If you are serious about color control in your books you may be interested in converting your book pages to the Pixopolis printer profile Fogra 27. Bear in mind that 95% of the time, your standard sRGB pages will look fine, but occasionally there will be problems, a bit like speaking Swedish with a Norwegian.

So rather than submitting sRGB pages, the next level is converting all your pages from sRGB to Fogra 27. Again, most of the time this will work fine and I’ve been doing this myself for many books with excellent results. But certain colors, very satured blue for instance, can not be reproduced on most printers, including the ones Pixopolis use. For images with such colors you want to use a technique called soft proofing which enables you to (a) see how the image will appear on paper; (b) see what part of your image has colors that cannot be reproduced; and (c) adjust your image to avoid surprises.

If you are familiar with color theory, you might want to skip ahead to the section on Soft Proofing below which tells you how to setup Photoshop for soft proofing with the Pixopolis Fogra 27 profile.

Let us first define a few terms.

A color space simply defines the range of colors that a screen or printer can show. Some color spaces, like Fogra 27, are tied to a particular machine - in Fogra 27’s case the printers that Pixopolis use. Other color spaces, like sRGB or Adobe RGB are device independent and used to allow consistent editing between computers.

A color profile is a small signature embedded in an image that describes how to correctly show the colors in a color space on a printer or display. When your camera takes a picture, a pixel may be recorded as having values red=200, green=150, blue=220. But such values are meaningless unless we define what red, green, and blue really are. It is a bit like saying that the distance between two houses on my street is 420 fingers and not saying how much a “finger” is.

Shown on the right is the horseshoe-shaped CIE chromaticity diagram. It is the international standard for describing colors, much like kilometers are used to describe distances. For our purposes we can assume that the CIE diagram contains all the colors. (Actually, the diagram doesn’t show brightness variations which is why brown - a darker version of orange - is not in the diagram).

Within the CIE color space are some smaller color spaces whose names you probably recognize. The Adobe RGB color space, for instance, is defined by picking values (CIE coordinates) for red, green and blue, thus defining a triangle of all possible AdobeRGB colors. These triangles are also called gamuts.

The sRGB color space is a smaller color space still, implying that there are certain colors that can be expressed in Adobe RGB but not in sRGB. Many computer screens have color spaces close to sRGB although some expensive ones can show the entire Adobe RGB color space. That is why the colors outside the Adobe RGB color space probably all look the same to you - you are after all looking at the CIE diagram on a device that can not show colors outside the AdobeRGB color space!

The 2200 Matt Paper is a typical CMYK color space for a printer and probably not too different from Fogra 27. (Printers, as you probably know, use cyan (C), magenta (M) and yellow (Y) to mix colors, hence the name CMY. The last letter K means black and is there because printers use a black ink to avoid having to mix cyan, magenta and yellow in equal proportions to get differents shades of grey - a very common color.)

Looking at the 2200 Matt Paper color space, you should now understand that there are a few colors that can be shown on your screen but not printed on a printer. This is exactly our problem: certain images with deep blue or green look great on the screen but can not be printed accurately on a printer with its smaller color space. You will sometime hear expressions like out-of-gamut colors which refers to all the RGB colors that can not be reproduced on a certain device, like a printer.

There are a few different ways to deal with out-of-gamut colors and these strategies are known as rendering intent: when converting an image from RGB to CMYK not all colors in the source space will have a corresponding CMYK color and therefore need to be translated to some other color. We will briefly describe two such rendering intents, useful for photographs: relative colorimetric and perceptual.

Relative colorimetric does not change the appearance of in-gamut colors. Out-of-gamut colors however are translated to the nearest possible color in the target CMYK space. (We do not define the exact meaning of “nearest” in this tutorial.) In other words, if a color can not be printed, use a color that is the closest reproducable, i.e. just inside the target space. Note there that can be a lot of colors mapped to the “border” of the target space and gradations in the RGB image may be lost.

Perceptual compresses the entire color space into the target space, thus maintaining the individual relationship between colors in the image. The exact conversion depends on the CMM (color management module) of which there are several to choose from: Adobe ACE, Apple ColorSync, Microsoft ICM, etc.

Which rendering intent is the best? This depends on your image. If there are no out-of-gamut colors in your image, relative colorimetric does not change the appearance of your image. For this reason, we recommend that you use relative colorimetric as your rendering intent. For images with intense colors, such as bright sunsets, perceptual may be better. We recommend that you use soft proofing to see if perceptual rendering produces a better conversion in these cases.

