FFmpeg Subtitles — Burn SRT/ASS into Video

There are two ways to add subtitles to a video: burn them into the picture (hard subs) or store them as a separate track (soft subs).

Differences Between Hard Subs and Soft Subs

Hard subs need a re-encode, so they take longer, but they show in any player. Soft subs usually need only a copy, so they are fast, but only players that support subtitle tracks show them.

Hard Subs (Burn-in)

Burning SRT Subtitles — subtitles Filter

The subtitles filter draws a subtitle file onto the video with the libass library.

ffmpeg -i input.mp4 -vf subtitles=subtitle.srt output.mp4

Replace subtitle.srt or subtitle.ass with the path to your own subtitle file.

To change the font size or position:

ffmpeg -i input.mp4 -vf "subtitles=subtitle.srt:force_style='FontSize=24,Alignment=2'" output.mp4

force_style overrides the style with ASS style fields such as FontSize, PrimaryColour and Alignment.

Burning ASS Subtitles — ass Filter

ASS (Advanced SubStation Alpha) is a subtitle format that can set fonts, colors and animations. The subtitles filter can read ASS files too.

ffmpeg -i input.mp4 -vf ass=subtitle.ass output.mp4

subtitles=subtitle.ass and ass=subtitle.ass both use libass, so the result looks the same. The difference: the ass filter reads only ASS files and has no force_style or charenc option.

Font Specification (ASS)

Set the font with Fontname in the [V4+ Styles] section of the ASS file. If the font is not installed on the system, point the fontsdir option at the folder that contains it:

ffmpeg -i input.mp4 -vf "ass=subtitle.ass:fontsdir=/path/to/fonts" output.mp4

Soft Subs (Embedding)

Embedding a Subtitle Track into MKV

An MKV file can hold several subtitle tracks. This command adds an SRT file as a subtitle track and copies the video and audio as they are:

ffmpeg -i input.mp4 -i subtitle.srt -map 0:v -map 0:a -map 1 -c copy -c:s srt output.mkv

• -map 0:v -map 0:a -map 1: selects the video and audio of input.mp4 and the subtitles in subtitle.srt. Without it, if input.mp4 already has a subtitle track, FFmpeg keeps that track and leaves out subtitle.srt
• -c copy: copies the video and audio without re-encoding
• -c:s srt: stores the subtitles in SRT format

For subtitles in several languages, list every stream with -map. Without -map, only one subtitle track is kept:

ffmpeg -i input.mp4 -i sub_ja.srt -i sub_en.srt \
  -map 0:v -map 0:a -map 1 -map 2 \
  -c copy -c:s srt \
  -metadata:s:s:0 language=jpn \
  -metadata:s:s:1 language=eng \
  output.mkv

Copying Subtitle Streams (Existing Tracks)

To convert an MKV file that already has subtitle tracks and keep them, use -c:s copy. Add -map 0 as well: without it, only one audio track and one subtitle track are kept.

ffmpeg -i input.mkv -map 0 -c:v libx264 -c:a aac -c:s copy output.mkv

Converting Subtitles from MKV to MP4

MP4 stores subtitles as mov_text (3GPP Timed Text, also called tx3g). To convert the subtitle tracks when you change an MKV into an MP4:

ffmpeg -i input.mkv -c:v copy -c:a copy -c:s mov_text output.mp4

Keep in mind:

• ASS/SSA styling is lost when you convert to mov_text
• Whether the subtitles show depends on the player
• If the target player has limited subtitle support, use hard subs instead

Checking and Extracting Subtitle Tracks

To list the subtitle tracks in a file, use ffprobe:

ffprobe -v error -show_streams -select_streams s input.mkv

To save the subtitles as a separate file:

ffmpeg -i input.mkv -c:s srt extracted_subtitle.srt

This works only for text subtitles such as SRT and ASS. Image subtitles, such as PGS from Blu-ray or VobSub from DVD, cannot be turned into SRT: FFmpeg stops with Subtitle encoding currently only possible from text to text or bitmap to bitmap.

Summary

Related Articles:

• Basics of Video Format Conversion
• FFmpeg Basic Syntax