James Cameron is planning to convert Titantic into 3D
Avatar director James Cameron has criticised the process some film-makers use to convert a film originally shot in 2D into the 3D format.
Cameron said filming a movie in 3D - as he did with the Oscar-winning movie Avatar - assures better quality over subsequent conversion.
He said directors "should have been pounding on the gates of the studios saying we want to make movies in 3D."
Cameron also expressed concerns the appetite for 3D films might soon waver.
The director said film-makers are now "paying the price" because the studios are calling the shots.
"The problem is these decisions should be made by film-makers, they shouldn't be made by studios, because if it was up to studios they're going to sacrifice quality for lower cost," Cameron said.
The film-maker has revealed he is planning to convert his 1997 blockbuster Titanic into a 3D release, but said it will be different because he plans to take his time instead of doing a "slapdash conversion".
Avatar was a huge commercial success and was reportedly the most expensive film ever made, with a budget of at least $300m (£185m).
Since the success of Tim Burton's 3D Alice In Wonderland, which was made into 3D after it was filmed, studios are looking to convert more movies.
Clash of the Titans is now expected to be released as a 3D film.
Speaking last month at the launch of Alice, Burton said it is more about technique, rather than the process.
"With all these tools, you can see good 3D, bad 3D, good conversion and bad conversions," he said.
Transformers director Michael Bay has also joined the debate, telling movie industry website Deadline.com that he is "not sold right now on the conversion process".