Burning and Embedding Subtitles — SRT, ASS, Hard Subs and Soft Subs

There are two main methods for adding subtitles to video: hard subs (burn-in) and soft subs (embedding). This article explains how each works and how to use them with FFmpeg.

Differences Between Hard Subs and Soft Subs

Hard subs require re-encoding and take longer to process, but are guaranteed to display in any player. Soft subs often only require a copy operation and are faster, but are limited to compatible players.

Hard Subs (Burn-in)

Burning SRT Subtitles — subtitles Filter

The subtitles filter uses libass internally to render subtitle files onto the video. Verified with FFmpeg 6.1.

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

.srt files are not in KNOWN_EXTS, so they are shown as a text block. Replace subtitle.srt with the actual file path when running.

To adjust font size or position:

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

You can override settings using ASS style format in force_style (FontSize, PrimaryColour, Alignment, etc.).

Burning ASS Subtitles — ass Filter

ASS (Advanced SubStation Alpha) is a subtitle format that supports advanced formatting including fonts, colors, and animations. While the subtitles filter can also read ASS files, the ass filter reproduces styles more faithfully.

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

subtitles=subtitle.ass and ass=subtitle.ass produce nearly identical output, but the ass filter uses an ASS-specific initialization path. For files with complex styles, the ass filter is recommended.

Font Specification (ASS)

Specify Fontname in the [V4+ Styles] section of the ASS file. If you are using a font not installed on the system, you can specify a font directory with the fontsdir option.

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

Soft Subs (Embedding)

Embedding a Subtitle Track into MKV

The MKV container supports multiple subtitle tracks. Here is the basic command for adding an SRT subtitle while copying the video and audio:

ffmpeg -i input.mp4 -i subtitle.srt -c copy -c:s srt output.mkv

• -c copy copies video and audio without re-encoding
• -c:s srt embeds the subtitle stream in SRT format
• Shown as a text block because the .srt input file is required

To embed multiple language subtitles:

ffmpeg -i input.mp4 -i sub_ja.srt -i sub_en.srt \
  -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 while preserving them, use -c:s copy.

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

Since the input file for CI testing does not have subtitle tracks, this command is shown as a text block.

Converting Subtitles from MKV to MP4

MP4 uses mov_text (SRT-based) as its subtitle format. To convert subtitle tracks from MKV to MP4:

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

Notes:

• Converting ASS/SSA subtitles to mov_text will lose style information
• MP4 generally has fewer players with subtitle support; MKV has higher compatibility
• If the playback environment is limited, consider switching to hard subs

Checking and Extracting Subtitle Tracks

To check the embedded subtitle tracks, use ffprobe (no output file needed):

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

To extract subtitles as an external file:

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

Summary

Related Articles:

• Basics of Video Format Conversion
• FFmpeg Basic Syntax

Tested with: ffmpeg 6.1.1 / Ubuntu 24.04 (GitHub Actions runner)
Primary sources: ffmpeg.org/ffmpeg-filters.html / ffmpeg.org/ffmpeg.html / trac.ffmpeg.org/wiki/HowToBurnSubtitlesIntoVideo