What Are PGS Subtitles?

PGS subtitles are image-based subtitles commonly found on Blu-ray discs and inside MKV files created from Blu-ray sources. PGS stands for Presentation Graphic Stream. Unlike SRT, ASS or VTT subtitles, PGS does not contain editable text. Each caption is stored as a bitmap image with its own timing, colors and screen position.

When a PGS subtitle track is extracted from a Blu-ray or MKV file, it is commonly saved with the .sup extension. This is why the terms PGS and SUP frequently appear together in subtitle conversion workflows.

What Does PGS Mean in Subtitles?

PGS means Presentation Graphic Stream. It is associated primarily with Blu-ray subtitle tracks and stores each subtitle cue as an image instead of a string of characters.

A PGS stream includes timing information, screen position data and graphical subtitle objects. The subtitle appearance can include specific fonts, colors, outlines and positions because the appearance is pre-rendered into the subtitle images.

Because the subtitle appearance is pre-rendered, compatible Blu-ray players can display the intended font, color and positioning consistently. This is different from text-based subtitle formats such as SRT, ASS and VTT, where the player must render the text.

Why Does Blu-ray Use Image-Based Subtitles?

Blu-ray uses image-based subtitles for practical playback and presentation reasons.

Consistent rendering

The appearance is stored in the subtitle images instead of being recreated differently by each player.

Complex typography

PGS can preserve custom fonts, outlines, symbols, vertical text, ruby characters and complex writing systems.

Precise positioning

Captions, signs and labels can appear in controlled areas of the screen instead of always appearing at the bottom center.

Forced subtitles and on-screen signs

PGS can be used for foreign-language dialogue, signs, songs and other captions that require deliberate styling or positioning.

Where Are PGS Subtitles Used?

PGS subtitles are commonly used on Blu-ray discs, Ultra HD Blu-ray sources where applicable, MKV files created from Blu-ray sources, Blu-ray remux files, home-media libraries and archived video collections.

They also appear in workflows involving media players and servers such as Plex, Jellyfin and Kodi. MKV is only a container, so not every MKV file contains PGS. An MKV file can contain many subtitle formats, including SRT, ASS, VobSub and PGS.

PGS is normally embedded in a disc structure or video container. When extracted as a standalone subtitle file, it is commonly saved with the .sup extension.

How Do You Identify a PGS Subtitle Track?

You can usually identify a PGS subtitle track by inspecting the media file in FFmpeg, MKVToolNix or a similar media information tool.

FFmpeg

In FFmpeg output, the codec may appear as:

Subtitle: hdmv_pgs_subtitle

This identifies an HDMV PGS image-based subtitle stream.

MKVToolNix

In MKVToolNix, the track may be labelled:

PGS

or:

HDMV PGS

Extracted file

After extraction, the file is commonly named with the .sup extension, for example:

subtitles.sup

What Is the Difference Between PGS and SUP?

PGS and SUP are closely related but describe different parts of the subtitle workflow. PGS, or Presentation Graphic Stream, is the image-based subtitle format. SUP is the file extension commonly used when a PGS subtitle stream is extracted from a Blu-ray disc or MKV container.

Because SUP files commonly contain PGS subtitle data, people often use “SUP subtitles” and “PGS subtitles” interchangeably.

PGS vs SRT: What Is the Difference?

PGS stores what the subtitle looks like. SRT stores what the subtitle says and when it appears.

Which Players Support PGS Subtitles?

PGS subtitle support depends on the player, container and playback device. Blu-ray players are designed for PGS subtitles, and desktop players such as VLC, Kodi and MPC-HC can display many PGS subtitle tracks depending on the file and environment.

Media servers such as Plex and Jellyfin can also work with PGS subtitles, but behavior depends on the playback client. Some clients can display PGS subtitles directly, while others require the server to burn the subtitle images into the video.

Whether PGS triggers transcoding depends on the media server, playback client, video container and device. External SRT subtitles are supported by more devices and can reduce subtitle-related transcoding. For more detail, see the guides for using converted SRT subtitles with Plex and using converted SRT subtitles with Jellyfin.

Why Does Converting PGS to SRT Require OCR?

PGS subtitles contain images, not characters. The visible words cannot be copied directly from the subtitle stream. Optical Character Recognition (OCR) analyzes each subtitle image, recognizes the characters and combines the resulting text with the original timing information to create an SRT file.

