Kate is an overlay codec, originally designed for karaoke and text, that can be
multiplixed in Ogg. Text and images can be carried by a Kate stream, and
animated. Most of the time, this would be multiplexed with audio/video to carry
subtitles, song lyrics (with or without karaoke data), etc, but doesn't have to
be.

Series of curves (splines, segments, etc) may be attached to various properties
(text position, font size, etc) to create animated overlays. This allows
scrolling or fading text to be defined. This can even be used to draw arbitrary
shapes, so hand drawing can also be represented by a Kate stream.

Homepage:
https://wiki.xiph.org/index.php/OggKate
