Kate is an overlay codec, originally designed for karaoke and text,
that can be multiplexed 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.