There are certain novels they say just can’t be filmed, but guess what? Most of them have been. “Dune”? “Naked Lunch”? “The Virgin Suicides”? “The 120 Days of Sodom”? “Ulysses”? All done — “Ulysses” twice, even.

Call it the inability of a filmmaker to resist a good story, or call it the weakness of a producer for a property with a built-in audience, but whatever the reason, this season sees a wave of “unfilmables” hitting our screens. There’s Ang Lee with the decade-in-development “Life of Pi,” Lana and Andy Wachowski codirecting “Cloud Atlas” with Tom Tykwer, and the Holy Grail of unfilmed books, “On the Road,” as imagined by Walter Salles.

Once upon a time, fantasy novels were the biggest challenge for filmmakers, but that’s rarely an issue anymore, post-“The Lord of the Rings.” Yet special effects were exactly what made author Yann Martel’s “Life of Pi” such a daunting project: Studios are loathe to green-light aquatic films, which have a rep for coming in late and over budget (“Waterworld” remains a traumatic memory). Lee, the third director to attempt the project, was himself almost shut down by 20th Century Fox, but the giant wave tank he built in an abandoned airplane hanger in Taiwan for the oceanic shots did the trick.

The other problem, and one common to many unfilmable novels, is — as Martel has said in relation to his book — “the reader settles in behind Pi’s eyes, like you would a passenger in a car. … It was very interior and you look out. Well obviously in a movie, the gaze is outside and we are looking at Pi.” Lee and his screenwriter David Magee finessed the problem by having a middle-aged Pi recounting his ordeal to a writer, thus allowing for some narration to frame the story. But, rather typically for the movies, Pi’s philosophical musings are glossed over, while something like the shipwreck, meriting only a relatively brief passage in the book, gets an expanded, heart-pounding 10 minutes.

“Cloud Atlas” (opening March 16 in Japan) presented a different sort of problem: Author David Mitchell had constructed a devilishly complex structure, set over centuries, with six separate yet interlocking stories like the layers of an onion. Mitchell himself has said, “I’m amazed that of all my books, this is the one (they’d) want to make.”

The Wachowskis and Tykwer — no strangers to heady material, having made “The Matrix” and “Run Lola Run,” respectively — weren’t even sure it was possible, and together they spent several months secluded at a Costa Rican beach house, constructing a scene-by-scene breakdown of the novel on a stack of colored index cards, which they would shuffle to see where they could find connections. Their major breakthrough was to have the same actors appear in all the different stories, weaving the varied strands together with a karmic thread; they would be “playing souls, not characters,” as Tykwer put it.

Filmmakers generally hate shifting perspectives, preferring to put the audience in the shoes of the “one man,” but there’s no reason it can’t be done. Robert Altman was famous for flitting from character to character (“Nashville” et al), while Quentin Tarantino did it to great effect in “Pulp Fiction,” yet it remains enough of a turnoff that “Cloud Atlas” found no major studio ready to bite, with Warner Bros. only kicking in a limited sum for distribution rights, resulting in “the most expensive indie film ever.”

Beat icon Jack Kerouac himself tried to interest Marlon Brando in a movie of “On the Road” shortly after it came out in 1957, but to no avail. Nothing happened until Francis Ford Coppola picked up the film rights at the height of his career in 1979. Yet he found no way to translate the book into a film for the next three decades, as various stars and directors came and went. Eventually it went to Brazilian director Walter Salles, who had proven his skill at period road movies with “The Motorcycle Diaries” (Coppola remains involved as an executive producer).

The difficulty with this one is clearly that little actually happens in Kerouac’s semiautobiographical account of crossing the U.S. by car and bus, and the beauty lies in the author’s rhythmic wordplay and descriptive reveries. Salles — who spent a few years just driving the same routes Kerouac took — makes it look authentic, and tries to find a similarly jazzy, improvised feel for his actors. Other beat films — such as “Naked Lunch” or “Howl” — have chosen to mix in the creation myth of the work, with the author as star, and Salles largely does the same, with Sam Riley pounding away at his typewriter and doing a mean impersonation of Kerouac in the prose-dense voice-over.

If one had to point to a Japanese author whose best-known works remain unfilmable, it would have to be Haruki Murakami. “Norwegian Wood” was made with mixed success, but surreal novels such as “The Wind-Up Bird Chronicle” or “Hard-Boiled Wonderland and the End of the World” seem like they’d be a nightmare to adapt. Then again, I can imagine the Wachowskis giving it a shot.

“Life of Pi” opens Jan. 26. “Cloud Atlas” opens March 15. A Japanese release date for “On the Road” has yet to be announced.