# Custom xfade Expression — effect skill for transitions

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Write your own transition as a per-pixel expression with xfade=transition=custom:expr=…

- **Type:** [Effect](https://varg.ai/how-to/effects.md) · [Transitions](https://varg.ai/how-to/browse/skills/transitions.md)
- **Slug:** `xfade-custom-expr`
- **Status:** live
- **Publisher:** varg (varg, verified)
- **License:** MIT
- **Source:** https://github.com/vargHQ/skills/blob/main/varg-video-edit-library/references/transitions.md#xfade-custom-expr
- **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

Anything you can describe per pixel from A, B, X, Y and progress P: a checkerboard, a diagonal band, a noise threshold.

**When:** When none of the 50+ named transitions fits and you still want a one-pass FFmpeg render.

**Watch out:** Slow renders on long clips: the expression runs for every pixel of every frame of the overlap.

**Timing:** As designed.

**Also called:** xfade expr

### In FFmpeg

Both inputs need the same size, frame rate, timebase, SAR and pixel format (`settb=AVTB` fixes the timebase error that `fps` alone leaves). `offset` is where the transition starts on the first clip's timeline (its length minus the overlap).

```bash
ffmpeg -i a.mp4 -i b.mp4 -filter_complex \
 "[0:v]fps=30,scale=1920:1080,setsar=1,settb=AVTB,format=yuv420p[a];[1:v]fps=30,scale=1920:1080,setsar=1,settb=AVTB,format=yuv420p[b];\
  [a][b]xfade=transition=custom:duration=0.5:offset=4.5[v];[0:a][1:a]acrossfade=d=0.5[a]" \
 -map "[v]" -map "[a]" out.mp4
```

Custom: replace `transition=…` with an expression. P runs from 1 to 0 across the overlap; A and B are the two pixel values.
```bash
# vertical bands that flip at different times
xfade=transition=custom:duration=0.8:offset=4.2:expr='if(gt(P, mod(floor(X/(W/8)),2)*0.5+0.25), A, B)'
```

### Where this comes from

- [FFmpeg filters: xfade](https://ffmpeg.org/ffmpeg-filters.html#xfade)
- [FFmpeg xfade gallery (trac wiki)](https://trac.ffmpeg.org/wiki/Xfade)

## Use when

- When none of the 50+ named transitions fits and you still want a one-pass FFmpeg render.

## Not for

- Slow renders on long clips: the expression runs for every pixel of every frame of the overlap.

## 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/45a4fa983b36/take-xfade-custom-expr.mp4 — custom between two shots, with the timeline strip of the edit. Footage: Sprite Fright (2021), Blender Studio | studio.blender.org, CC BY 4.0; WING IT! (2023), Blender Studio | studio.blender.org, CC BY 4.0. (credit: Rendered by varg · Sprite Fright (2021) + WING IT! (2023))

## More transitions skills

- [editly](https://varg.ai/how-to/skills/editly.md): Declarative JSON5 video editing CLI/API: clips, titles, transitions, audio mixing, rendered with ffmpeg.
- [Mix (Cross Dissolve)](https://varg.ai/how-to/skills/mix-dissolve.md): The two shots blend through each other. The quietest transition: "time passes" or "this continues".
- [Dip to Black](https://varg.ai/how-to/skills/black-fade.md): A fades to black, then B fades up. Marks the end of a section or a bigger jump in time.
- [White Flash](https://varg.ai/how-to/skills/white-flash.md): The frame blows out to white for a moment and opens on the next shot. Energy, memory or a camera snap.
- [Pull In (Zoom In)](https://varg.ai/how-to/skills/pull-in-zoom.md): The camera rushes into shot A and lands, still moving forward, in shot B. The signature short-form transition.
- [Pull Out (Zoom Out)](https://varg.ai/how-to/skills/pull-out-zoom.md): The camera jerks backwards out of shot A and keeps pulling back as shot B settles in.