Optimized File Delivery

Delivering repro-ready RGB or CMYK image files

Delivering finished RGB or CMYK image files provides the greatest degree of control over how the images will display or be reproduced. Delivering completely finished files requires a high level of communication of precise instructions between provider (you) and receiver.

This process usually involves:

• flattening the master file – no layers, channels or paths
• converting it to the delivery color profile and file format
• almost always 8-bit depth

The image receiver may need images for a variety of uses, including offset reproduction, the web, digital projection, or for prints. When a file will be reproduced in multiple mediums, steps must be taken to ensure the file matches the specs of the different media. The first step for delivery of finished files is to nail down the exact media requirements. Hopefully, this occurred during the planning step – collect as much information as possible about the deliverables or file requirements from the service provider.

The minimum information required to deliver finished files is:

• the reproduction size
• the media the file will be output to – ie print, web, projection, digital prints
• if it is for print, the type of screening, line screen and the general paper specification (ideally, the exact paper)
• the type of press (offset, web, gravure, digital, etc.)

In today’s fluid and fast-paced production environment, getting answers to this seemingly simple set of questions is akin to asking your broker if the stock market will go up or down tomorrow. Many times, design directors do not determine final sizes until they have the images in hand. It is often impossible to know who will be printing any particular project. Sometimes, it goes to multiple printers — especially in the case of magazine ads that may run in a variety of publications. It is also increasingly common for a set of images to appear in all media. If you can get this information, it is much easier to create appropriately-prepared delivery files.

One strategy that works well for some is to deliver files in sets of small, medium and large, at 300ppi for print, and small, medium, and large, at pixel dimensions to match for web or projection. Although this is slightly more work, it does enable output sharpening since the image receiver should be able to adjust the size of the most appropriately-sized image without impacting the sharpening very much. It is important for the image files to be sized down and not up, unless the up-res is 10% or less.

Preparing image files for the web

The web designer should provide pixel dimensions for the needed image files; this makes your job fairly easy.

Master files should be:

• flattened – no layers, channels or paths, and
• converted to sRGB color space

It is good practice to embed the profile, even if most web browsers won’t see it – a few, such as Safari, do and color accuracy will be enhanced in those cases.

Trip wire: When embedding the profile, one caveat to be aware of is that color consistency is more important than color accuracy. In a non-color managed browser, un-tagged images are assigned the viewer’s monitor profile. This means that identical RGB triplets, tagged and un-tagged, display the same. In a color managed browser such as Safari, tagged and un-tagged RGB triplets do not display the same since the untagged RGB will display in monitor color and the tagged RGB will display in sRGB color.

Image size

The image files should then be resized to the correct width and height (in pixels). The ppi is irrelevant. But, if you want to visualize the final size on screen in terms of inches (or centimeters), set the resolution to 72 ppi and adjust the inches or centimeters to the desired size. Since this usually involves making the image files smaller, there are two resampling choices to consider: bicubic, or bicubic sharper. Many prefer bicubic sharper since it sharpens and resizes in one action. For more control, use bicubic resampling, then sharpen to taste using smart sharpen, unsharp mask, or high-pass sharpening. Since you are sharpening for screen, the effect you see at 100% on your monitor is accurate.

Figure 1 Photoshop CS4 general preferences for resizing image files.

The Image Interpolation choice set shown here is used to calculate any dynamic changes (crop, or any of the functions within the free transform options) in Photoshop.

The final step is to save the image files to the JPEG, PNG or, more rarely, GIF format. JPEG is the usual choice since it offers the best quality/size ratio for photographic images. Photoshop offers two methods of creating JPEGs: “Save as” JPEG, or “Save For Web and Devices”. Save for Web and Devices offers more control over compression options as well as previewing the final compression effects before you convert.

