mercredi 27 octobre 2010

Prepress : My CMYK Images Change When I Print or Export PDF!

A color management mystery (with a solution!)

My CMYK Images Change When I Print or Export PDF!

[Note that this article originally appeared in InDesign Magazine, Issue 37. For a free sample issue of InDesign Magazine, or for subscription information, see www.indesignmag.com]


If you are going to use CMYK images from Photoshop in your InDesign documents, there are some things you need to know. Unless you understand the pitfalls, they’ll cause you untold grief sooner or later.

The genesis of this article was a problem posed to me by a production editor at a major book publisher (let’s call her Liz) who was couldn’t figure out a mystery involving a book she was preparing for print: While some CMYK images from Photoshop were showing up in the final PDF files correctly, others–the majority, in fact–were changing. The numbers inside the CMYK image were changing dramatically–for example, a 80-percent black would change into 50-percent black with a bunch of cyan, yellow, and magenta thrown in.

Like an airplane disaster, there is virtually never just one problem in these prepress mysteries–it’s almost always a combination of factors, of misunderstandings, of hidden settings and incorrect assumptions. But by slowly unraveling the issues, we gain a better understanding of InDesign and Photoshop, and we can learn how attack the next unique situation more efficiently.

So here are some lessons learned, and advice I can offer.

CMYK Is Not Bad

Often, creative professionals fall into either a CMYK or RGB camp, each arguing with the other than their color model is the One True Answer. But I’m here to tell you that both models have their pros and cons, and there is no “right answer.” I love RGB workflows as much as anyone–the ability to place an RGB image into InDesign and have it perform the high-quality conversion to CMYK (exactly the same as Photoshop!) when I print or export is miraculous and a huge time-saver. I recommend RGB workflows to almost everyone, but they do have their limitations.

For example, in Liz’s book project (the one that was causing problems), most of the images were computer screen captures. Screen captures tend to work best when converted to CMYK in Photoshop using a “max-K” generation–that is, a custom GCR curve with GCR set to Maximum, which replaces as much cyan, magenta, and yellow with black ink as possible.

You could get InDesign to use a Max-K profile, but there’s no way to tell InDesign to convert photographic images with one kind of profile and screenshots with another. So the best practice is usually to convert screen shots in Photoshop first.

There are other situations where converting to CMYK in Photoshop is preferable, such as when you need to make little tweaks to just the black channel in a photograph. Suffice it to say that CMYK is not bad, and you shouldn’t feel bad if your workflow brings CMYK images into InDesign. (You should, however, feel bad if you have not yet investigated whether using RGB images would be a more efficient workflow.)

By the way, I want to be clear that I’m talking about pixel-based images in this article, not vector images. If you’re creating graphics in Illustrator that will be sent to press, they should almost certainly be CMYK, not RGB.

Embedding Profiles is Problematic

Say you’re saving a CMYK image in Photoshop. You have the Save As dialog box open, and chances are the Embed Color Profile checkbox is selected for you. If you’re like most people, you just click Save and get on with it. If it’s on by default, it must be the correct choice, right? Not necessarily.

Here’s an InDesign secret: By default, InDesign ignores embedded CMYK color profiles, so adding that CMYK profile when saving a file in Photoshop is generally unnecessary. Plus, it fattens files a bit: Embedding the profile can add 750K or even as much as 2 MB.

More dangerous, though, is that InDesign might actually use the embedded CMYK profile. And in those cases, your CMYK values can change, sometimes radically, and not always in the way you expect or want.

For this reason, I generally do not embed profiles in images I’ve converted to CMYK, unless one of the following conditions is true:

  • I know that I do need them color-managed in InDesign (pretty rare); or
  • I will be editing the images again later in Photoshop (or giving them to someone else to edit).

Note that you should always embed profiles in RGB images. It takes up hardly any space and the RGB image is virtually useless without it.

Tip: To remove an embedded profile from a CMYK image, open the file in Photoshop, go to File > Save As, and uncheck Embed Color Profile. Alternatively, you can open the image in Photoshop, choose Edit > Assign Profile, and choose Don’t Color Manage This Document. Click OK and save the file.

The Danger of Cross Rendering

Again, if a profile is embedded, InDesign might use it. That means InDesign might try to color-manage your CMYK images–changing the color values in an effort to make the color better. For example, a solid 50% cyan in your image would end up on press as cyan with yellow, magenta, and perhaps even black mixed in. Gah!

