What is a Slice?

Have you ever viewed an Acrobat file that appeared to have several thin white outlined boxes in it? What you are seeing is an artifact (aka…product) of saving as or exporting to PDF from a software application. Fair warning: this is a complicated concept, so get ready for a lot of pictures with this posting.

Just for a starting point, take a look at the “Banner Art” screenshot above. The art was created in Illustrator and involves a photographic background overlayed by several vector objects with effects, drop shadows, and a white glow. Just to get a better idea of the structure of the art, let’s take a look at the art in Illustrator’s outline view (shortcut: command/control y) – see “Banner Art Outlines” screenshot below.

Now, let’s cut this up into slices. First, we’ll save as a PDF. And just to simplify matters, let’s save as a specific flavor of PDF, the PDF/X-1a:2001. (See “Save As PDF” screenshot below.)

Now, let’s open the PDF file in Illustrator. (See “Banner Art PDF” screenshot below.) You probably noticed several white lines in the art. How odd; they weren’t there before.

What you are seeing is a collection of image slices. When you save as or export to PDF, raster objects are created where transparent and semi-transparent objects and effects like drop shadows and glows overlay another object. This process is sometimes called stitching. Now, take a look at the “Banner Art PDF Outlines” screenshot below. This is what the file looks like in Illustrator’s outline view (shortcut: command/control y).

Now, why would Illustrator, Photoshop, Corel Draw, Quark, or InDesign, for that matter, do this? Image slices have a purpose. That purpose is to create a common, standardized file for every variety of computer application called a RIP (raster image processor). This standardized strategy enables RIPs to print to a wide variety of devices such as offset printing presses, plate-makers, large format printers, computer-driven photo images, and desktop printers.

You may have noticed the PDF/X1a is a relatively old standard, circa 2001. It may be old, but it’s still an enforced standard by the magazine and 4-color print industry. The reason for the longevity of the X/1a standard is some RIPs cannot interpret files which include transparent objects and effects. Before sending Acrobat files to your printer, it’s always a good idea to verify your save as or export settings.

One last word on slices before we close. The PDF process is automated and will produce redundant objects called clipping paths. A clipping path defines the visible boundary of an image slice whether the slice is vector or raster. Sometimes these redundant paths are out of registration or misaligned. This is called a slicing error. We always recommend checking your PDFs in Acrobat with the zoom tool (shortcut: command/control spacebar). If the slice lines disappear or remain as a single-pixel hairline when you zoom in, it is likely they will not print. If the white line grows as you zoom in, then you should either make a new PDF or try to edit your PDF with an application like Photostop or Illustrator.

Diving Deep in Image Management

Our workflow begins with client supplied images and proceeds along separate parallel tracks : one for CMYK and one for RGB. For the sake of argument, let’s assume you have several unaltered digital photos to manage. And, just to narrow the focus a little more, let’s confine the purpose of this entry to print production, not web or internet. Therefore, the beginning image management section of the RGB workflow is the appropriate place to start.

As you may have noticed, most RGB images have an embedded profile of sRGB IEC61966-2.1. Unfortunately, this is not a trustworthy production color space. It’s fine for web, but if you’re going to design for print you should convert all your images to Adobe RGB (1998). Why? Some RIPs have an out-dated version of this profile and will image poorly if you print directly from sRGB IEC61966-2.1. Furthermore, many higher-end digital cameras have a color space that is described as sRGB IEC61966-2.1 (it’s what shows up in the profile information in Photoshop) but is actually larger and closer too Adobe RGB (1998). Predictable color is dependent on having a common reference for conversion and output. Some service providers recommend the profile sRGB, so far, we have more predictable and reliable results with Adobe RGB (1998).

To convert your images in Photoshop, go to the top menu bar and select Edit > Convert to Profile. Make sure to check the “Use Black Point Compensation” and “Use Dither” boxes in the “Conversion Options” of the dialog box. This will mitigate any posterizing of your image. And finally, don’t hit “Okay” without clicking the preview button a few times. There is one caveat concerning “Black Point Compensation”; if your image has little to no shadow content you might want to uncheck this option. That’s what the Preview option is for. Preview is your chance to see if you are going to have any issues with your color conversion. Finally, save your converted files as new Photoshop files. You never know when you might want to go back to your original customer provided files. Sometimes the client needs them because they’ve lost track of their original images. Occasionally, there’s a concern over the appearance of the printed piece and your client will want to examine your work against what was provided. CYA, conserve your assets.

You can use Photoshop to convert all your files as a batch action. Photoshop batch processing, actions and droplets can save you a lot of time. When we employ these features, we always open the new files and compare the appearances to the original files. A blind process does not always yield the best results. We’ll talk more about the dangers of blind processing when we cover PDF creation.

Now that all your images are in a common color profile perform your edits (clone brush retouch, color range adjustments, addition of other elements, sizing and cropping). You’ll want to save these as multilayer Photoshop files. Do your alterations in a non-destructive manner with layer copies, masks and adjustment layers. You never know when you’ll have to go back and tweak something for a current or future project. Once all edits have been finished, save copies of your files as flattened TIF’s. DO NOT save over or discard your multilayer Photoshop files. That is the digital equivalent of painting yourself into a corner. Work that way and one of these days you’ll be sorry.

You may have heard about a loss-less version of jpeg called JPEG 2000. The problem with that format is not every RIP can process it. It’s no fun getting a call late at night from a disgruntled printer and having to perform an emergency fix. Don’t gamble if you can help it.

Now that all your images have been edited and saved as flattened tiffs, it’s time to pick which side of the street you’re going to drive on. If you’re dealing with art that must be printed on a conventional 4 to 6 color press it’s time to change over to the CMYK workflow. And we hope you have properly calibrated your monitor. The shift to CMYK will need to be managed. Save your CMYK art as a new file, don’t overwrite your RGB file. You may need it at some future date. BPI always adds the color space and profile to our file names (for example, FileName_SWOPv2, FileName_Gracol06, FileName_PrinterNameG7, etc…).

Now, you’re probably wondering what an appropriate CMYK profile is. In the United States, US Web Coated SWOP v2 is almost a default. However, a number of printers and large format providers have moved to Coated Gracol 2006. Some print providers have a custom G7 profile and can provide you with one to install in Photoshop. It’s always a good idea to call and ask.

Finally, let’s spend a little time talking about color conversion. Whether your converting from RGB to CMYK or CMYK to RGB you should treat conversion as a one-way street. Please take another look at the workflow diagram. Notice the arrow from RGB to CMYK. RGB is larger in color space then any CMYK profile in existence. BPI usually, about 90% of the time, has to perform color adjustments on a CMYK image to match (as close as possible) the appearance of the RGB counterpart. And, we do our CMYK color adjustments in a non-destructive manner with adjustment layers and masks. Once those edits are complete, we save a flattened tiff copy of the CMYK multi-layer Photoshop file.

You are now ready to construct your art.