# FFmpeg Concat Filter (Re-encode) — effect skill for cuts & pacing

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Join any sources frame-accurately by decoding, normalizing and re-encoding them in one filter graph.

- **Type:** [Effect](https://varg.ai/how-to/effects.md) · [Cuts & pacing](https://varg.ai/how-to/browse/skills/cuts.md)
- **Slug:** `ffmpeg-concat-filter`
- **Status:** live
- **Publisher:** varg (varg, verified)
- **License:** MIT
- **Source:** https://github.com/vargHQ/skills/blob/main/varg-video-edit-library/references/cuts.md#ffmpeg-concat-filter
- **Works in:** Claude Code, Claude.ai, Codex, Cursor, OpenClaw, Gemini CLI, Any MCP client, varg Studio
- **Requires:** ffmpeg
- **Cost:** Free — the take renders locally; generated footage, if any, bills by the model

## What it does

Plain hard cuts at exact frames, from mixed cameras, phones and AI generations.

**When:** Mixed resolutions, fps or codecs; whenever trims must be frame-accurate.

**Watch out:** Forgetting to normalize: concat needs matching size, SAR and pixel format per stream, and the same stream count per segment.

**Timing:** Hard cuts

**Also called:** concat filter, filter_complex concat, re-encode join

### In FFmpeg

Three segments with video + audio each, normalized to 1080×1920 at 30 fps:
```bash
ffmpeg -i a.mp4 -i b.mp4 -i c.mp4 -filter_complex \
"[0:v]scale=1080:1920:force_original_aspect_ratio=decrease,pad=1080:1920:-1:-1,setsar=1,fps=30[v0];[0:a]aresample=48000[a0];\
 [1:v]scale=1080:1920:force_original_aspect_ratio=decrease,pad=1080:1920:-1:-1,setsar=1,fps=30[v1];[1:a]aresample=48000[a1];\
 [2:v]scale=1080:1920:force_original_aspect_ratio=decrease,pad=1080:1920:-1:-1,setsar=1,fps=30[v2];[2:a]aresample=48000[a2];\
 [v0][a0][v1][a1][v2][a2]concat=n=3:v=1:a=1[v][a]" -map "[v]" -map "[a]" -c:v libx264 -crf 18 -c:a aac out.mp4
```
Input order is segment by segment: v,a of segment 1, then v,a of segment 2. A clip without audio needs a silent stream (`anullsrc` + `atrim`) so every segment has the same streams.

### Where this comes from

- [FFmpeg filters: concat](https://ffmpeg.org/ffmpeg-filters.html#concat-filter)
- [FFmpeg filters: trim](https://ffmpeg.org/ffmpeg-filters.html#trim)

## Use when

- Mixed resolutions, fps or codecs; whenever trims must be frame-accurate.

## Not for

- Forgetting to normalize: concat needs matching size, SAR and pixel format per stream, and the same stream count per segment.

## Install

- Claude Code: `claude plugin marketplace add vargHQ/skills && claude plugin install varg@varg`
- Codex: `npx skills add vargHQ/skills --skill varg-video-edit-library -a codex -y`
- Cursor: `npx skills add vargHQ/skills --skill varg-video-edit-library -a cursor -y`

## Example output

https://s3.varg.ai/how-to/previews/c993e5c5b3cf/take-ffmpeg-concat-filter.mp4 — concat between two shots, with the timeline strip of the edit. Footage: Spring (2019), Andy Goralczyk / Blender Foundation (Blender Studio) | studio.blender.org, CC BY 4.0; Coffee Run (2020), Blender Studio | studio.blender.org, CC BY 4.0. (credit: Rendered by varg · Spring (2019) + Coffee Run (2020))

## More cuts & pacing skills

- [SmartCut (capcut-ai-editor)](https://varg.ai/how-to/skills/capcut-smartcut-mcp.md): MCP that reads CapCut auto-captions to cut silences and duplicate takes right inside your CapCut project.
- [Auto-Editor](https://varg.ai/how-to/skills/auto-editor.md): CLI that cuts dead space by loudness or motion and exports MP4 or editor timelines; now also an agent skill.
- [AutoCut](https://varg.ai/how-to/skills/autocut.md): Edit video in a text editor: Whisper transcribes, you delete lines in Markdown, AutoCut cuts the video.
- [Cut Video](https://varg.ai/how-to/skills/cut-video.md): Claude Code skill that tightens long recordings with forced-aligned cuts, keeping laughs and comedic pauses.
- [Videogrep](https://varg.ai/how-to/skills/videogrep.md): Search spoken words across videos and auto-build supercuts of every match.
- [PySceneDetect](https://varg.ai/how-to/skills/pyscenedetect.md): Find cuts, fades and scene changes in a video, then split it or export scene lists for the edit.