Video Assembly

When to Use

• Combine multiple production outputs (video clips, audio files, subtitles) into a final deliverable.
• Final stage of the video production pipeline: concatenation, audio mixing, transitions, and format conversion.
• Trigger keywords: assemble video, combine clips, add audio, mix audio, burn subtitles, video transitions, format conversion, normalize audio.

Prerequisites

• FFmpeg installed and available in system PATH.
• For complex compositions: Remotion and Node.js installed (delegate to remotion-best-practices).
• Windows host primary (PowerShell). Commands below use PowerShell syntax.

Procedure

Mode 1: FFmpeg (Simple, Fast)

Best for: Concatenation, audio mixing, basic transitions, format conversion.

1. Concatenate Video Clips

# Create file list (PowerShell)
"file 'clip1.mp4'" | Out-File -FilePath filelist.txt -Encoding ascii
"file 'clip2.mp4'" | Out-File -FilePath filelist.txt -Encoding ascii -Append
"file 'clip3.mp4'" | Out-File -FilePath filelist.txt -Encoding ascii -Append

# Concatenate (same codec/resolution)
ffmpeg -f concat -safe 0 -i filelist.txt -c copy output.mp4

# Concatenate (different codecs/resolutions - re-encode)
ffmpeg -f concat -safe 0 -i filelist.txt -vf "scale=1920:1080:force_original_aspect_ratio=decrease,pad=1920:1080:(ow-iw)/2:(oh-ih)/2" -c:v libx265 -crf 23 -preset medium -c:a aac -b:a 192k output.mp4

2. Add Audio to Video

# Replace audio
ffmpeg -i video.mp4 -i audio.wav -c:v copy -c:a aac -map 0:v:0 -map 1:a:0 output.mp4

# Mix audio (keep original + add music)
ffmpeg -i video.mp4 -i music.mp3 -filter_complex "[0:a][1:a]amix=inputs=2:duration=first:dropout_transition=3" -c:v copy output.mp4

# Add audio with volume control
ffmpeg -i video.mp4 -i bgm.mp3 -filter_complex "[1:a]volume=0.3[bg];[0:a][bg]amix=inputs=2:duration=first" -c:v copy output.mp4

3. Add Subtitles

# Burn subtitles (SRT file)
ffmpeg -i video.mp4 -vf "subtitles=subs.srt:force_style='FontName=Arial,FontSize=24,PrimaryColour=&HFFFFFF,OutlineColour=&H000000,Outline=2'" -c:a copy output.mp4

# Burn subtitles (ASS file for styled)
ffmpeg -i video.mp4 -vf "ass=subs.ass" -c:a copy output.mp4

4. Transitions Between Clips

# Crossfade (2 second transition) - PowerShell line continuation
ffmpeg -i clip1.mp4 -i clip2.mp4 -filter_complex `
  "[0:v]trim=0:5,setpts=PTS-STARTPTS[v0]; `
   [1:v]trim=0:5,setpts=PTS-STARTPTS[v1]; `
   [v0][v1]xfade=transition=fade:duration=2:offset=3[outv]" `
  -map "[outv]" output.mp4

5. Extract/Manipulate Frames

# Extract frames
ffmpeg -i video.mp4 -vf "fps=1" frames/frame_%04d.png

# Create video from frames
ffmpeg -framerate 24 -i frames/frame_%04d.png -c:v libx265 -crf 23 -preset medium -pix_fmt yuv420p output.mp4

# Speed up/slow down
ffmpeg -i video.mp4 -filter:v "setpts=0.5*PTS" -filter:a "atempo=2.0" fast.mp4   # 2x speed
ffmpeg -i video.mp4 -filter:v "setpts=2.0*PTS" -filter:a "atempo=0.5" slow.mp4   # 0.5x speed

6. Format Conversion

# MP4 (H.265) - universal compatibility
ffmpeg -i input.mov -c:v libx265 -crf 23 -preset medium -c:a aac -b:a 192k output.mp4

# WebM (VP9) - web delivery
ffmpeg -i input.mp4 -c:v libvpx-vp9 -crf 30 -b:v 0 -c:a libopus output.webm

# GIF (short clips)
ffmpeg -i input.mp4 -vf "fps=15,scale=480:-1:flags=lanczos" -loop 0 output.gif

7. Audio Normalization

# Normalize to -16 LUFS (YouTube standard)
ffmpeg -i input.mp4 -af "loudnorm=I=-16:TP=-1.5:LRA=11" -c:v copy output.mp4

# Two-pass normalization (more accurate)
ffmpeg -i input.mp4 -af loudnorm=I=-16:TP=-1.5:LRA=11:print_format=json -f null -
# Read measured values from JSON output, then apply:
ffmpeg -i input.mp4 -af "loudnorm=I=-16:TP=-1.5:LRA=11:measured_I=-20:measured_TP=-2:measured_LRA=8:measured_thresh=-30" -c:v copy output.mp4

Mode 2: Remotion (Complex Compositions)

Best for: Animated captions, data visualizations, complex motion graphics, React-based templates. Delegate to remotion-best-practices skill for guidance.

Use Remotion when:

• Animated text/captions needed
• Data visualizations in video
• Complex motion graphics
• Programmatic/template-based video generation
• React-based UI elements as video overlays

Pass to Remotion:

• Clip paths and timings
• Text content and styles
• Animation specifications
• Output format requirements

Quality Presets

Assembly Pipelines

Standard Video

1. Gather clips from comfyui-video-pipeline outputs.
2. Gather audio from comfyui-voice-pipeline outputs.
3. Normalize audio levels (-16 LUFS for YouTube).
4. Trim/arrange clips to match script timing.
5. Add transitions between clips (if multiple).
6. Add background music (optional, lower volume).
7. Add subtitles (if applicable).
8. Export in target format.

Talking Head Video

1. Lip-synced video from comfyui-voice-pipeline.
2. Add intro/outro (if applicable).
3. Add lower thirds or name cards.
4. Add background music at -20dB below speech.
5. Normalize speech to -16 LUFS.
6. Export.

Tutorial/Walkthrough

1. Screen recordings + talking head overlay.
2. Use Remotion for animated annotations.
3. Add chapter markers.
4. Add subtitles (auto-generate from audio).
5. Export with chapters embedded.

Pitfalls

• Do not use for initial video generation or AI clip creation (use comfyui-video-pipeline).
• Do not use for voice synthesis (use comfyui-voice-pipeline).
• Do not use for complex 3D rendering or high-end VFX that requires a dedicated NLE (Non-Linear Editor) like DaVinci Resolve or Premiere Pro, unless the requirements can be met via Remotion.
• Avoid using deprecated FFmpeg filters and codecs. Always check the latest FFmpeg documentation for updates and best practices.
• Be cautious with FFmpeg's concat demuxer, as it can be unstable with certain file formats. Prefer using the concat filter for more robust concatenation.
• Ensure that all input files are properly encoded and compatible with the chosen FFmpeg or Remotion workflow to avoid unexpected errors or quality issues.
• PowerShell line continuations: Use backtick (`) instead of backslash (\) for multi-line FFmpeg commands in PowerShell.

Verification

• Run ffmpeg -i output.mp4 to verify stream integrity and resolution.
• Check audio levels to ensure normalization to -16 LUFS.
• Verify that no black frames exist at cut points.
• Confirm subtitles are correctly timed and burned in.
• Validate that the final file size is within the target platform's limits.
• Ensure all input files are compatible and properly encoded.
• Test the final video on the target platform to ensure compatibility and performance.

Related skills

• comfyui-video-pipeline
• comfyui-voice-pipeline
• remotion-best-practices