If you’re converting CMYK to RGB (for web graphics, perhaps), this might be welcome. But generally, if you have an image that was already converted to CMYK using one CMYK profile (like SWOP), and you’re printing it to a different CMYK space (like a custom profile from your printer), you’ll probably get yourself in trouble. This is called “cross-rendering” and it’s unadvisable. At best, it probably won’t improve your quality much, and at worst, it ruins your day.

Instead, it’s generally preferable to send the CMYK image you have, as is, to the printer, or–even better–open the original RGB image and re-separate to the new CMYK profile.

When CMYK Gets Altered

As I’ve said, I generally don’t want InDesign to alter my CMYK values–I’d rather pass them through as is. So that takes me back to Liz’s book example. In her case, she had a bunch of images that did have embedded color profiles and the colors were unexpectedly changing. But why?

Here’s where the discussion starts to get somewhat technical. But take it slowly and you’ll understand where the pitfalls lie.

There are several places in InDesign where you can tell the application to stop ignoring embedded profiles and start using them to cross-render.

Override the Defaults. When you import a CMYK image, you can turn on the Show Import Options checkbox in the Place dialog box. The Color tab of the import options dialog box offers you the chance to choose a color profile (Figure 1). But unfortunately, the interface is quite confusing. The Profile pop-up menu is usually set to Use Document Default, which means ignore any embedded profile! That is, the “document” it refers to is the InDesign document, not the file you’re importing.

ProfileImportOptions.jpg

Figure 1: The import options dialog box lets you choose to ignore the embedded profile or honor it.

If there is a profile listed above Use Document Default, it means there is one embedded in the image. If you choose that, you’re telling InDesign that you want to honor the embedded profile, not ignore it–you want InDesign to color manage (cross-render) this CMYK image. You would do this only when you’re feeling particularly confident of your color management skills.

The list of profiles below Use Document Default are your typical profiles installed on your computer. It is extremely rare that you would ever choose one in that list. (Chris Murphy, author of Real World Color Management, calls this the “hurt me” option.)

If you have already imported an image, you can find the same options by selecting the image on the page and choosing Object > Image Color Settings. For example, if you think someone chose to honor an embedded profile and you want to tell InDesign to ignore it, you can set the Profile pop-up menu back to Use Document Default.

In the mysterious case that we’re looking at, Liz knew that no one used either of these features to honor the profiles. But still, InDesign was honoring them. So…

Preserve Embedded Profiles. Let’s say you are in a fully color-managed workflow, and you really like CMYK cross-rendering for some bizarre reason, but you have become tired of always choosing the embedded profile in the import options dialog box each time you imported a CMYK image. You can change InDesign’s settings so that it will stop ignoring embedded profiles. To do this, you need to choose Edit > Color Settings and select Preserve Embedded Profiles from the CMYK pop-up menu in the Color Mangement Policies dialog box (Figure 2).

ColorSettings.jpg

Figure 2: The Color Management Policies section lets you set InDesign's default color profile behavior. For CMYK, Preserve Numbers is safe and sane. Preserve Embedded Profiles is great for RGB, but can be treacherous for CMYK, unless you really know what you're doing.

Again, to be clear, this means you’re telling InDesign that you like cross-rendering. It’s the same as using Image Color Settings to pick the embedded profile for every image you import.

In my opinion, the “Preserve Numbers (Ignore Linked Profiles)” setting is far safer. This tells InDesign to ignore those embedded profiles in CMYK images (unless you override it on a case-by-case basis, as I mentioned above).

When I asked Liz about this possibility, she did the most obvious thing in the world: She opened the document, then chose Edit > Color Settings. She then informed me that she definitely had Preserve Numbers selected.

A ha! “A” for effort, but Fail.

The most important thing you need to know about the Color Settings dialog box is that it does not reflect or affect the currently open document, or any document you have already created. It goes against all intuition and expectation. Instead, this dialog box only shows the defaults for new documents you will create.

To see or change the settings that were in force when a document was made, you need to know a secret trick. I’ll tell you that in a moment, but first I’ll explain one last place in InDesign where you can control whether CMYK values will be cross-rendered.

Preserve Numbers. When you print or export your InDesign document to PDF, you have the option to convert your colors–to color manage them, as it were. In the Export PDF dialog box, this option lives in the Output pane and is called the Color Conversion pop-up menu (Figure 3). The default selection is the sane one: “Convert to Destination (Preserve Numbers)”. That means InDesign should leave CMYK numbers unchanged (pass them through), if:

  • Your image has no embedded profile; or
  • Your image has an embedded profile, and your document’s color settings policy is set to the default setting of Preserve Numbers (Ignore Linked Profiles). As we learned earlier, Preserve Numbers (Ignore Linked Profiles) means InDesign should treat CMYK images as though they don’t have an embedded profile (unless you specifically override that for an image).
PDF_colormgmt.jpg

Figure 3: The Preserve Numbers option is generally the sane option when exporting a PDF because colored text, objects, and CMYK images tend to be left alone.