Preserving your metadata when using Photoshop Save For Web and Devices

Save for Web and Devices strips most metadata from files by default as you save them, which can be desirable if you need the smallest possible file size, but undesirable if you wish to keep your copyright and other IPTC information in the image file. Photoshop CS3 saved only the image file description and the creator’s copyright notice by default. It was possible to save all metadata (XMP format only) in CS3 if the “include XMP” option box was checked. In CS3, this option saved an XMP copy of the EXIF data as well, so stripping EXIF data had to be done prior to using this option.

Photoshop CS4 improved the situation considerably with four metadata options (XMP format only):

• None
• Copyright and contact information
• All metadata except EXIF data
• All metadata

Figure 2 CS4 and 5 Save for Web and Devices metadata options

Preparing image files for digital projectors and other screen devices

Digital projectors come in a variety of resolutions. You should deliver flattened image files that have the correct pixel dimensions for the projection device.

The common resolutions are:

• VGA (640x480) — primarily used on small graphic tablets
• SVGA (800x600) — only seen now on older projectors
• XGA (1024x768) — the most common resolution for Power Point presentations and projected video
• SXGA (1280x1024)
• SXGA+ (1400x1050) — newer projectors
• Widescreen (1920x1080) — HDTV

Figure 3 The crop tool in Photoshop CS4 and later can be set to crop at specific pixel dimensions to match a screen.

Images should be cropped to fit the varying aspect ratios. If cropping is undesirable, images can be dropped into a canvas of the correct aspect ratio that is filled with a compatible color (or black). If the image is to be zoomed in on, or panned across, it will need to be sized accordingly. Projected images should be sharpened for screen just as they are for the web.

Although digital projectors can be calibrated and profiled using the same or similar tools and software as for computer monitors, they seldom are. For this reason, it is a good idea to convert the delivery files to the sRGB color space. Conversion to sRGB is particularly important if the images are to be incorporated into other software, such as Power Point, Keynote or MS Word, since these programs vary in their ability to read and use embedded color profiles.

Preparing RGB files for digital printing

Digital print devices include inkjet, continuous tone printers such as those found in digital color labs, and commercial digital presses. Files delivered for these machines need to be “high resolution”, which may mean 300ppi at the final printed size in inches or centimeters. In our testing, we discovered that some laser print devices show smoother tonal rendition and better detail if given 400ppi files.

Files should be:

• Flattened, with no channels
• Converted to 8 bit unless 16-bit printing is supported, as is the case with some of the newer printer drivers and RIPs.

Inkjet printers can print at very high dpi resolutions such as 720, 1440, and 2880. There has always been some confusion between the terms “dots per inch” (dpi) and “pixels per inch” (ppi). PPI is for pixels in the file, and DPI is for ink dots on the page. For instance, you would prepare a document to 8x10 inches at 300ppi and print it at 1440dpi on an inkjet printer.

As far as ideal input ppi resolution, most agree that there is no more detail to be realized beyond 480ppi, and that 360ppi is a very practical upper limit. As a rule, large inkjet prints can be made from lower-resolution files, although 180ppi is often mentioned as the practical lower limit — depending on print size and viewing distance.

Continuous tone printers, such as the Kodak Durst Lamda, and Cymbolic Sciences Lightjets, print at different resolution than inkjets (either 200dpi or 400dpi). These devices expose photographic paper with laser light generated from digital files and, despite their lower native resolution compared to inkjets, they create continuous tone images. This technology is flexible in terms of input resolution. A one-to-one matching of input to output resolution is ideal, but good results can be achieved with an input resolution one-half that of the output resolution. This makes this type of printer ideal for very large print sizes.

Commercial digital presses are becoming an increasingly common adjunct to offset presses in the commercial print world. Since they are much easier to set up and run than offset presses, they are ideal for short runs and print-on-demand applications. In general, they print with a linescreen equivalent of around 200dpi. Most printers recommend 300ppi resolution files for input, although anything above 200ppi is quite safe. Depending on the image, you may see a smoother tone as a result of resolution up to 400ppi.

