.SUP

Introduction

Blu-ray discs store subtitles as pictures, not text, in a format called PGS (Presentation Graphic Stream). When the subtitle track is extracted from a disc or an MKV file, it is saved as a .sup file. Each subtitle is a run-length encoded bitmap with a colour palette, placed at a position on screen.

Because the subtitles are images, they cannot be edited or restyled as text. Tools such as Subtitle Edit use optical character recognition to turn them into SRT.

Structure

A SUP file is a series of segments. Each starts with a 13-byte header: the magic bytes PG (50 47), a 32-bit presentation timestamp and a 32-bit decoding timestamp on a 90 kHz clock, a segment type byte and a 16-bit size. All numbers are big-endian.

The segment types, in hexadecimal, are 16, the presentation composition, which says what to show and when; 17, the window definition; 14, a palette; 15, an object holding the run-length encoded bitmap; and 80, the end of a display set. A display set, from a composition segment to an end segment, puts one subtitle on screen or clears it.

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