***ANOTHER EDIT*** Most of the info below is out of date, since you can now set up YouTube 3D videos without the use of tags.
While uploading, go to the "Advanced Settings" tab, under 3D Video select "This video is already in 3D", and then choose from either side-by-side or top/bottom. I recommend prepping your video as side-by-side squeezed with the left eye on the left. It's more commonly used than all the other options, and given YouTube's multiple reinventions of their 3D player, you might find in a few months that your top/bottom or side-by-side full videos don't work.
***EDIT*** - Despite Youtube posting this info earlier this month, it looks like these tags don't all work yet. Based on our quick test of a side-by-side full left/right video, changing yt3d:aspect=16:9 to yt3d:aspect=1.78:1 fixed the aspect ratio problem. The yt3d:swap tag was also still necessary to get the player to recognize the left eye frame was on the left. No word yet on when the switch to the new tags will be fully implemented, but keep these handy for when it happens. If there's some magic to these new tags that we're missing, please let us know in the comments.
From the help page:
YouTube recommends that you upload video in side-by-side squashed Left/Right. We also currently support the following formats:
- yt3d:enable=LR: Side by side squashed left eye frame on left, right eye on right -recommended
- yt3d:enable=RL: Side by side squashed, right eye frame on left, left eye frame on right - old format but no longer recommended
- yt3d:enable=LonR: Top frame is left eye, bottom frame is right eye
- yt3d:enable=RonL: Top frame is right eye, bottom frame is left eye
Those of you posting your own 3D films on Youtube have likely been as frustrated as me by the recent random aspect ratio changes on Youtube 3D videos. It sounds like they're eliminating support for side-by-side full left/right videos.
With the widest user base of any video service delivering 3D content, and the ability for anyone to upload content that can be watched in a dozen different formats, Youtube has done a great service to the 3D community by creating this 3D player, and a disservice by not announcing the changes to their player, or continuing support for early adopters.
Thanks to Greg Penn for sending us this info!