Color profiles for RGB printing

Inkjets, continuous tone printers, and commercial digital presses can all be profiled. In addition, they can be used with Raster Image Processors (RIPs), which usually provide a means to calibrate as well as profile the device. Obtaining a specific profile for any of these printers is ideal. Unless you own the device and have the means to profile it, you will need to rely on good communication with the people who run the machine to find out if there is a custom profile available, or whether they use a RIP to apply an input profile on the fly. When that is the case, delivering a good RGB file in a standard RGB space such as sRGB or Adobe RGB (1998) will suffice. If the shop running the device shrugs off color management questions, your best bet is to deliver sRGB files and hope for the best. If you have the time and inclination, you can pay to have test targets printed and then create your own profiles. Be aware that the longevity of these profiles depends entirely on the level of process control at the print shop or service bureau.

Preparing digital image files for CMYK delivery

The majority of photographers shy away from providing CMYK files. Perhaps it’s because many of them remember making color separations from the film days as an arcane process requiring $50,000.00 drum scanners, specialized RIP software, and printers who are unnecessarily secretive. Meanwhile, many design directors blithely push the “convert to profile” button without a second thought. The truth is somewhere in between. Good RGB to CMYK conversion is both an art and a science, but it is not rocket science. We highly recommend Rick McCleary’s CMYK 2.0 (Peachpit, 2009) as a good guide to this process, as well as the Adobe white paper by Jeff Schewe and of course the Real World Color Management books by Bruce Fraser.

The big difference between preparing image files for RGB output versus CMYK output is the CMYK color mode has a much smaller gamut (range of possible colors) than the RGB color mode. Consequently, certain colors that appear in RGB simply don’t translate into CMYK, which is why vivid blue skies often become dull or take on a magenta hue. It also explains why detail disappears in very saturated colors. For example, the weave in a saturated red fabric may turn into undifferentiated red mush. Various strategies can be employed to adjust the hue and saturation of RGB files to achieve reasonable CMYK rendering. Fancier strategies, such as replacing a weak black (K) channel with one derived from the Cyan or Magenta channel, can be used to bring back detail in out-of-gamut colors. The truth is that these problems only surface occasionally and the norm is for RGB images to convert fairly easily to CMYK.
Read more about converting RGB to CMYK in the CMYK output section

Color profiles for CMYK printing

One potential stumbling block is the concern that specific CMYK profiles are needed for best results. Offset presses are different from other printing devices in that they have much greater calibration control. Newer presses are controlled by computers that can adjust the mix of inks at many points across the press sheet with a high degree of accuracy. Just as we gray-balance monitors and adjust white balance on cameras, good press operators gray balance their presses. The gray balancing methodology outlined in the GRACol/SWOP specifications has the huge benefit of making custom CMYK profiles largely irrelevant except in the unusual case of non-standard ink colors. Three profiles, one for sheetfed presses and two for web presses, cover the range of papers from premium coated #1 sheets, to wood pulp based #5 sheets. Adobe’s version of these three profiles shipped with CS4, or the GRACoL/SWOP versions can be downloaded from the SWOP.org website. The GRACoL Coated profiles can be used in place of U.S. Sheetfed Coated and the Web Coated SWOP 2006 Grade 3 or Grade 5 paper profiles can replace the U.S. Web Coated v2 profiles that shipped with earlier versions of Photoshop up to CS3. We have discovered that even non color-managed print shops print better from the new GRACoL or SWOP profiles (as long as they don’t assign a different profile to the image files). If you are provided with a specific CMYK profile, by all means use it; however, don’t assume that just because the printer is unknown you are unable to deliver good CMYK files. An additional benefit of the GRACoL/SWOP methodology and the accompanying CMYK profiles is that they maximize the CMYK gamut, getting the most color out of the press and making conversion from RGB easier, since fewer colors get clipped. Although the new profiles are all formulated for coated stock, and have correspondingly high Total Ink Limits, most printers can easily bring those values down for uncoated stock via Photoshop, a device link profile, or by means of the RIP.

