VobSub

VobSub, short for Video Object Subtitles, is a bitmap-based subtitle format used in DVD-Video. It stores subtitle data as graphic images that are overlaid on video during playback.

Files and structure: VobSub subtitles are typically distributed as a pair of files with the same base name: a binary subtitle image file with the .sub extension and a companion index file with the .idx extension. The .idx file is an ASCII index describing each subtitle's timing, palette, and reference to the corresponding image data in the .sub file. Each entry specifies when a subtitle should appear and disappear, the location and size of the image, and the color palette used to render the image.

How it works: The video player reads the time stamps from the index file and overlays the corresponding bitmap from the .sub file on top of the video at the designated position. The color palette is defined in the index or embedded in the image data, allowing for simple color rendering with transparency.

Usage and compatibility: VobSub subtitles are common with ripped DVD content and are supported by many media players, including VLC and MPC-HC, when the .idx and .sub pair is present. They are an example of a raster subtitle format, as opposed to text-based subtitle formats like SubRip (.srt) or WebVTT. In containers such as Matroska, VobSub streams can be imported as external subtitle tracks.

Limitations: Because they are bitmap-based, VobSub subtitles require more storage than text-based subtitles and are not easily searchable or editable; editing typically requires specialized tools. They remain tied to the DVD subtitle specification and its descendants, such as PGS in Blu-ray.