Thursday, August 21, 2008

CMYK and Trapping

I'm rerunning a post below on the topic of "trapping" that I originally wrote in August 2005, when it came up while we were readying Mom's Cancer for press. It's an apt time for this particular rerun because it's trapping time again with Whatever Happened to the World of Tomorrow? But first some foundation....

Most books with color art are printed in CMYK, whose letters stand for cyan, magenta, yellow and black (I believe the "K" actually stands for "key color"--which is always black). When printed in various intensities, those four inks can make most (but not all) other colors. For example, equal amounts of yellow and magenta make red. Cyan plus yellow is green, magenta plus cyan is purple, etc.

The next voice you hear will be me from three years ago. Ah, I was so young and innocent then....

"Trapping" is an obscure but interesting part of the pre-press process. In CMYK printing each color of ink is printed separately, one after the other, so that a page actually goes through the press four times. If the paper lines up perfectly with each pass, all the colors align and you get perfect registration. Very often, though, if you look at four-color printing closely enough, you can see that the inks are just a bit off. You'll see a colored halo on one side, or colors slopping out of their black boundaries, or a gap where colors don't meet up.

Good registration (left) and bad (right)

Trapping helps minimize registration problems by spreading out the non-black colors a few pixels wherever they meet a black line. That way, even if registration is a little bit off, they still have some "wiggle room" to overlap as intended. With Photoshop, trapping is as easy as pushing a button (I can't imagine how anyone did it pre-digitally, or whether they bothered at all). Coincidentally, a private cartoonists' board I frequent just had a long discussion about trapping.

That discussion came in handy when I got word late last week that the printer wasn't happy with my color registration. It wasn't coming out right. Not lining up. Within half a second I realized the problem: no trapping. When I submitted my final image files to Abrams they were trapless. Trap-free. Bereft of trap. My trapping had shuffled off this mortal coil, run down the curtain, and joined the choir invisible. The subject never came up and I never thought to ask. My bad.

So I spent a few hours this morning speedily trapping the 26 color pages scattered throughout Mom's Cancer. I envisioned the overseas printer tapping his toe, glancing nervously at his watch, paying overtime while the presses waited in idle silence for my upload.

Assuming my trapping worked, I should have first proofs to review in a few days. Next book, I'm hiring a high school kid to take care of this.


Me back in the present again (old and cynical). I learned a lot from my mistakes on Mom's Cancer, which might have made me a little cocky going into WHTTWOT. What I've realized, of course, is that now I have whole new opportunities to make entirely different mistakes. Despite my threat above, I did not hire any high school kids to do my trapping this time. However, I did hire some college kids to--but that's a subject for a future post.



grace said...

CMYK is usually used in 4 color printing for offset printing.

Paul said...

Useful article on CMYK and trapping. Thank you!