CMYK guide prints

CMYK guideprints can be a very useful tool, especially when the printer is unknown. These are easily produced on desktop printers. The main criteria is that you have a good profile for the printer/paper combination, and that you make the print from either the CMYK proof space (RGB file with the appropriate CMYK profile set as the destination space), or from the CMYK derivative file. Doing either will restrict the colors to the target CMYK color gamut, giving a realistic preview of how the image file will print on the offset press. Using the printer driver will give a good visual match; using a RIP will get you even closer, especially if the RIP has a linearization function. Linearization calibrates the printer and makes the profile even more accurate. RIP-driven inkjets are, in fact, what printers now use for making proofs.

One advantage of delivering CMYK files is that if the image files will be reproduced in a variety of media, from web to print (screen to substrate), it is possible to achieve a visual match across the various media if you create the CMYK files first. Then when you convert them back to RGB for the other uses, they will match since the colors have already been reduced to the CMYK gamut.

Sharpening for output

All image files, regardless of reproduction method, require output sharpening for optimal reproduction. This targeted output sharpening occurs when the image is at the final size for the specific display or printing device and substrate. Sometimes the only piece of missing information preventing delivery of a completely finished image file is the exact reproduction size. While it is possible to sharpen delivery files “generically”, any resizing done after this will resample the pixels and result in some loss of sharpness. As mentioned previously, a work-around is to deliver sets of files at different sizes, but this does add to the workload. However, use of Photoshop actions can automate and speed up this process.

There are two fundamental ways to sharpen digital images:

• sharpening the pixels that make up an image, and/or
• edge sharpening, which is sharpening the edges of shapes within an image.

Pixel sharpening can be done in Photoshop using Unsharp Mask or Smart Sharpen filters. This do-it-yourself method requires much knowledge and experience to get it right. Different images require different amounts of radius and threshold. We feel that most users will get more optimal results by using a dedicated sharpening plug-in such as PhotoKit Sharpener, Nik Sharpener Pro, Focal Blade or others.

Although pixel-based sharpening can certainly work well, many in the CMYK pre-press business prefer edge sharpening. Edge sharpening avoids emphasizing noise or other artifacts. In addition, edge sharpening holds up better if an image is resized slightly — often an unavoidable occurrence when image files are put into page layouts. The best technique for edge sharpening is to use one of the many variations of combining the Photoshop high-pass filter with an overlay, hard light, soft light, or linear light-blending mode. A nice advantage of this method is that it requires a duplicate layer, which makes the process non-destructive and infinitely adjustable by tweaking the opacity of the high pass layer. This technique also allows for adding a layer mask to prevent some areas from being sharpened or to minimize selective sharpening.

When judging sharpening for print, the image should be viewed at 50% or even 25% (if a very large image), and not at 100%. Viewing at 50% gives a much better approximation of the actual effect of the sharpening whereas 100% view will be largely misleading. Appropriate sharpness is definitely a subjective decision. Our advice is to try many techniques until you find one that gives good results and is repeatable. Keep a record of what you like best so you do not have to reinvent this part of the wheel each time. Remember that different output devices as well as different substrates may each require very different approaches and levels of output sharpening.
Read more about sharpening digital image files in the sharpening section

Delivering repro-ready, but unsized, RGB or CMYK image files with no output sharpening

When the final size for an image is unknown, best practice is to submit the file without output sharpening. This is a very common delivery situation since information about final size can be hard to come by. All the file preparation outlined above is completed with the exception of sizing and sharpening. We’ve outlined the strategy of delivering sets of files at varying sizes, but this requires more work and uses up more delivery bandwidth so it may not be practical for large numbers of files. In addition, the image receiver may prefer to resize the files and do the output sharpening. If the delivery format is TIFF or PSD, these files can be delivered with a sharpening layer. The layer should be labeled to indicate that it is a sharpening layer. This method allows the image receiver to flatten the file, retaining the sharpening, as long as the image is not drastically resampled. Alternatively, they can discard the layer and redo the sharpening if the resample requires different sharpening.