Video toolbox (ffmpeg)

A small ffmpeg how-to that doubles as a tour of meshfox's variable types and forms (see SPEC.md's "Variables", "File variables" and "Form fences"). Everything you would otherwise retype on a command line is a variable, and one form sets them all:

Variable	Type	What it shows
`INPUT`	`file` (`mode="open"`, `ext=` hint)	pick an existing video — Browse… (web) / `Ctrl-O` (terminal)
`OUTPUT`	`file` (`mode="save"`)	name a file that does not exist yet
`OUTPUT_DIR`	`dir`	a folder; `@tmp` is the canvas's own scratch directory
`PRESET`	`select`	x264 speed/size trade-off
`CRF`	`int`	quality, validated as a whole number
`KEEP_AUDIO`	`bool`	a checkbox
`START`, `DURATION`	`string`	free text

Needs ffmpeg and ffprobe on PATH. Out of the box everything works on a generated test clip (block sample); pick your own video with Browse… and the same blocks run on it.

Test clip

Ten seconds of a generated test pattern with a tone, written to OUTPUT_DIR (once — it is left alone if it is already there). It is what INPUT points at by default; convert below runs it first, so you never have to. Pick a video of your own with Browse… and it is simply unused.

set -euo pipefail
mkdir -p "$OUTPUT_DIR"
if [ -f "$OUTPUT_DIR/sample.mp4" ]; then
  echo "$OUTPUT_DIR/sample.mp4 is already there"
  exit 0
fi
ffmpeg -y -hide_banner -loglevel error \
  -f lavfi -i "testsrc2=size=640x360:rate=25" \
  -f lavfi -i "sine=frequency=440:sample_rate=44100" \
  -t 10 -c:v libx264 -pix_fmt yuv420p -c:a aac "$OUTPUT_DIR/sample.mp4"
echo "wrote $OUTPUT_DIR/sample.mp4"

Inspect

ffprobe prints the container and streams as JSON into OUTPUT_DIR. The file node below it reads that report back through $OUTPUT_DIR — a dir variable's value is a trusted root, so a path outside the canvas directory is fine once the folder is confirmed.

set -euo pipefail
[ -f "$INPUT" ] || { echo "input not found: $INPUT (run 'sample' or pick a video)" >&2; exit 1; }
mkdir -p "$OUTPUT_DIR"
ffprobe -v error -show_entries format=format_name,duration,size,bit_rate:stream=index,codec_type,codec_name,width,height \
  -of json "$INPUT" > "$OUTPUT_DIR/probe.json"
echo "wrote $OUTPUT_DIR/probe.json"

Report

$OUTPUT_DIR/probe.json

Convert

The form sets the variables; Apply commits them for the session and — because convert is marked autorun — re-encodes right away, no run button. Nothing else reacts: probe, extract-audio and gif read the very same values (INPUT, OUTPUT_DIR, START, DURATION) when you run them. The default paths live under @tmp — the canvas's scratch directory, which session reset clears — so they need no confirmation; a file or folder you choose elsewhere becomes a trusted location for this canvas once chosen.

field var="INPUT" label="Input video"
field var="OUTPUT" label="Converted video"
field var="OUTPUT_DIR" label="Output folder"
field var="PRESET" label="x264 preset"
field var="CRF" label="Quality (CRF)"
field var="KEEP_AUDIO" label="Keep audio"
field var="START" label="Start"
field var="DURATION" label="Duration (s)"

Re-encodes to H.264 with the chosen preset and quality, optionally cutting START/DURATION out first. OUTPUT is a mode="save" file: its folder is created, the file itself is overwritten without asking.

set -euo pipefail
[ -f "$INPUT" ] || { echo "input not found: $INPUT (run 'sample' or pick a video)" >&2; exit 1; }
mkdir -p "$(dirname "$OUTPUT")"
args=(-y -hide_banner -loglevel error -ss "$START")
[ -n "$DURATION" ] && args+=(-t "$DURATION")
args+=(-i "$INPUT" -c:v libx264 -preset "$PRESET" -crf "$CRF" -pix_fmt yuv420p)
if [ "$KEEP_AUDIO" = "true" ]; then args+=(-c:a aac); else args+=(-an); fi
ffmpeg "${args[@]}" "$OUTPUT"
echo "wrote $OUTPUT ($(du -h "$OUTPUT" | cut -f1))"

Audio

The audio track alone, as MP3, named after the input.

set -euo pipefail
[ -f "$INPUT" ] || { echo "input not found: $INPUT (run 'sample' or pick a video)" >&2; exit 1; }
mkdir -p "$OUTPUT_DIR"
name="$(basename "${INPUT%.*}")"
ffmpeg -y -hide_banner -loglevel error -i "$INPUT" -vn -c:a libmp3lame -q:a 2 "$OUTPUT_DIR/$name.mp3"
echo "wrote $OUTPUT_DIR/$name.mp3"

GIF

A looping GIF of the same START/DURATION window (five seconds when DURATION is empty), with a generated palette so it does not look like mud.

set -euo pipefail
[ -f "$INPUT" ] || { echo "input not found: $INPUT (run 'sample' or pick a video)" >&2; exit 1; }
mkdir -p "$OUTPUT_DIR"
name="$(basename "${INPUT%.*}")"
ffmpeg -y -hide_banner -loglevel error -ss "$START" -t "${DURATION:-5}" -i "$INPUT" \
  -vf "fps=12,scale=480:-1:flags=lanczos,split[s0][s1];[s0]palettegen[p];[s1][p]paletteuse" \
  "$OUTPUT_DIR/$name.gif"
echo "wrote $OUTPUT_DIR/$name.gif"