NEARLY TWO DECADES after he last sent Calvin and Hobbes exploring, Bill Watterson got an offer he decided not to refuse.

Webcartoonist Dave Kellett had done a voice-only interview with Watterson for his new comics documentary, “Stripped.” Now, Kellett had an even bolder proposal: Would Watterson — who retired his beloved “Calvin and Hobbes” strip in 1995 — consider providing the film’s poster art?

“Aside from supplying a few sentences to the documentary, I’m not involved with the film, so Dave’s request to draw the poster came completely out of the blue,” Watterson tells Comic Riffs early Wednesday afternoon. “It sounded like fun, and maybe something people wouldn’t expect, so I decided to give it a try.

“Dave sent me a rough cut of the film and I dusted the cobwebs off my ink bottle.”

Soon, Watterson was rendering an image that never would have passed the syndicate censors during his “Calvin and Hobbes” days: An adult springing to full-color life in all his dorsal nudity.

“Given the movie’s title and the fact that there are few things funnier than human nudity, the idea popped into my head largely intact,” Watterson tells The Post. “The film is a big valentine to comics, so I tried to do something really cartoon-y. I had thought of having it colored with off-registered printing dots like newspaper comics, but Dave asked if I’d paint it instead, and I think he made the right call.”

“Stripped,” by Kellett and fellow L.A.-based filmmaker Frederick Schroeder, features more than 60 cartoonists who talk about the state of the comic-strip industry. In the film, Watterson eloquently speaks to the emotional bond that readers form with comic-strip characters as a function of the daily strip ritual.

In a rare interview, Comic Riffs first heard from Watterson in 2011, when the cartoonist spoke to The Post about his professional admiration for “Cul de Sac” creator Richard Thompson. That same year, in support of the Team Cul de Sac charity for Parkinson’s research, Watterson created his first public art since the mid-’90s.

Next month, the work of Watterson and Thompson will go on exhibit in a two-man show at the Billy Ireland Cartoon Library & Museum at the Ohio State University in Columbus. “His originals are just incredible to see up close,” Watterson said to Comic Riffs last year of Thompson’s work.

As for his second public artwork since 1995, Watterson tells The Post today: “It’s a silly picture that sums up my reaction to the current publishing upheaval, so I had a good time, and I hope it brings the film some attention.”

The “Stripped” DVD will be available April 2.