The makers of the original "Jurassic Park" film are recalling the "nightmare" of using pioneering visual effects, as a 3D version of the Oscar-winning movie is released.

Steven Spielberg's 1993 blockbuster used a combination of stop motion filming with dinosaur models and primitive computer generated technology, which was unprecedented at the time.

The filmmakers talked about the challenges involved at a screening of "Jurassic Park 3D" hosted by the Academy of Motion Picture Arts and Sciences (AMPAS), ahead of its US release on Friday, 20 years after the original.

William Sherak said the most difficult scene was when the T-Rex attacks the visiting guests in their car, in the rain, calling it "my personal nightmare" because of the complexity of effects needed.

"The best moment is the sequence of the car falling down the tree," he added, referring to a later scene in the landmark movie.

Adapted from a Michael Crichton novel, the movie starred Sam Neill, Laura Dern and Jeff Goldblum and won three Oscars in 1994, while making $920 million at the box office globally.

There have been two sequels, in 1997 and 2001, and "Jurassic Park 4" is due out next year.

The 3D version gives extra depth and color to the original, although critics may argue that the adaptation -- rather than making a film in 3D from scratch -- adds little to what was already a gripping and spectacular film.

"This was really the first time the computer graphics were used to make what appeared to be a living animal in a feature film. And we didn't know if we could do it," visual effects specialist Dennis Muren told AFP.

"We started very slowly doing tests to see if we could have a computer image like a dinosaur because if you can do that, you can give them a performance that you can never see with robotics or stop motion or any other way."

He also pointed to his work on 1991's "Terminator 2: Judgment Day," which helped pave the way for the Jurassic Park special effects.

Aged 66, Muren is a Hollywood institution -- he has won six Oscars, notably for special effects for Spielberg's "E.T." (1982), and some of the "Star Wars" and "Indiana Jones" movies.

"The learning curve for me was very steep on Jurassic Park, because I didn't know the tools," said Phil Tippett, another special effects guru who has worked on "Star Wars" and the recent "Twilight" movies.

"It was a time where computer graphic animation wasn't at a very high level, so we developed technology that allowed stop motion animators to be able to manipulate (dinosaurs) -- it wasn't software, it was more mechanical device."

Presenting the Beverly Hills screening, David Cohen said: "If we didn't have 'Jurassic Park' in 1993, we wouldn't have had Richard Parker last year" -- referring to the computer-generated tiger in Ang Lee's stunning "Life of Pi."

Tippett, who shared the 1994 Oscar for special effects with Muren for "Jurassic Park," added: "Technology is always a pain in the ass."

"What you need is creative people, an artist. The technology doesn't do anything. It gets in the way," he said.