Nintendo has admitted there are problems remaking old games in stereoscopic 3D.

Designers got stuck adding the extra dimension when they found out that it altered gameplay. Nintendo's Takao Nakano, leader of the 3D Classics remakes, told Satoru Iwata that large portions of game code had to be completely re-written.

"In the original version, the game unfolds on a flat surface," Nakano explained, talking of upcoming re-release Xevious. But in the new version, the player's ship Solvalou is seen to be floating in mid-air.

"If the bomb suddenly appears - zhing! - at the same altitude as Solvalou... We were like, 'Huh? Something doesn't feel right!' Everything was off!

"The actual game was made in 2D, so the bombs strike Solvalou the moment they're fired. But for the Nintendo 3DS system, we had to create an interval between when they are fired and when they strike. There were all kinds of discrepancies like that," Nakano explained.

The projects already take "20 times the work" of a simple port, Nakano revealed - but the remake plans were once even more elaborate.

"At first, since it was new hardware, we tried making the graphics richer, but that didn't impress much," Nakano said. So he, Iwata and Miyamoto decided to leave the vintage visuals alone.

Also culled was a 3D Classics version of NES Tennis. It fell foul of court perspective problems and programming issues after changing collision detection for the ball and racket into 3D. "It took as much work as making a tennis game from scratch," Nakano elaborated.

Despite these problems, four unannounced 3D Classics games are still in the works.

Xevious joins existing 3D Classics title Excitebike for download in Europe this month.