YouTube is announcing today a new feature to convert existing uploaded videos to 3D. By tonight, users clicking the Edit Info button on an upload’s Watch page will see the option to convert already-uploaded or new standard two-dimensional clips to 3D.

Google engineers wanted to simplify three-dimensional video after noting a steep rise in uploads from a new crop of 3D-capable cellphones and camcorders. Their output can be viewed on a variety of PC, television, game and cellphone screens with differing and sometimes incompatible technologies. Jonathan Huang, YouTube’s product manager for 3D, said “We wanted to make the barriers evaporate”.

By tracking a clip’s motion and color cues for depth and distance calculations, a second set of images are constructed as seen from an offset angle as with a pair of human eyes, making a true stereoscopic video pair. Click here to see an example.

Viewers will see a small red 3D button at the lower right of frame with choices for different types of colored glasses, monitors or televisions and as plain old two-dimensional video, or even in black and white, useful when content color matches glasses colors.

The net effect with existing videos can be striking, even on the screens of 3D-capable cellphones.

But save any red-green paper glasses that come your way, as they do even better.

Be ready to duck.