SubtitleSafeArea
data class SubtitleSafeArea(val left: Float = 0.0f, val top: Float = 0.0f, val right: Float = 0.0f, val bottom: Float = 0.0f)
The part of the output that subtitles must stay inside, as an inset from each edge.
Each inset is a fraction of the output: left and right of its width, top and bottom of its height. Use it to keep text out of a display cutout, rounded corners, the overscan of a television, or a control bar drawn over the video. Fractions mean the same on every renderer, whatever the pixel density of its output. Set a new value when the output changes shape, for example after a rotation.
The built-in text drawing lays cues out inside what is left. A typeset ASS track keeps its author's placement. docs/subtitle-placement.md has the whole rule.