FFmpeg Captions and Subtitles (2025) Skill

CRITICAL GUIDELINES

Windows File Path Requirements

MANDATORY: Always Use Backslashes on Windows for File Paths

When using Edit or Write tools on Windows, you MUST use backslashes (\) in file paths, NOT forward slashes (/).

Documentation Guidelines

NEVER create new documentation files unless explicitly requested by the user.

Quick Reference

| Task | Command | |------|---------| | Burn SRT | ffmpeg -i video.mp4 -vf "subtitles=subs.srt" output.mp4 | | Burn ASS | ffmpeg -i video.mp4 -vf "ass=subs.ass" output.mp4 | | Add soft sub | ffmpeg -i video.mp4 -i subs.srt -c copy -c:s mov_text output.mp4 | | Extract sub | ffmpeg -i video.mkv -map 0:s:0 output.srt | | Style subs | -vf "subtitles=s.srt:force_style='FontSize=24,PrimaryColour=&HFFFFFF'" | | Text overlay | -vf "drawtext=text='Hello':x=10:y=10:fontsize=24:fontcolor=white" |

| Format | Extension | Best For | |--------|-----------|----------| | SRT | .srt | Simple, universal | | ASS | .ass | Styled, animated, anime | | VTT | .vtt | Web/HTML5 video |

When to Use This Skill

Use for subtitle and caption operations:

Hardcoding (burning) subtitles into video
Adding soft subtitle tracks to containers
Extracting subtitles from MKV/MP4
Styling captions (font, color, position)
Dynamic text overlays

FFmpeg Captions and Subtitles (2025)

Complete guide to working with subtitles, closed captions, and text overlays using FFmpeg.

Subtitle Format Reference

Supported Formats

| Format | Extension | Features | Use Case | |--------|-----------|----------|----------| | SubRip | .srt | Simple timing + text | Universal, web | | Advanced SubStation Alpha | .ass/.ssa | Rich styling, positioning, effects | Anime, styled subs | | WebVTT | .vtt | Web standard, cues, styling | HTML5 video | | TTML/DFXP | .ttml/.dfxp | Broadcast, accessibility | Streaming services | | MOV Text | .mov (embedded) | QuickTime native | Apple ecosystem | | DVB Subtitle | (embedded) | Bitmap-based | European broadcast | | PGS | .sup | Blu-ray bitmap subtitles | Blu-ray | | CEA-608/708 | (embedded) | Closed captions | US broadcast, streaming |

Format Characteristics

SRT (SubRip):
- Simple text-based format
- Supports basic HTML tags (<b>, <i>, <u>)
- Widely compatible
- No positioning or advanced styling

ASS/SSA:
- Advanced styling (fonts, colors, outlines)
- Precise positioning anywhere on screen
- Animation and effects support
- Karaoke timing

WebVTT:
- HTML5 standard format
- CSS-like styling
- Cue settings for positioning
- Speaker identification

Burning Subtitles (Hardcoding)

Basic Subtitle Burn-in

# Burn SRT subtitles
ffmpeg -i video.mp4 -vf "subtitles=subs.srt" output.mp4

# Burn ASS/SSA subtitles (preserves styling)
ffmpeg -i video.mp4 -vf "ass=subs.ass" output.mp4

# Burn subtitles from MKV container
ffmpeg -i video.mkv -vf "subtitles=video.mkv" output.mp4

# Burn specific subtitle track (index 0)
ffmpeg -i video.mkv -vf "subtitles=video.mkv:si=0" output.mp4

# Burn subtitles with stream index
ffmpeg -i video.mkv -vf "subtitles=video.mkv:stream_index=1" output.mp4

Styled Subtitle Burn-in

# Force style (overrides subtitle styling)
ffmpeg -i video.mp4 \
  -vf "subtitles=subs.srt:force_style='FontName=Arial,FontSize=24,PrimaryColour=&HFFFFFF,OutlineColour=&H000000,Outline=2,Shadow=1'" \
  output.mp4

# Yellow subtitles with black outline
ffmpeg -i video.mp4 \
  -vf "subtitles=subs.srt:force_style='FontSize=28,PrimaryColour=&H00FFFF,OutlineColour=&H000000,Outline=3'" \
  output.mp4

# Larger font for accessibility
ffmpeg -i video.mp4 \
  -vf "subtitles=subs.srt:force_style='FontSize=36,Bold=1'" \
  output.mp4