The typical workflow is:

1. Parse the PGS stream.
2. Extract subtitle images and timing information.
3. Reconstruct each image using its palette.
4. Run language-specific OCR.
5. Combine the recognized text with the original timestamps.
6. Create the SRT file.
7. Review possible OCR errors.

OCR accuracy depends on the language, font, outline, contrast, resolution and source quality. Clean, high-resolution Blu-ray subtitles generally produce better results than stylized or low-resolution subtitle images. Converted text should be reviewed before professional use.

What Is Preserved When PGS Is Converted to SRT?

PGS to SRT conversion preserves the parts that SRT can store and drops the visual properties that only exist in the image stream.

Usually preserved

• Text recognized by OCR
• Start time
• End time
• Cue order
• Basic line breaks when the conversion option supports them

Not preserved in standard SRT

• Original font
• Font color
• Graphical effects
• Exact screen position
• Bitmap styling
• Image details
• Blu-ray palette information

SRT is designed to store text and timing. It cannot reproduce every visual property contained in a PGS image stream.

How Do You Work With PGS Subtitles?

Most PGS workflows have four stages: identify the track, extract it as SUP, convert the subtitle images to text and review the result.

Step 1: Identify the PGS track

Determine whether the PGS subtitle is embedded in an MKV or Blu-ray source, or whether it has already been extracted as a SUP file.

Step 2: Extract it as SUP

If the subtitle is embedded in an MKV file, use FFmpeg, MKVToolNix or the appropriate Blu-ray workflow to extract it.

With FFmpeg, this command copies a subtitle stream into a standalone SUP file:

ffmpeg -i input.mkv -map 0:s:0 -c:s copy output.sup

The 0:s:0 selector chooses the first subtitle stream, which may not always be PGS. Run ffmpeg -i input.mkv first and identify the hdmv_pgs_subtitle stream. Adjust the stream mapping when necessary. FFmpeg extracts the PGS stream. It does not perform OCR. For a longer walkthrough, see the guide to extract a PGS stream with FFmpeg.

With MKVToolNix, extraction is performed with mkvextract, not with the Start Multiplexing action. Example only:

mkvextract input.mkv tracks 2:output.sup

Track number 2 is only an example. Identify the correct PGS track ID first. MKVToolNix track IDs are not necessarily the same as FFmpeg stream indexes. For a full workflow, see the guide to extract PGS subtitles with MKVToolNix.

For disc sources and remux workflows, read the full guide to extract SUP subtitles from Blu-ray or MKV.

Step 3: Convert the subtitle images to text

After the PGS track is available as a SUP file, use the SUP and PGS to SRT converter to recognize the subtitle images with OCR and create an editable SRT file.

Step 4: Review the result

Open the resulting SRT file in a text editor or subtitle editor such as Subtitle Edit or Aegisub. OCR is not guaranteed to be perfect, so review names, punctuation, line breaks and unusual characters before using the file professionally. For a desktop review workflow, read the Subtitle Edit OCR guide.

Technical Structure of a PGS/SUP File

Most users do not need to understand these components to convert PGS subtitles. However, the structure explains how images, colors, timing and positioning are stored.

A SUP file containing PGS subtitle data is a binary stream made of display sets. Each display set can include several segment types:

• PCS. Presentation Composition Segment: Defines timing, screen dimensions, composition state and which subtitle objects and palettes are used.
• WDS. Window Definition Segment: Defines the window area where subtitle graphics can appear.
• PDS. Palette Definition Segment: Stores palette entries used to turn indexed image data into visible colors and transparency.
• ODS. Object Definition Segment: Stores the actual subtitle bitmap object, compressed with run-length encoding.
• END segment: Marks the end of a display set.

Run-length encoding, often shortened to RLE, compresses repeated pixels in the subtitle image. The player reads the object data, applies the palette information, places the subtitle image at the defined screen position and displays it during the cue timing.

Frequently Asked Questions About PGS Subtitles

Recommended resources

• SUP and PGS to SRT converter
• Extract SUP from Blu-ray or MKV
• FFmpeg PGS extraction guide
• MKVToolNix PGS extraction guide
• PGS subtitles for Plex
• PGS subtitles for Jellyfin