shrinkvid

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# shrinkvid - batch shrink videos with ffmpeg
# --------------------------------------------
# Usage:
#   shrinkvid [directory] [format] [crf] [scale_pct]
#
# Examples:
#   shrinkvid                 # defaults: ~/Downloads, mp4, CRF=25
#   shrinkvid . mp4 25 50     # 50% size
#   shrinkvid ~/Movies        # custom directory
#   shrinkvid . mkv 24        # current dir, mkv files, CRF=24
#
# Notes:
# - Originals are renamed with `_Original` suffix.
# - Skips files already ending in `_Original`.
# - Does not overwrite existing `_Original` files.
#
# Important:
# - libx264 commonly requires width/height divisible by 2 (e.g. yuv420p).
#   Some inputs (or scaled results) can end up odd-sized, e.g. 1728x1117.
#   This function forces the final dimensions to be even by rounding DOWN
#   to the nearest multiple of 2 (changes at most 1px in each dimension).

shrinkvid() {
  emulate -L zsh
  set -o pipefail
  setopt null_glob

  local dir="${1:-$HOME/Downloads}"
  local fmt="${2:-mp4}"
  local crf="${3:-25}" # 28 is very aggressive, 23 is almost visually the same as input
  local scale_pct="${4:-}"

  # Video filter:
  # - If scale_pct is provided, scale by that percentage.
  # - Always ensure the output width/height are even numbers by truncating down:
  #     trunc(x/2)*2  -> nearest even <= x
  #
  # This avoids: "height not divisible by 2" and similar libx264 encoder errors.
  local vf=""
  if [[ -n "$scale_pct" ]]; then
    # Scale *then* coerce to even dimensions (rounding down).
    vf="-vf scale=trunc(iw*${scale_pct}/100/2)*2:trunc(ih*${scale_pct}/100/2)*2"
  else
    # No scaling requested, but still coerce to even dimensions if needed.
    # If input is already even-sized, this is a no-op.
    vf="-vf scale=trunc(iw/2)*2:trunc(ih/2)*2"
  fi

  builtin pushd -q "$dir" || { echo "Directory not found: $dir"; return 1; }

  for i in *.${fmt}; do
    [[ "$i" == *_Original.${fmt} ]] && continue
    local original="${i%.*}_Original.${fmt}"
    mv -n -- "$i" "$original" || { echo "Skipping $i (backup exists)"; continue; }

    echo "Shrinking $i (CRF=$crf${scale_pct:+, scale=${scale_pct}%})..."
    echo "  Applying filter: ${vf#-vf }"
    echo "  Output dims will be forced to even numbers (rounded down if needed)."

    ffmpeg -y -i "$original" \
      -c:v libx264 -preset slow -crf "$crf" \
      ${=vf} \
      -c:a copy \
      "$i"
  done

  popd -q
}