Must float on top.
Stereoscopic 3D subtitles are a bit more complex than their 2D counterpart.
The apparent depth of any 3D content can range from infinity behind the screen all the way to the theater space.
Conventional subtitles typically use single color font with an optional drop shadow or an extrusion. Since such subtitles have no stereoscopic parallax their apparent 3D depth places them on the screen plane.
This would be fine if all other content were placed behind the screen and none projected inside the theater space, but this is rarely the case. Placing subtitles on the screen plane while other content is seen in front of the subtitles creates a powerful visual conflict.
To address this issue, S3D subtitles’ depth must change from shot to shot, and on a rare occasion may even change dynamically to accommodate for depth changes in a shot. The challenge is to avoid depth conflicts while keeping the text legible.
3D Blu-ray supports subtitle parallax rendering using depth information contained in the metadata. Theatrical digital projectors currently do not support this feature and require composited (burned-in) subtitles for each foreign language DCP.