This script creates A4 (DIN A1) collage pages from any number of PNG images, automatically resizing and arranging them into printable pages using ImageMagick.

convert.sh

#!/bin/bash

# -----------------------------------------------------------------------------
# Script: convert.sh
#
# Description:
#   This script processes all PNG images in the specified source folder by:
#     1. Squaring and resizing them to 1000x1000 pixels (centered, white background).
#     2. Arranging them into A4-sized (2480x3508 px, 300 DPI) collage pages,
#        with 3 images per row and 4 rows per page (12 images per page).
#     3. Saving the resulting collages in the 'result' folder.
#
# Usage:
#   ./convert.sh
#
# Requirements:
#   - ImageMagick (convert, montage)
#   - Bash shell
#
# Output:
#   - Resulting collage pages are saved as result/page_1.png, result/page_2.png, ...
#
# Folder structure:
#   - Source images: REPLACE_FOLDER_NAME/
#   - Output:        REPLACE_FOLDER_NAME/result/
#
# -----------------------------------------------------------------------------

# Cleanup previous results
RESULTFOLDER="REPLACE_FOLDER_NAME/result"
SQUAREFOLDER="$RESULTFOLDER/squared"
rm -rf "$RESULTFOLDER"
mkdir -p "$SQUAREFOLDER"

# Folders
SRCFOLDER="REPLACE_FOLDER_NAME/"

# --- 1. Square all PNG images (centered, 1000x1000 px for processing, keep ratio) ---
echo "Squaring images..."
find "$SRCFOLDER" -maxdepth 1 -iname '*.png' -print0 | while IFS= read -r -d '' f; do
  b=$(basename "$f")
  convert "$f" -resize 1000x1000 -background white -gravity center -extent 1000x1000 "$SQUAREFOLDER/$b"
done

# --- 2. Montage settings for A4 portrait at 300 DPI ---
IMGS_PER_ROW=3
ROWS_PER_PAGE=4
IMGS_PER_PAGE=$((IMGS_PER_ROW*ROWS_PER_PAGE))  # 12

# Final A4 size in pixels
WIDTH=2480
HEIGHT=3508

# --- 3. Create collages ---
cd "$SQUAREFOLDER"
mapfile -d '' files < <(find . -maxdepth 1 -iname '*.png' -print0 | sort -z)
total_images=${#files[@]}
num_pages=$(( (total_images + IMGS_PER_PAGE - 1) / IMGS_PER_PAGE ))

echo "Creating $num_pages A4 collage pages..."

for ((page=1; page<=num_pages; page++)); do
  imgs=()
  for ((i=0; i<IMGS_PER_PAGE; i++)); do
    idx=$(( (page-1)*IMGS_PER_PAGE + i ))
    [ $idx -ge $total_images ] && break
    # Remove leading ./ from filenames for montage
    img="${files[$idx]}"
    img="${img#./}"
    imgs+=("$img")
  done

  # Create the montage without -extent
  montage "${imgs[@]}" \
    -tile ${IMGS_PER_ROW}x${ROWS_PER_PAGE} \
    -geometry 827x827+0+0\! \
    -background white \
    "montage_tmp.png"

  # Center and extend/crop to exact A4 size at 300 DPI
  convert "montage_tmp.png" -gravity center -background white -extent ${WIDTH}x${HEIGHT} -density 300 "$RESULTFOLDER/page_${page}.png"
  rm -f "montage_tmp.png"

  echo "Created page_${page}.png in result folder"
  if [ -f "$RESULTFOLDER/page_${page}.png" ]; then
    echo "$RESULTFOLDER/page_${page}.png"
  else
    echo "Warning: $RESULTFOLDER/page_${page}.png was not created!"
  fi
done

echo "All done! Collages saved as result/page_1.png ... result/page_${num_pages}.png"