Presentation Graphics Stream (PGS or PG) is a bitmap graphics format commonly used for subtitles.

Presentation Graphics

Type of format	Graphics & Subtitle Format
First released	2006
Developer	Blu-ray Disc Association (Panasonic)
Filename extension	None*
Limits	32 Streams
Open Format?	Yes
Free Format?	?
Magic Number	"PG" 0x5047

The format is similar to Interactive Graphics, but instead of using it for interactive menus, it is used for subtitles or rarely decorations. It is an vast improvement over DVD subtitles.

Presentation Graphics are displayed on its own independent layer behind the Interactive Graphics; it is similar to Interactive Graphics (IG) used for menus (both layers are on top of the Primary Video). The graphics (along with its data and X-Y coordinates) are stored inside M2TS files that contains the video and audio streams. The graphics are decoded from the Blu-ray player's special graphics decoder. It can only use 32 streams. The max graphics memory buffer size is 4 MB. Presentation Graphics are available on both HDMV and BD-J.**

Example of PGS on top of the Primary Video using different colors and sizes.

Features:   

• It uses simple 8-bit colors and transparency (256 color palette), this is useful for simple graphics but complex ones like photos will cause dithering a la GIF.  

•  Unlike Java Graphics (PNG, JPEG), Presentation Graphics use simple animations: Fade In/Out, Color changing, Wipe In/Out, and Scrolling. Supports frame rates up to 30 Hz. Despite being simple, they take a lot of space in the graphics memory buffer if pushing it to its limits.

•  It's resolution plane size supports 1920x1080/1280x720/720x480/720x576. 

•  Uses Run-length encoding, a simple compression method.

Presentation Graphics cannot be "opened" on a normal image program on a computer, it must be opened by demuxing the M2TS file or a specialized Blu-ray/subtitling editing software such as BDedit, BDReauthor, Scenarist, etc. When a PGS is extracted/demuxed from the M2TS file, it's usually saved in a file with the .sup extension (Subtitle Presentation).

Data

Data inside a PGS is made of several sections, each have the following header:

Presentation Graphics are divided 5 segments:

• Presentation Composition Segment (PCS)
• Window Definition Segment (WDS)
• Palette Definition Segment (PDS)
• Object Definition Segment (ODS)
• End of Display Set Segment (END)

Presentation Composition Segment (PCS) is used for composing the graphic's sub-picture. It is also called the Control Segment because it indicates a new Display Set (DS) definition, composed by definition segments (WDS, PDS, ODS) until an END segment is found.

In a Display Set, there can be several windows, palette and object definitions, and the composition objects define what's to be shown on screen.

Presentation Composition Segment fields:

Composition Objects define the position for each image to be displayed:

Window Definition Segment (WDS) defines the rectangular area on the screen where the graphic's sub-picture will be shown. This rectangular area is called a Window. This segment defines several windows, and all the fields from Window ID up to Window Height will repeat each other in the segment defining each window.

Palette Definition Segment (PDS) is used to define a palette for color conversion. Multiple palette entries are allowed, each with a unique palette ID, enabling the last five fields to repeat.

Object Definition Segment (ODS) defines the graphics object. These are images with rendered text on a transparent background.

End of Display Set Segment (END) always has a segment size of zero and indicates the end of a Display Set (DS) definition. It appears immediately after the last ODS in one DS.

Example of how subtitles are display. An Epoch in the drawing includes a DS1 (Epoch Start), a DS2 (Normal Case), and a DS3 (Normal Case). The DS1 contains a WDS for specifying the Window in which the subtitles are displayed, an ODS for specifying the line “Actually ... I was hiding my feelings.”, and a first PCS. The DS2 contains a second PCS, and the DS3 contains a third PCS.

Subtitles

Presentation Graphics are commonly used for subtitles for the deaf/hard-of-hearing (SDH), translations, karaoke, and sing-a-along lyrics on screen. Basic subtitles for SDH in mainstream movies typically use white fonts and black outlines. Subtitles for foreign movies typically use either white or yellow colored fonts with black outlines, not only for dialog, but also signs and lyrics.

Example of speaker ID in SDH (voices are off-screen)

Example of non-speech information in SDH (source is off-screen)

Example of non-speech information in SDH (two sources: off-screen and on-screen)

Example of music description in SDH

Example of lyrics transcription in SDH

For diffrent languages, Presentation Graphics Streams are tied with its language code defined in ISO 639-2/T. Each code represents names of language by three characters, e.g. ENG, JAP, etc. 

Forced Subtitles
Some commercial Blu-rays contain forced subtitles, meaning they cannot be turned off due to the HDMV programming, this can be "fixed" by ripping the BD, modifying the code (or M2TS file), and making a new version to burn on a blank BD-R. The reason why some PGS cannot be turned off is either artistic intent or using it for decorative menus. 

Footnotes

• *It doesn't use a file extension in a traditional sense. BD editing programs use other image formats and converts them to PG. When a PGS is extracted/demuxed from the M2TS file, it's usually saved in a file with the .sup extension (Subtitle Presentation). Scenarist also uses the .pes file format for import, export, and editing (doesn't contain any magic numbers).
• ** Presentation Graphics Streams and Text Subtitle Streams cannot be used and displayed at the same time. 

Sources

• TheScorpius666 (2017-07-15). "Presentation Graphic Stream (SUP files) BluRay Subtitle Format". blog.thescorpius.com.
• Scenarist BD Development manual, BD-ROM Whitepaper (2010), and original research.
• Blu-ray Disc Demystified
• BDedit 
• PGS specification is defined in the US Patent US 20090185789 A1.

Author(s) : Æ Firestone