ASS Style Parameters

| Parameter | Description | Example | |-----------|-------------|---------| | FontName | Font family | FontName=Arial | | FontSize | Size in points | FontSize=24 | | PrimaryColour | Text color (AABBGGRR) | PrimaryColour=&HFFFFFF | | SecondaryColour | Karaoke color | SecondaryColour=&H00FFFF | | OutlineColour | Outline/border color | OutlineColour=&H000000 | | BackColour | Shadow/background color | BackColour=&H80000000 | | Bold | Bold text (0/1) | Bold=1 | | Italic | Italic text (0/1) | Italic=1 | | Underline | Underlined text (0/1) | Underline=1 | | Outline | Outline width | Outline=2 | | Shadow | Shadow depth | Shadow=1 | | Alignment | Position (numpad style) | Alignment=2 | | MarginL/R/V | Margins in pixels | MarginV=50 |

Color Format (ASS)

ASS uses &HAABBGGRR format (Alpha, Blue, Green, Red):
- White: &HFFFFFF or &H00FFFFFF
- Black: &H000000 or &H00000000
- Yellow: &H00FFFF (00-Blue, FF-Green, FF-Red)
- Red: &H0000FF
- Blue: &HFF0000
- 50% transparent black: &H80000000

Adding Subtitle Tracks (Soft Subs)

Embed SRT as Track

# Add SRT to MP4 (MOV text)
ffmpeg -i video.mp4 -i subs.srt \
  -c copy -c:s mov_text \
  output.mp4

# Add SRT to MKV
ffmpeg -i video.mp4 -i subs.srt \
  -c copy -c:s srt \
  output.mkv

# Add SRT to WebM (WebVTT)
ffmpeg -i video.webm -i subs.srt \
  -c copy -c:s webvtt \
  output.webm

Multiple Subtitle Tracks

# Add multiple languages
ffmpeg -i video.mp4 -i subs_en.srt -i subs_es.srt -i subs_fr.srt \
  -map 0:v -map 0:a -map 1 -map 2 -map 3 \
  -c copy -c:s mov_text \
  -metadata:s:s:0 language=eng -metadata:s:s:0 title="English" \
  -metadata:s:s:1 language=spa -metadata:s:s:1 title="Spanish" \
  -metadata:s:s:2 language=fra -metadata:s:s:2 title="French" \
  output.mp4

# Add ASS subtitles to MKV (preserves styling)
ffmpeg -i video.mp4 -i styled.ass \
  -map 0 -map 1 \
  -c copy -c:s ass \
  output.mkv

Set Default Subtitle Track

# Set subtitle as default
ffmpeg -i video.mp4 -i subs.srt \
  -c copy -c:s mov_text \
  -disposition:s:0 default \
  output.mp4

# Set forced subtitles (always display)
ffmpeg -i video.mp4 -i forced.srt \
  -c copy -c:s mov_text \
  -disposition:s:0 forced \
  output.mp4

Extracting Subtitles

Extract to External File

# Extract first subtitle track to SRT
ffmpeg -i video.mkv -map 0:s:0 output.srt

# Extract specific subtitle stream
ffmpeg -i video.mkv -map 0:s:1 output.srt

# Extract to ASS format
ffmpeg -i video.mkv -map 0:s:0 output.ass

# Extract to WebVTT
ffmpeg -i video.mkv -map 0:s:0 output.vtt

# Extract all subtitle tracks
ffmpeg -i video.mkv -map 0:s subs_%d.srt

List Available Subtitle Tracks

# Show all streams including subtitles
ffprobe -v error -show_entries stream=index,codec_name,codec_type:stream_tags=language,title \
  -of csv=p=0 video.mkv

# Show only subtitle streams
ffprobe -v error -select_streams s \
  -show_entries stream=index,codec_name:stream_tags=language,title \
  -of csv=p=0 video.mkv

Convert Subtitle Formats

# SRT to ASS
ffmpeg -i subs.srt subs.ass

# ASS to SRT (loses styling)
ffmpeg -i subs.ass subs.srt

# SRT to WebVTT
ffmpeg -i subs.srt subs.vtt

# WebVTT to SRT
ffmpeg -i subs.vtt subs.srt

drawtext Overlays and Whisper AI Integration

Detailed examples for drawtext overlays (fonts, positioning, escaping, time expressions, boxes, outlines, animated text) and Whisper AI transcription / subtitle generation workflows live in references/drawtext-and-whisper.md. Load that reference when creating burned-in text graphics or generating subtitles from speech.

