Show image thumbnails using Sixel graphics.

list-images.sh

#!/bin/bash
#
# Usage:
#
#   $ list-images [files]*
#
# This script is a demo to show how to use hyperlinks with images in Alacritty.
# It can be used on any terminal emulator with Sixel support, but it is not
# intended to be used for regular use, mostly because it lacks a proper way to
# handle errors (it just dies).
#
# Dependencies:
#
#  - A relatively recent version of Bash.
#  - identify(1), from ImageMagick 6.
#  - img2sixel(1), from libsixel.
#  - nproc(1), from coreutils.
#
# img2sixel(1) is used instead of ImageMagick's convert(1) because the
# conversion to Sixel is much faster with it.
#
# There are two environment variables that can be used to change the behaviour
# of the script:
#
#   THUMBNAIL_SIZE    Number of rows to determine the size of the thumbnail.
#
#   BORDER_COLOR      Color of the highlight border. It is expected in
#                     hexadecimal form (i.e., "FF0000" is red).

set -euo pipefail

: "${THUMBNAIL_SIZE:=5}"

: "${BORDER_COLOR:=FF7700}"

printf -v BORDER               \
  '\e[38;2;%d;%d;%dm'          \
  $(("0x${BORDER_COLOR:0:2}")) \
  $(("0x${BORDER_COLOR:2:2}")) \
  $(("0x${BORDER_COLOR:4:2}"))


# Terminal dimensions.

IFS=';' read -p $'\e[14t' -rs -dt _ WIN_HEIGHT WIN_WIDTH
IFS=';' read -p $'\e[18t' -rs -dt _ ROWS COLUMNS

CELL_WIDTH=$((WIN_WIDTH / COLUMNS))
CELL_HEIGHT=$((WIN_HEIGHT / ROWS))

IMAGE_WIDTH=$((CELL_WIDTH * THUMBNAIL_SIZE * 2))
IMAGE_HEIGHT=$((CELL_HEIGHT * THUMBNAIL_SIZE))

NPROC=$(nproc)


# Main program.

declare -a workers=()
declare -a tmpfiles=()

trap cleanup EXIT

cleanup() {
  if [ "${#tmpfiles[@]}" -gt 0 ]
  then
    rm -f "${tmpfiles[@]}"
    tmpfiles=()
  fi
}

declare -i CURRENT_ROW_OFFSET_X="$WIN_WIDTH"
declare -i CURRENT_ROW_HEIGHT=0

# Reserve vertical space and save the cursor at the beginning.
start_row() {
  if [ "$CURRENT_ROW_HEIGHT" -gt 0 ]
  then
    printf '\e8\e[%dB' $((CURRENT_ROW_HEIGHT / CELL_HEIGHT + 1))
  fi

  for (( n = 0 ; n <= THUMBNAIL_SIZE ; n += 1 ))
  do
    printf '\n'
  done

  printf '\e[%dA\e7' "$THUMBNAIL_SIZE"

  CURRENT_ROW_OFFSET_X=0
  CURRENT_ROW_HEIGHT=0
}

# Draw a single image from the workers array.
draw() {
  if [ "${#workers[@]}" -eq 0 ]
  then
    return
  fi

  # Extract first worker from the array.
  local worker="${workers[0]}"
  workers=("${workers[@]:1}")

  read -rs width height < "$worker.size"

  local next_offset_x=$((CURRENT_ROW_OFFSET_X + width))

  if [ "$next_offset_x" -ge "$WIN_WIDTH" ]
  then
    start_row
  fi

  if [ "${height}" -ge "$CURRENT_ROW_HEIGHT" ]
  then
    CURRENT_ROW_HEIGHT=${height}
  fi

  if [ "$CURRENT_ROW_OFFSET_X" -gt 0 ]
  then
    printf '\e8\e[%dC' $((CURRENT_ROW_OFFSET_X / CELL_WIDTH + 1))
  fi

  printf '%s\e]8;;%s\a' "$BORDER" "$(cat "$worker.image")"

  cat "$worker"

  printf '\e]8;;\e[m\a'

  CURRENT_ROW_OFFSET_X+=$((width + CELL_WIDTH * 2))
}

for image in "$@"
do
  if [ "${#workers[@]}" -ge "$NPROC" ]
  then
    # We have to wait for the process before calling draw() because the
    # subshell created for the function can't wait for a process created
    # in this scope.

    wait "$(cat "${workers[0]}.pid")"

    draw
  fi

  image=$(realpath "$image")

  # The “worker” is a background shell where the image is rescaled
  # and converted to Sixel.

  worker=$(mktemp)
  printf "%s" "$image" > "$worker.image"

  (
    identify -format '%w %h\n' "$image" > "$worker"
    read -rs width height < "$worker"

    if [ "$width" -gt "$height" ]
    then
      scale=(-w "$IMAGE_WIDTH" -h auto)
    else
      scale=(-h "$IMAGE_HEIGHT" -w auto)
    fi

    img2sixel \
      "${scale[@]}" \
      -r nearest    \
      -q low        \
      -o "$worker"  \
      "$image"

    identify -format '%w %h\n' sixel:"$worker" > "$worker.size"
  ) &

  printf "%s" "$!" > "$worker.pid"

  workers+=("$worker")
  tmpfiles+=("$worker" "$worker.image" "$worker.pid" "$worker.size")
done


# Show pending images.

wait
while [ "${#workers[@]}" -gt 0 ]
do
  draw
done

echo