The first three Shrek films are to be converted into 3D, Dreamworks studio has announced.Ahead of the release of the fourth film in the series, Shrek Forever After, the studio said it was converting the first three for a 3D Blu-ray release.Shrek Forever After will open the Tribeca Film Festival on 21 April and be released in cinemas in July.It is the final film in the series and reunites stars Eddie Murphy, Cameron Diaz, Mike Myers and Antonio Banderas.Shrek Forever After will be released as a 3D film in cinemas, while Shrek, Shrek 2 and Shrek the Third, will all be converted for home release.Dreamworks chief executive Jeffrey Katzenberg said all of DreamWorks' animated films were created in 3D format from the start and was confident of a good final product."Our movies exist in digital files to begin with," he told the annual National Association of Broadcasters convention."To go back and rebuild to a quality 3D experience is not inexpensive, but we are about to achieve a pretty high quality result."The tales of the loveable but grumpy green ogre have made more than $2bn (£1.3bn) worldwide since 2001.The latest in the franchise sees Shrek duped into signing a contract and transported to an alternate version of Far Far Away.Cheap processKatzenberg said he was hopeful extra box office revenue from 3D films could more than make up for the money lost from falling sales of DVDs."Suddenly, the theatrical marketplace has turned in the opposite direction," he said."Whatever the decline would be for [the latest] Shrek when it goes to the home video market will be more than offset by the incremental revenue we'll see from 3D," he said.But he urged studios to keep up the quality of 3D, criticising the recent Clash of the Titans, which was converted from 2D using what he called a "quick and cheap post-production process".CautionHe compared it to Avatar which was was shot with 3D cameras, with actors in special suits that allowed their movements and expressions to be recreated in 3D using computer software."I just want us all to be cautious and to err to the side of delivering more than expected, not the minimum level or less than expected," Katzenberg said."You can't ask people to pay more without giving them more."Clash of the Titans has grossed $231m (£149.9m) worldwide since it was released earlier this month April.Source: BBC