Sunday, 24 November 2019

Limiting Ink Density

If you're printing at Lightning Source, e.g. because you're doing a POD product at DriveThruRPG, you have to abide by LS's printing specifications. One of those is a so-called "ink density limit."

What is an ink density limit, and how do you stay within it?

As I wrote in the last post, printing presses use four colors of ink to replicate the colors in your digital files, cyan, magenta, yellow, and black.. CMYK. Most colors are reproduced using a combination of multiple inks. Darker colors need more ink, as do more saturated colors.

For especially deep, saturated colors like inky maroons or really dark, muddy greens, you can need quite a lot of ink - black, yellow, and magenta. Just like coloring over the same spot over and over again with a marker, too much ink can wrinkle the paper, or cause smudging and bleeding before the ink has had time to dry.

For this reason, Lightning Source imposes a blanket restriction on the total amount of ink any one spot on the paper can take, to 240%. Their darkest, richest black allowable is achieved by 100% black ink, with 60% cyan, 40% magenta, and 40% yellow.

Do we have a problem?

Waiting for Lightning Source to tell you that you have an ink density problem is going to ratchet up your turnaround time. It takes several days for DTRPG and Lightning Source together to check, process and then reject your PDF. Better to check it yourself.

The best way to check I've found so far isn't great, as you have to skim your whole document by hand in InDesign. Even so, it beats losing half a week to turnaround time.

Open up the so-called 'Separations Preview' panel, in the menu: Window | Separations Preview.

(It's called that, because it's a preview of how your document colors will be separated into the four different inks.)

Turn the 'View' to 'Ink Limit', and then change the numeric value to '240%', like so:


This causes your document to be re-rendered in a washed-out gray, but with any areas that violate the 240% limit picked out in red.

For example, here's an ink-heavy map from my compendium in the normal view:


Here's how it looks with the Ink Density limit preview on:


If you click on that to zoom in you can see a few areas have been called out by InDesign as busting the ink density limit.

Why those areas? There are some dark, muddy browns there - they're close to gray but not exactly gray, which means you need a combination of cyan, magenta, and yellow to reproduce them on paper. They're also really dark, which means a bunch of black ink goes on as well. Soggy.

InDesign doesn't seem to have any sort of whole-document pre-flight check for ink density, which means there's no simple way to find these problems without scanning the whole document. (If you know of one, let me know!)

Checking Ink Density in Photoshop

Unfortunately, Photoshop only allows you to check a single pixel at a time. Open up the Info panel, and click the eye droppers to change its display options. You want to see the CMYK color, and Total Ink views.


As you mouse around your image, Photoshop will show you the total ink density. This isn't really useful for finding problems, but it's a decent way to check that you have fixed them. (See below.)

Fixing Ink Density

Fixing ink density problems can be tricky for indie publishers, particularly if the publisher/artist roles are played by separate people and the publisher only has InDesign, and the artist only has Photoshop.

If you're the publisher:

  1. You may need to explain to the artist what ink density is, and how to spot check it.
  2. Send the artist a screenshot of the ink density view from InDesign, so they can see the problem areas.

If you're the artist:

  1. Learn what ink density is, and how to spot check it.
  2. Find out from your publisher what the problem areas of the image are, so you can check them out
  3. Keep your original art piece separate, because many of the methods of fixing ink density are destructive, making permanent changes to the art.

Method 1: Tweak the image

This involves some trial and error, but some combination of level adjustments, curves, and desaturating dark colors can fix ink density problems. This may keep you in your comfort zone, but it strikes me as a little haphazard for two reasons:
  • Photoshop doesn't seem to have any built-in adjustments that target ink density. Because density is a subtle combination of high saturation and low brightness, there's no blend mode or levels solution that's a guaranteed fix that won't wash out the image (as far as I can tell).
  • Checking your work isn't isn't easy, because you can only check one pixel at a time. By the time you've fixed the problems, but then re-adjusted so your image isn't washed out, you might have caused new problems elsewhere.
I welcome tips or links to articles about this approach.

Method 2: Convert to the printer's recommended color profile

Converting to some specially designed color profiles seems to fix ink limits.. sometimes.

For this method, you should probably learn what color profiles are so you have some idea of what's happening.

Applying the profile is easy (once it's installed). In Photoshop, use Edit | Convert to Profile, and pick the printer's profile.

A key limitation of this approach is that ink density limits are not a feature of color profiles. Reducing the ink density can be a side effect of a profile conversion, but there's nothing in the profile that sits there as an ongoing safeguard against ink density problems.

Photoshop does its best, but if you have any layers in the multiply blend mode, while each layer individually might respect ink density limits, the resulting final pixels can still blow past the limits.

For this reason, if you rely on a color profile conversion to fix ink density you should:
  • Flatten the image first (to get rid of blending mode interactions)
  • Limit the amount of adjusting you do afterwards, e.g. fixing the contrast
  • Spot check problem areas before you declare victory
Currently, DriveThruRPG is recommending that art be converted to the CGATS21_CRPC1.icc profile, which you can download from them.

Method 3: Convert to custom CMYK profile settings

If you can't figure out the printer's target color profile, Photoshop will let you manually specify an ink density limit if you convert to a 'custom' CMYK profile.

Choose Edit | Convert to Profile..



Chose the CMYK destination space, then pick 'Custom CMYK' as the profile. When the profile customization dialog pops up, type in '240' as the ink density limit:


..and that's it.

Finally, spot check any problem areas your publisher highlighted for you, and any other likely problem areas if you do any adjustments after the conversion.

As with using the printer's color profile, there's nothing magical about converting to CMYK that will keep your image within the ink density limit if you adjust it. It may already be out of whack if you have blend mode interactions, so try this with a flattened version of the image if your spot checks.

Send it back to the publisher and.. good luck!

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