Encode for smooth scrubbing.

#!/usr/bin/env bash
# flythrough-encode.sh - encode a clip for smooth scroll scrubbing (Flythrough for Framer).
# Free to use and share. Needs ffmpeg + ffprobe (brew install ffmpeg).
#
#   ./flythrough-encode.sh input.mp4 [options]
#   ./flythrough-encode.sh "frames/%04d.png" --name approach --mobile-src "frames-portrait/%04d.png"
#
# Options
#   --name NAME        output base name (default: input file name)
#   --gop N            keyframe every N frames (default 6). A seek decodes at most N-1 extra frames.
#   --max-smooth       every frame a keyframe (-g 1): smoothest seeking, about 2.4–4× the size
#   --fps N            frame rate (default 30)
#   --width N          desktop width (default 1920); height follows the source aspect
#   --mobile           also write NAME-mobile.mp4: 720x1280 portrait, centre-cropped from the input
#   --focus F          horizontal crop position for --mobile, 0 = left, 0.5 = centre, 1 = right
#   --mobile-src FILE  a separate portrait source for the mobile clip (better than cropping)
#   --mobile-size WxH  mobile size (default 720x1280)
#   --reverse          also write reversed clips (NAME-reverse.mp4, NAME-mobile-reverse.mp4) for Glide mode
#   --crf N            quality, lower = better and bigger (default 22)
#   --poster-at S      poster frame time in seconds (default 0)
#   --out DIR          output folder (default: current folder)
set -euo pipefail

die() { echo "error: $*" >&2; exit 1; }
command -v ffmpeg >/dev/null || die "ffmpeg not found (brew install ffmpeg)"

[ $# -ge 1 ] || { sed -n "2,22p" "$0"; exit 1; }
IN="$1"; shift
NAME=""; GOP=6; FPS=30; WIDTH=1920; MOBILE=0; FOCUS=0.5; MSRC=""; MSIZE="720x1280"
REVERSE=0; CRF=22; POSTER=0; OUT="."
# flags that take a value fail clearly when the value is missing
val() { [ $# -ge 2 ] && [ -n "$2" ] && [ "${2#--}" = "$2" ] || die "$1 needs a value"; }
while [ $# -gt 0 ]; do
  case "$1" in
    --name) val "$@"; NAME="$2"; shift 2;;
    --gop) val "$@"; GOP="$2"; shift 2;;
    --max-smooth) GOP=1; shift;;
    --fps) val "$@"; FPS="$2"; shift 2;;
    --width) val "$@"; WIDTH="$2"; shift 2;;
    --mobile) MOBILE=1; shift;;
    --focus) val "$@"; FOCUS="$2"; shift 2;;
    --mobile-src) val "$@"; MSRC="$2"; MOBILE=1; shift 2;;
    --mobile-size) val "$@"; MSIZE="$2"; shift 2;;
    --reverse) REVERSE=1; shift;;
    --crf) val "$@"; CRF="$2"; shift 2;;
    --poster-at) val "$@"; POSTER="$2"; shift 2;;
    --out) val "$@"; OUT="$2"; shift 2;;
    *) die "unknown option $1";;
  esac
done
[ -n "$NAME" ] || NAME="$(basename "${IN%.*}")"
mkdir -p "$OUT"
MW="${MSIZE%x*}"; MH="${MSIZE#*x}"

# arrays keep paths with spaces intact; image sequences (frames/%04d.png) need an input frame rate
input_args() { case "$1" in *%*) IA=(-framerate "$FPS" -i "$1");; *) IA=(-i "$1");; esac; }

X264=(-an -c:v libx264 -pix_fmt yuv420p -profile:v high -preset slow -crf "$CRF"
  -g "$GOP" -keyint_min "$GOP" -sc_threshold 0 -bf 0 -r "$FPS" -movflags +faststart)

enc() { # enc <input> <filter> <output>
  input_args "$1"
  ffmpeg -hide_banner -loglevel error -y "${IA[@]}" -vf "$2" "${X264[@]}" "$3"
}

DESK="$OUT/$NAME.mp4"
enc "$IN" "scale=$WIDTH:-2:flags=lanczos" "$DESK"
OUTS=("$DESK")

if [ "$MOBILE" = 1 ]; then
  MOB="$OUT/$NAME-mobile.mp4"
  if [ -n "$MSRC" ]; then
    enc "$MSRC" "scale=$MW:$MH:force_original_aspect_ratio=increase:flags=lanczos,crop=$MW:$MH" "$MOB"
  else
    enc "$IN" "scale=-2:$MH:flags=lanczos,crop=$MW:$MH:(in_w-$MW)*$FOCUS:0" "$MOB"
  fi
  OUTS+=("$MOB")
fi

if [ "$REVERSE" = 1 ]; then
  for f in "${OUTS[@]}"; do
    r="${f%.mp4}-reverse.mp4"
    ffmpeg -hide_banner -loglevel error -y -i "$f" -vf reverse "${X264[@]}" "$r"
    OUTS+=("$r")
  done
fi

ffmpeg -hide_banner -loglevel error -y -ss "$POSTER" -i "$DESK" -frames:v 1 -q:v 3 "$OUT/$NAME-poster.jpg"
[ "$MOBILE" = 1 ] && ffmpeg -hide_banner -loglevel error -y -ss "$POSTER" -i "$OUT/$NAME-mobile.mp4" -frames:v 1 -q:v 3 "$OUT/$NAME-mobile-poster.jpg"

size() { stat -f %z "$1" 2>/dev/null || stat -c %s "$1"; }
echo "keyframe every $GOP frames, crf $CRF, $FPS fps"
for f in "${OUTS[@]}"; do
  b=$(size "$f"); mb=$(awk "BEGIN{printf \"%.2f\", $b/1048576}")
  lim=12; case "$f" in *-mobile*) lim=6;; esac
  warn=""; awk "BEGIN{exit !($b/1048576 > $lim)}" && warn="  <- over $lim MB, try --crf 26 or a shorter clip"
  echo "  $f  $mb MB$warn"
done
echo "  $OUT/$NAME-poster.jpg"
[ "$MOBILE" = 1 ] && echo "  $OUT/$NAME-mobile-poster.jpg"
exit 0