However, if you choose Convert to Destination, you’re telling InDesign to convert all CMYK images to the output CMYK destination–you want it to cross-render, changing the CMYK values. In this case, InDesign will either use an image’s embedded profile or the default document CMYK profile (the profile listed in Color Settings when the document was first created). This is dangerous stuff. I can’t think of a single time I’ve ever used Convert to Destination.

Inline Sidebar Note: By the way, I should point out that if the destination (“target”) CMYK profile is exactly the same as the image or document profile, then the numbers won’t change. For example, let’s say an image was converted with the SWOP color profile, had that profile embedded, told InDesign to honor the profile, and then printed using the SWOP profile as the destination. In that case, the color values would not change because there’s no such thing as cross-rendering from SWOP to SWOP. (That’s like saying “translate between French and French.”)

You have the same options in the Print dialog box, but they live in the Color Management pane (Figure 4). The main choice is the Preserve Numbers checkbox. Selecting the checkbox is the same as the Preserve Numbers option described above. Deselecting it is the same as using Convert to Destination.

print_colormgmt.jpg

Figure 4: The Preserve Numbers checkbox is grayed out unless the Destination profile is different than the mysterious and hard-to-discover document CMYK profile.

What’s Your Profile?

Liz was baffled. She could tell that the colors were changing by opening the PDF in Acrobat Pro and looking at the Output Preview panel. (You can get the same feedback by placing the PDF back into InDesign and use the “colorimeter” readings in the Separations Preview panel.) She knew the images had embedded profiles, but she checked and double-checked and she was using Preserve Numbers when making the PDF file. Why was InDesign cross-rendering?

The first clue came when we selected an image and chose Object > Image Color Settings. The embedded profile was selected! If no one did that manually, then how did that happen?

While pondering, I called upon scripting maven Olav Martin Kvern to write a short script that reset all the image color settings to Use Document Default. You can download it here, if you want it. However, Liz wanted to get to the bottom of the problem, not just run a script.

As we learned earlier, if InDesign honors the embedded profiles of all your placed images, then it’s probably your Color Settings Policies. Opening Color Settings won’t tell you what the current document’s policies are, so how do you find out?

Here’s the secret trick:

  1. Choose Edit > Color Settings and turn on the “Profile Mismatches: Ask When Opening” checkbox. Also, make sure that the CMYK policy is set to Preserve Numbers (Ignore Linked Profiles).
  2. If the document you want to check is open, close it. Now open it.
  3. If the built-in document policies are different than the current setup in Color Settings, you’ll get a mismatch dialog box. The first dialog box you’ll see reflects the RGB mismatch. Leave it set to “Leave the document as is” and click OK.
  4. Now InDesign displays the CMYK mismatch (Figure 5). This is the only place where you can see the document’s built-in policy and profile. In Liz’s case, she could finally see that the CMYK policy had been set to Preserve Embedded Profiles! Mystery solved.
  5. To alter this policy, choose “Adjust the document to match current color settings”. And, to force InDesign to ignore all those embedded image profiles, choose Disable All Profiles from the Placed Content pop-up menu.
DisableProfiles.jpg

Figure 5: The Policy Mismatch dialog box is the only place InDesign where you can look inside and find the hidden profiles and policies attached to your document.

When you click OK, InDesign reconfigures the document. For Liz, that meant all those images stopped cross-rendering and she could get on with business.

Extenuating Circumstances

To be honest, I simplified Liz’s situation a bit, for the sake of teaching these topics. Every publication bound for press poses its own particular set of issues, often based on the (often bizarre) requirements of the printer. This case was no different. The book contained a combination of color photographic and screencapture images. It had to be sent to the printer as a PDF file, but the PDF had to be created by running a PostScript file (which used a custom Print preset and a custom PPD file) through Adobe Distiller (using a custom .joboptions file). The whole thing gave me a headache just considering it.

However, no matter what the extenuating circumstances of a job, you always have to keep coming back to the basics. Understanding how, why, and when color management settings, policies, and overrides apply is the key to solving color mysteries and saving the day.

Posted via email from Steve Prud'Homme Prepress Blog Collage

Aucun commentaire:

Enregistrer un commentaire