Paper to Pixel to Paper Again, part two: Inking and Screening and Resolution, Oh My!

Sean Michael Robinson:

Greetings!

This is the second installment of Paper to Pixel to Paper Again, a series that explains (in an overly thorough manner) the how-to's of preparing line art (and later in the series, color art!) for print.

If you haven't done so yet, please go back and read the first installment.

Last week we started this series by throwing out some caveats, stating some goals, and peeping at my work space and equipment. This week we'll be laying down the basic knowledge you need on order to know the behind some of the things we'll be discussing and doing. f you're impatient to actually, you know, get to it, then sit on your hands for a week and skip this installment!

The past one and a half centuries have seen tremendous changes in the ways images are created, and the ways those same images are reproduced. Since the late 1800s, almost all technical innovation in printing has involved improving the reproduction of what are (misleadingly) called continuous tone images: that is, images that, when viewed in the right circumstances, appear to have smooth gradations of tone and value. If you're viewing a postcard with a reproduced photograph, or looking at a color diagram in a text book, in most viewing distances, these images appear to be smoothly changing values of color. But in reality these images are made up of tiny cells, distributed in an array, that through some miraculous flaw in human vision, work together to create those gradated illusions.





Above: a scanned detail of a Dave Sim commission. Below — an extreme closeup of the resulting print. Notice the array of dots that create the image.

But there are limits to this illusion. Certain people (myself included) have close vision that's significantly sharper than the mean, and are able to see individual printing dots when they're anything other than the finest pitches. More significantly, the sharpness of human vision increases with a corresponding increase in contrast, meaning that extremely high contrast images (say, black on white) represent much sharper visual acuity than a field of color. Additionally, we see another corresponding increase when we're presented with edges. Lastly, fine information that is near or beyond the fine-ness of the screen itself, or oriented in direction in a way that is not perfectly aligned with the screen, can cause all kinds of unintentional visual oddities. 

This is why you will never see a professional publication that has a large chunk of text that is screened and intended to be read. And this is why you should NEVER, NEVER SCREEN LINE ART.

I'm going to belabor this point (who, me?) because it seems to have been forgotten or ignored as screening methods have improved, or as expertise (and money!) have drained from the print fields. Unless you're reproducing in color and intending to show the artist's process as the intent of the print (a la IDW's Artist Series, the Cerebus Archive portfolio series, etc), LINE ART SHOULD NOT BE SCREENED FOR REPRODUCTION.

Imagine, if you will, the simplest of print methods, something along the lines of a stamp. A chunk of potato in which you've carved out an object in relief, maybe your name, carving back the wood from the sections of the images that you wish to remain uninked, and then pressing it against a flat surface slightly moist with ink, and then finally transferring that ink with light pressure onto another surface.

Your resulting print, any unevenness in the ink aside, is a binary. Either a particular area of the paper is inked, or it is not. On, or off, no in-between. And, really, how would you represent in between? You examine your print some more, consider that you really would like to have a “gray” area of, say, 30 percent to augment your totally black and totally white portions of your image. So you cut some parallel hatching lines into a previously fully inked area of your potato, and you print again.

This is line art in microcosm, line art at the beginning, just beyond scraping lines in the sand with a stick, just beyond taking an old torch and dipping it in bison blood and dragging it along the surface of your cave. Primal, black marks on white, any “gray” an illusion created by finer marks of black on white.

—

Okay, let's skip ahead at least a millennium, where we arrive at the present day.

No longer satisfied with the speed of your potato print, you're now interested in taking your paper line drawings and reproduce them with all the bells of modern technology. A terrifyingly fast, abominably loud web press, running off a thousand copies of your masterpiece in an hour. Between you and that copy are a good dozen technicians and a veritable space-shuttle level of switches and knobs and little blinky lights. How do you ensure your drawing survives the process? How can you signal to these strange, unknowably distant beings what it is you want out of your print?

(And please don't tell me that your desktop laser printer, or the, ahem, helpful staff at your local copy center, are any more knowable or accessible :) )

You need to know how to prepare your files. You need to know what to ask for. And you need to know about resolution.

RESOLUTION

In order to actually, you know, get to the part of this series where we actually DO something, I'm going to need you to take for granted a few facts. Rest assured I'll come back to them in future installments, and rest assured, I'll be happy to argue with you about them in the comments.

When you're preparing color or grayscale images for print, that is, images intended to be reproduced as (not actually) continuous-tone images, the limit of your effective resolution is the screen that these images will pass through. The fineness of a screen is measured in LPI — lines per inch. A printer printing on an extremely coarse surface — a cardboard box, newsprint, some kind of screenprinting application etc — will use a really coarse screen, sometimes as coarse as 40 LPI. Printing on a sheetfed offset press on coated paper, or on a very good one-off digital press on coated paper, the LPI might be as high as 300 LPI.

A good rule of thumb for supplying files that WILL be half-toned is, the maximum effective resolution is twice the line screen resolution. So, if your printer will be screening your final image at 200 LPI, 400 pixels per inch is the highest effective resolution you can supply. Anything above that is pointless, as it's lost in the screen. (This is not the case if you're suppling some elements separately, a in a PDF, where you can have images with different resolutions and color spaces coexisting in the same document. More on this later!)

Conversely, when you're printing WITHOUT a screen — whether that's in black, or using a spot color — your only resolution limit is your vision, and the resolution of the output device, whether that's a laser printer or a plate setter at the printer.

Without further ado, here are the resolutions you should be aiming for for suppling files to your printer —

as low as 100 PPI in some extreme circumstances, as high as 600 PPI on coated stock with good printing should be dependent on the destination LPI. Remember, it's easier to downscale than upscale! I always scan any color art that's going to leave me permanently (go to a client, etc) at-size at 600 ppi, as a safety.

Line art/bitmap--
1200 ppi for laser printers and other digital printers (600 PPI might be acceptable on rough paper if there are no very fine lines or repeating tones present)
1200 or 2400 for web or sheetfed offset with fine lines and tones.

But, fortunately, since we're dealing with line art, just because you're SUPPLYING line art files at that resolution, doesn't mean you need to scan at that resolution!

… and that's all we have time for next week. Next week — we finally (for real!) get to it!Thoughts? Questions? Quibbles? Hit me up in the comments!