Randomly compose images by layering pngs

Question

I am trying to use Imagemagick v7 to batch create images by randomly combining layers which are transparent pngs, all at the same size.

I am a total newbie so I copied the code I found here: https://stackoverflow.com/a/27621140/17835805

#!/bin/bash

# Number of output files - edit freely :-)
NFILES=10

# Build arrays of filenames in each layer, assume directories are "Layer0", "Layer1" etc
IFS=$'\n' L0files=($(find "Layer 0" -name "*.png"))
IFS=$'\n' L1files=($(find "Layer 1" -name "*.png"))
IFS=$'\n' L2files=($(find "Layer 2" -name "*.png"))
IFS=$'\n' L3files=($(find "Layer 3" -name "*.png"))

# Produce NFILES output files
for i in `seq 1 $NFILES`; do

   # Choose random index into each array of filenames
   index0=$( jot -r 1  0 $((${#L0files[@]} - 1)) )
   index1=$( jot -r 1  0 $((${#L1files[@]} - 1)) )
   index2=$( jot -r 1  0 $((${#L2files[@]} - 1)) )
   index3=$( jot -r 1  0 $((${#L3files[@]} - 1)) )

   # Pick up files as specified by the random index
   f0=${L0files[index0]}
   f1=${L1files[index1]}
   f2=${L2files[index2]}
   f3=${L3files[index3]}

   # Generate output filename, "output-nnn.png" 
   # ... where nnn starts at 0 and goes up till no clash
   i=0
   while :; do
      out="output-$i.png"
      [ ! -f "$out" ] && break
      ((i++))
   done

   echo $f0, $f1, $f2, $f3 "=> $out"
   convert "$f0" "$f1" -composite "$f2" -composite "$f3" -composite "$out"
done

I managed to make it randomly pick my layers but:

1. No files are created in the output folder.
   
2. No matter what I put for NFILES, it always stops at the first one ("output-o.png").

Would be great if I could also make sure that, once a layer has been randomly picked and used, it is not picked again. Perhaps deleting the file with ephemeral?

Answer 1

ok, thanks to the great help of Mark Setchell, finally figured out a solution.

#!/bin/bash

# Number of output files
NFILES=20
BASE="$HOME/foldername"



# Function to read a random line from file, deleting it if required

randomLine () {
   filename=$1; delete=$2

   # Check file is not empty
   declare -i nlines
   nlines=$(wc -l < "$filename")
   if [ $nlines -eq 0 ] ; then
      2>&1 echo "ERROR: $filename is empty"
      exit 1
   fi
   # Read lines into array
   IFS=$'\n' read -d '' -r -a lines < "$filename"

   # Choose a random line number
   ((sel = RANDOM % nlines))

   # Output result for caller in global variable
   retval="${lines[sel]}"

   # Delete the line if required
   if [ $delete -eq 1 ] ; then
      lines[sel]="JUNK"
      printf "%s\n" "${lines[@]}" | grep -v "JUNK" > "$filename"
   fi
}


# Main program


# Create output directory
mkdir -p "${BASE}/export"

# Build lists of filenames in each layer - write lists in /tmp
for ((layer=1; layer<=6; layer++)) ; do
   find "${BASE}/parts/p${layer}" -name "*.png" > /tmp/L${layer}
done

# Produce NFILES output files
for ((n=0; n<$NFILES; n++)) ; do

   # Pick up files randomly, deleting 2, 4 and 6, reusing others
   randomLine "/tmp/L1" 0; f1="$retval"
   randomLine "/tmp/L2" 1; f2="$retval"
   randomLine "/tmp/L3" 0; f3="$retval"
   randomLine "/tmp/L4" 1; f4="$retval"
   randomLine "/tmp/L5" 0; f5="$retval"
   randomLine "/tmp/L6" 1; f6="$retval"

   # Generate output filename
   for ((i=0;;i++)) ; do
      out="${BASE}/output-$i.png"
      [ ! -f "$out" ] && break
   done

   echo $f1, $f2, $f3, $f4, $f5, $f6 "=> $out"
   convert "$f1" "$f2" -composite "$f3" -composite "$f4" -composite "$f5" -composite "$f6" -composite "$out"

done