Above: scans of page 225 of the aborted February 2013 printing of High Society, which, as you can see, was half-toned, as it was supplied to the printer as 8-bit grayscale files. Below is an image from the restored files, produced from the exact same scans as the above images, but treated much differently in the prepress stage. Notice that the dots in the Cerebus image above are strange, gradually changing shapes, moire that is a result of the original screen tone applied to the art being reproduced through a second screening process at the printer.
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 potato from the sections of the images that you wish to remain un-inked, 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 on the embedded YouTube videos.
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 —
Color or Grayscale--
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 measure.
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!
***
When we left off prior to those three little dots, we were discussing how you NEVER EVER NEED TO HALF-TONE LINE ART, and how not half-toning allows you to work in a much finer resolution space.
All of the Cerebus Restoration projects have been delivered to the printers as 1-bit (line art) 2400 ppi originals. (In a later installment I'll get into why I arrived at that resolution, and all of the benefits we'd had as a result. But for now, let's just take this fact for granted.)
But just because we're delivering 2400 ppi bitmaps doesn't mean we're scanning at that resolution. This wouldn't be practical or even desirable, for a few different reasons.
Instead we've been scanning at different resolutions (and different formats) depending on the source material, and then upscaling and finally sharpening to arrive at the delivered size. I'll spell all of the how-to of this out in the next installment.
For the most part, there are three sources for materials in the project so far—print materials, photo negatives used to produce the printing plates in the initial run of the book, and original artwork. The print materials and the photo negatives are both at size (or close to the size) of the printed books they will be used to generate, so after much testing, these have been scanned at 1200 pixels per inch. The original art, however, is much larger, and so for the most part these can safely be scanned at 600 pixels per inch and retain all of their detail in the final result.
(How much larger? For most of the run of the book, the active image area of any given printed Cerebus page was around 6" x 9", whereas the corresponding area of the original art was around 10" x 15". For most books the original art was reduced to 60 percent of the original size, with the exception of the latter half of Church & State I through Jaka's Story, which was reduced to 56 percent instead, presumably to try to address any future problems with binding etc in collected editions of the books.)
And why try to skimp on resolution at the scanning stage? There are a few answers to this, but the biggest issue is sheer time. Every time you double your scanned resolution, you're quadrupling the actual time it takes for the scanner to scan the image. (It's not precisely quadruple, as there's a certain amount of setup, warm-up, etc that remains the same no matter how fast the scanner operates).
With the Cerebus restoration project, I was already dealing with a six thousand page problem. Quadrupling your scan time for no good reason isn't ideal.
As to why, exactly, you can safely upscale so much for line art applications when it remains such a general no-no for color reproduction — the very short version is, the color scan contains much more information per pixel than the resulting line art pixel, and that information enables the upscale and attendant bitmap conversion to be very precise to the source document despite the resolution change. More (I seem to be saying this a lot?) in a future installment.
***
Okay, you've drawn your masterpiece, you've read way too much information about resolution and line art, and now you're impatient to get on with it. Now how do you get all your inky dinky little lines into this here computer box?
Shooting line art used to be a complex process, involving a huge stat camera loaded with orthochromatic film, and an expert operator with a good eye and a ton of experience.