CEA-608/708 Closed Captions

Extract Closed Captions

# Extract CEA-608 captions from ATSC stream
ffmpeg -f lavfi -i "movie=broadcast.ts[out0+subcc]" -map 0:1 captions.srt

# Extract from video with embedded CC
ffmpeg -i video_with_cc.mp4 \
  -filter_complex "[0:v]format=yuv420p[v];[0:v]crop=1:1:0:0[c]" \
  -map "[c]" -c:v libx264 -f null - 2>&1 | grep -A 1 "Closed caption"

Add CEA-608 Captions

# Embed CEA-608 captions (requires eia608 line)
ffmpeg -i video.mp4 -i captions.scc \
  -c:v libx264 -c:a copy \
  -vf "movie=captions.scc[captions];[0:v][captions]overlay" \
  output.mp4

Subtitle Positioning

ASS Alignment Values

7 (top-left)     8 (top-center)     9 (top-right)
4 (mid-left)     5 (mid-center)     6 (mid-right)
1 (bottom-left)  2 (bottom-center)  3 (bottom-right)

Position Examples

# Top center subtitles
ffmpeg -i video.mp4 \
  -vf "subtitles=subs.srt:force_style='Alignment=8,MarginV=20'" \
  output.mp4

# Left-aligned subtitles
ffmpeg -i video.mp4 \
  -vf "subtitles=subs.srt:force_style='Alignment=1,MarginL=50,MarginV=30'" \
  output.mp4

# Right side for speaker identification
ffmpeg -i video.mp4 \
  -vf "subtitles=subs.srt:force_style='Alignment=3,MarginR=50'" \
  output.mp4

Multiple Subtitle Positions

# Two subtitle tracks at different positions
ffmpeg -i video.mp4 \
  -vf "[in]subtitles=speaker1.srt:force_style='Alignment=1,MarginL=50'[tmp];\
       [tmp]subtitles=speaker2.srt:force_style='Alignment=3,MarginR=50'" \
  output.mp4

Batch Processing

Burn Subtitles to Multiple Videos

#!/bin/bash
# burn_subs_batch.sh

for video in *.mp4; do
    base="${video%.mp4}"
    if [ -f "${base}.srt" ]; then
        ffmpeg -i "$video" \
          -vf "subtitles=${base}.srt:force_style='FontSize=24,Outline=2'" \
          -c:a copy \
          "output/${base}_subbed.mp4"
    fi
done

Convert Subtitle Format Batch

#!/bin/bash
# convert_subs.sh

for srt in *.srt; do
    base="${srt%.srt}"
    ffmpeg -i "$srt" "${base}.vtt"
done

Troubleshooting

Common Issues

"Unable to find a suitable output format"

# Specify output format explicitly
ffmpeg -i video.mkv -map 0:s:0 -f srt output.srt

Garbled characters in subtitles

# Force UTF-8 encoding
ffmpeg -i video.mp4 \
  -vf "subtitles=subs.srt:charenc=UTF-8" \
  output.mp4

Font not found

# Specify fonts directory
ffmpeg -i video.mp4 \
  -vf "subtitles=subs.srt:fontsdir=/path/to/fonts" \
  output.mp4

# List available fonts
fc-list : family | sort | uniq

Subtitle timing offset

# Delay subtitles by 2 seconds
ffmpeg -i video.mp4 \
  -vf "subtitles=subs.srt:itsoffset=2" \
  output.mp4

# Or use setpts to adjust
ffmpeg -i subs.srt -itsoffset 2 delayed.srt

Subtitle not showing on high-res video

# Scale subtitle rendering to video resolution
ffmpeg -i video_4k.mp4 \
  -vf "subtitles=subs.srt:force_style='FontSize=48,Outline=3'" \
  output.mp4

Verification Commands

# Check if subtitles are present
ffprobe -v error -select_streams s -show_entries stream=codec_name -of default=nw=1 video.mp4

# Count subtitle lines
grep -c "^[0-9]" subs.srt

# Validate SRT format
ffmpeg -i subs.srt -f null -

Best Practices

Accessibility

Use high contrast colors (white on dark, yellow on dark)
Minimum font size of 24pt for standard video
Include speaker identification for multiple speakers
Position subtitles to avoid obscuring important visual content
Keep lines short (42 characters max per line)
Display duration: minimum 1 second, maximum 7 seconds per caption

Quality

Use ASS format for styled subtitles (anime, music videos)
Use SRT for simple dialogue
Use WebVTT for web delivery
Preserve original styling when possible
Test on target devices before distribution

Performance

Burn subtitles only when necessary (streaming, compatibility)
Prefer soft subs for archival and flexibility
Use hardware encoding when burning subtitles to large files
Process in parallel for batch operations

This guide covers FFmpeg subtitle and caption operations. For text overlays with shapes and graphics, see the shapes-graphics skill.

Agent Skills: FFmpeg Captions and Subtitles (2025)

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