I did not see a previous post regarding this feature, so if this has already been requested, I apologize.
I'm not sure how much love this request will get but I would love to see it implemented. I recently bought a 3D TV and so I have been adding split image 3D movies to a separate library section in Plex that I am calling "Movies (3D)." When viewing split image 3D content on Plex I just have to hit the 3D button on my Samsung TV remote to put it in the appropriate mode. This works well for the video, but if I pause, hit info, hit menu, or need to use external subtitles it looks like crap because all of the OSD parts span the whole width of the screen when not in 3D mode, and thus the two different sides overlap when in 3D mode.
My feature request is that when watching a 3D movie in Plex, Plex would enter it's own 3D mode. This would mean that all parts of the OSD be shown on both halves of the screen so that when they are combined and overlaid on top of each other so you still see the normal OSD as if you were watching a non-3D movie. Even better would be if you offset the OSD just a little to give it a 3D effect of floating in front of the movie. This could also be done with external subtitles, which right now are absolutely useless for a 3D movie. You could have Plex apply this to specific library sections, or to all files with certain media tags in their filenames. This seems to have already been implemented to some extent in the Samsung TV client, as it recognizes files with H-SBS or H-TAB as 3D movies split half side by side or top and bottom, respectively, and plays them in the correct mode. Of course, the Samsung Plex client has some major limitations compared to Plex HT and I would much rather use Plex HT on my Mac Mini for all of my Plex viewing. It would be awesome if Plex can also send a signal to my TV through the HDMI connection to tell it to switch to the appropriate 3D mode. However, I'm not sure if that last part is possible, though I don't see why the rest of this proposed feature could not be done.
