VobSub (IDX) is an image-based subtitle format used primarily for DVDs. The IDX file contains timing and coloring information and references the corresponding SUB file, which holds the subtitle images in MPEG-2 streams.

The VobSub Index file, developed in the late 1990s, is part of the VobSub format used for DVDs. The IDX file's ability to support multiple subtitle tracks and detailed timing information made it essential for the diverse language needs of DVD video. While development ceased in the early 2000s, the IDX format remains in use for DVD ripping and archival applications.