subpicture

Subpicture refers to a graphics overlay stream used in digital video playback to render subtitles and other on-screen graphics. It is separate from the main video and audio streams and is composited by the player onto the video image.

Subpicture data is typically bitmap-based and timed, with each subpicture image associated with a time interval and stored alongside the video data in container formats such as DVD-Video and Blu-ray. A subpicture consists of one or more graphic images with a color palette and transparency information; pixels can be foreground shapes with defined colors and per-pixel or per-palette transparency, allowing smooth edges and see-through areas.

In DVD-Video, subpictures are used to display subtitles and to indicate menu selections or closed-captioning. The data is delivered as a separate subpicture stream (often rendered as VOBSUB) and is synchronized with playback by the player's subtitle timing. In Blu-ray, the similar functionality is provided by Presentation Graphic Streams (PGS).

Subpictures differ from text-based subtitle tracks (e.g., SRT) in that they are raster images, allowing complex styling, backgrounds, and non-Latin scripts with consistent rendering on all devices. They can also implement effects such as highlighting, color changes, or animation of the subtitle graphics. Applications extend to other overlays: chapter markers, on-screen prompts, or UI highlights in menus. The term “subpicture” designates the overlay layer responsible for these graphics, separate from the core video and audio streams.