the heels of digital photography’s growing acceptance. Digital cameras and scanners capture images through red, green and blue (RGB) filters, producing RGB images. Of course, high-end CMYK drum scanners (now mostly a thing of the past) capture color in the RGB color space as well, but the the scanner’s computer handles the conversion to CMYK automatically. Consequently, longtime time prepress experts are used to working with CMYK images, and many printing companies still prefer to receive CMYK files from their clients, perhaps believing this absolves them from responsibility for the potentially "bad" color that might result from performing the conversion in-house.

As the growing use of images captured from digital cameras or designer-driven scanners has all but eliminated scanning as a prepress function, printers will be getting more and more incoming RGB image files. It is critical to determine how to successfully convert these RGB files into the cyan, magenta, yellow and black colors that will be used to print them.

Color management is absolutely essential. While color reproduction has always been "managed" for print production, color management today is almost purely electronic. Simply put, every device in the workflow that touches color—computers, monitors, digital cameras, scanners, proofers and printers—captures, displays or outputs color a little differently. Color management takes into account the different capabilities of image capture devices, display devices and output devices, and helps ensure the color displayed on one computer monitor can closely match another, or that a proof from an inkjet printer approximates what will be produced on a press.

Profiles—descriptions of each device in the workflow’s color capabilities—are the key to good color management. Profiles can be proprietary, but acceptance of ICC profiles, based on the specifications put forth by the International Color Consortium, is nearly universal. By combining source profiles with destination profiles, input color can be transformed via rendering intents to the proper color on output. The mapping of these profiles is performed by a color management/matching module (CMM). CMMs exist in many places in the production workflow, including image editing and page layout software, the computer operating system, printer drivers and RIPs.

RGB ‘s big three
In a print production workflow, there are three primary places where RGB data can be converted to CMYK:

At an individual desktop computer.
With a server-based system.
In the RIP.

There also is at least one color conversion option available online at www.colorcentric.com, an ASP-style service that offers RGB to CMYK conversion of individual images, accomplished by color experts on a pay-per-conversion basis. The Colorcentric.com system includes a unique compression method that allows very large images to be color corrected via a Web connection without a long upload/download time.

At the personal computer level, Adobe Photoshop reigns as the supreme image-editing tool used universally by photographers, designers and prepress providers. Like my friend, the majority of these users employ Photoshop to make color space conversions. Using consistent color settings and the right profiles, Photoshop can convert images from RGB to CMYK successfully. (The process can even be automated through batch processing and scripting options.) Photoshop, however, is not the only option for desktop-level color conversion. You can use an image editing application, such as the Windows-based Picture Window Pro offered by Digital Light & Color (Cambridge, MA) or Binuscan’s (Hartsdale, NY) CMYK+. Scanner interface software (Binuscan ColorPro, LaserSoft Imaging’s SilverFast, Creo oXYgen, or Heidelberg’s LinoColor) naturally allow for conversion to CMYK, much like the drum scanners of old. The latest version of Adobe’s Acrobat Distiller includes an option to convert color to CMYK. And the Mac operating system’s "Save as PDF" allows savvy users to create ColorSync filters to be used to convert images, text or vector artwork to CMYK using ICC profiles in the process of creating a PDF file.