To summarize what we have just said, we can conclude that the key to getting good prints is to start with an RGB image without out-of-gamut colors and - if that is not possible - select a rendering intent that will convert your image without problems.

Soft proofing is used to get a preview of what the image will look like on paper. With the preview we can usually adjust the RGB image to match the capabiltities of the printer. Notice that we adjust the RGB image so that the following CMYK conversion looks about the same. We do not try to adjust the converted CMYK image to match the original RGB image.

The requirements for soft proofing is that you have access to the Fogra 27 ICC profile. In Photoshop under  Edit>Color Settings, try to locate Europe ISO Coated FOGRA27 in the CMYK dropdown menu.

If Fogra 27 doesn’t appear in the list, you can download it from http://www.pixbookdesign.com/FOGRA27.

The ICC profile should be stored in C:\WINDOWS\system32\spool\drivers\color on a Windows machine and Library/ColorSync/Profiles on a Mac, in the user’s home directory. You will have to restart Photoshop to see the change.

You also need a calibration device for your screen, such as Gretag Macbeth’s Eye-One. Without a calibrated screen, soft proofing is not very meaningful.

Step 1: Color settings

In Photoshop, select Edit>Color Settings again and click on More Options. Make sure you have the same settings as in the picture on the right.

The Gray setting is obtained by first clicking on Gray, then Load Gray, and then finding the Fogra 27 profile.

Once you have these settings you can save them under the name Pixopolis if you like.

First make sure all Photoshop documents are closed, then select View>Proof Setup>Custom from the View menu.

Make sure you have the same settings as in the picture on the right.

We don’t recommend checking the Simulate Paper Color as we think the preview is overly pessimistic anyway and your image will look washed out. But if you like to try it we recommend that you first remove everything white on the screen (menus etc) so that the whitest part of the screen is in your image. Also, look away when checking the box to allow your eyes to adapt to the change.

Save your profile under the name Pixopolis so that you can quickly select it under View>Proof Setup in the future.

While it is possible to adjust your book pages after they have been designed, we think it is much better to adjust images separately before you design your book. There are three advantages to adjusting before:

1. •If you decide to re-design your book, you don’t have to adjust your page again.
   

2. •Adjusting pages might affect other images on the same page.
   

3. •In the book design, your images will be smart objects and they have limited adjustment options.
   

So open an image that you intend to use in your book. The example on the right has some saturated green colors in the sky and is difficult to print accurately.

Now follow these steps:

1. •Select Image>Duplicate
   

2. •Select Window>Arrange>Tile Vertically
   

3. •Select the original image.
   

4. •Select View>Proof Setup>Pixopolis
   

Press cmd-Y (or ctrl-Y on a PC) and you will see that the image (on the left in the screen dump below) changes a bit. If you do this on your own images you might notice that the image looks more flat, has less saturation, lacks contrast etc. Each image will have a different change.

Soft preview: the preview of the print (left) and the duplicate RGB image (right)

We can now actually see how the color gamut of the original image (on the right) is going to be compressed when printed (on the left). If you have an image with really satured greens and blues, the result of this may be very depressing. In reality, we have noticed that images print a little better than you would think but it is always a good idea to check beforehand.

The cmd-Y (ctrl-Y on a PC) toggles between the soft preview and the original image - the title bar of the window will tell you what state you are in.

Furthermore, you can press shift-cmd-Y (shift-ctrl-Y on a PC) to get a gamut warning. You will get the same gamut warning regardless of whether you are soft previewing or not. With the gamut warning, all un-printable areas are greyed out. Press again to turn it off.

Step 4: Adjusting the image

The intention of the final step is to adjust the RGB image so that the printed image will look about the same.

Make sure you have the original image (left) selected, not the duplicate. The duplicate will be used for reference.

A simple adjustment is to select Image>Adjustments>Hue/Saturation, select the Green channel and reduce the saturation; you will notice that the gamut warning disappears. You can play with levels, curves, hue/saturation - preferably on an adjustment layer - and try to match the duplicate image. You probably won’t be 100% successful but you are likely to get a much better printed image than without adjustments.

Step 5: Batch converting pages

When your book is finished you can safely batch convert all pages from sRGB to Fogra 27. This is described in the FAQ.

Notice that the FOGRA 27 action used in the batch conversion does a relative colorimetric rendering intent. If you have discovered that perceptual is better, you need to convert those pages individually. If one of your images needs perceptual rendering, all the other images on that page will be affected as well.