Two years after breakthrough 3D megahit "Avatar," Hollywood is still struggling to decide how best to use the new technology, as filmgoers tire of the novelty and say no to annoying glasses.

While 2011 ends with a couple of well-received 3D movies -- including Steven Spielberg's holiday smash "Tintin" and Martin Scorsese's "Hugo" -- filmmakers need to focus on what works in three dimensions and what doesn't, say experts.

Following a series of 3D flops over the last 12 months, the coming year will see a new crop of releases, including a suped-up version of "Avatar" director James Cameron's record-breaking "Titanic" in April.

But experts say filmmakers can no longer count on the simple fact of putting "Now playing in 3D" on the posters to attract cinema-goers wary of paying a few extra bucks for a questionably improved experience.

"3D film distribution in 2011 has been a lesson in learning for studios and theaters alike," Jeff Bock, senior box office analyst at industry data provider Exhibitor Relations told AFP.

"While there were nearly 40 (3D) films released this year, studios have been scaling back in terms of the type of films released in this new format. The reason? Audiences won't pay for a 3D engagement that isn't a premium picture."

The 3D revolution -- or the latest attempt to bring 3D to cinema, following earlier failed efforts -- comes as the film industry is struggling to reinvent itself as the ways of watching movies multiply.

Hollywood could arguably be said to be seeking its "iTunes moment," like the arrival of the Apple song purchasing site for the music industry, as pirate copies of films proliferated from DVDs to illegal downloads and online streaming.

But signs that 3D has struggled came this year not only from summer box office flops -- "Fright Night," "Conan the Barbarian," "Glee," and "Spy Kids -- but also from flagging sales of 3D televisions.

"Two of the major pain points for consumers are still the price of the TV and the need to wear glasses," market research company NPD said in April, although sales figures later in the year looked better.

Japanese games giant Nintendo was forced to slash the price of its new 3DS console by up to 40 percent in July, following disappointing sales of the new version of its popular console.

Nevertheless filmmakers keep embracing the new technology: at the Busan International Film Festival in South Korea in October, low-budget 3D productions were keen to take on the big studios at their own game.

"Budget doesnt matter, it is story that matters in cinema and its the same when you are using 3D," said South Korean Choo Sang-rok, director of the 3D feature "Persimmon."

Back in Hollywood, Spielberg says that 3D should be used only when appropriate. "I disagree with my colleagues who believe that every film should be in 3D. It's another tool in a very large tool chest," he told the industry daily Variety.

"I think 3D should be used when there is something to be achieved from it, not just to be able to slam the 3D brand on a movie ad," he said.

There have been reports that Spielberg is contemplating making a 3D version of his iconic movie "Jurassic Park" by converting it -- a process which critics say is purely a money-making ploy.

Proper 3D movies are filmed using two slightly off-set cameras, the images from which are fed into a moviegoer's left and right eyes by the glasses handed out to watch the film.

Converted 3D movies -- or "fake 3D" -- are films made with normal cameras, but then processed using computers in post-production to give the impression of multiple layers of depth.

A number other other classic films are also set to be revived in three dimensions, including "Star Wars," said Exhibitor Relations expert Bock. Others, like "The Lion King," have already appeared in 3D.

Scorsese meanwhile said he was apprehensive after deciding to make "Hugo" in 3D. "We were scared," he told the BBC. "It was like walking a tightrope, the whole picture."

"People start to have rules: You can't do this, don't do that," he recalled. "I said, 'I don't want to hear what can't be done." "I'm not usually that intrepid but I wouldn't let people confuse me," he said.

Analysts say Hollywood is not going to turn its back on 3D, but will learn from its mistakes.

"Studios are going to be much more savvy as to what they choose to release in 3D in the years to come. Make no mistake -- 3D is not going anywhere, it's just adapting, as long as the audience responds," said Bock.

Hollywood 3D movies expected out in 2012 include "Men in Black III," "The Amazing Spider Man," "The Avengers" and "The Hobbit," he said, but added that one of the biggest films of the year will only be available in 2D.

"The one that won't be: 'The Dark Knight Rises.' Which will probably be the highest grossing film of the year. So, what does that tell you? It still comes down to story," said the Exhibitor Relations expert.

"Always does. Always will," he said.