star_field

#!/bin/sh
#
# star_field [options] XxY output_image
#
# Generate a random 'hex' star field (tilable) of the size given.
#
# Options:
#    -r   angle of rotation of the hex star (def=15)
#    -b   how bright (percentage) should the stars be (def=100)
#
####
#
# WARNING: Input arguments are NOT tested for correctness.
# This script represents a security risk if used ONLINE.
# I accept no responsiblity for misuse. Use at own risk.
#
# Anthony Thyssen    March 2006
#
PROGNAME=`type $0 | awk '{print $3}'`  # search for executable on path
PROGDIR=`dirname $PROGNAME`            # extract directory of program
PROGNAME=`basename $PROGNAME`          # base name of program
Usage() {                              # output the script comments as docs
  echo >&2 "$PROGNAME:" "$@"
  sed >&2 -n '/^###/q; /^#/!q; s/^#//; s/^ //; 3s/^/Usage: /; 2,$ p' \
          "$PROGDIR/$PROGNAME"
  exit 10;
} 

# defaults
angle=15
bright=100

while [ $# -gt 0 ]; do
  case "$1" in
  --help|--doc*) Usage ;;
  -r) shift; angle=$1 ;;    # rotation angle
  -b) shift; bright=$1 ;;   # How bright (percentage)

  --) shift; break ;;    # end of user options
  -*) Usage "Unknown option \"$1\"" ;;
  *)  break ;;           # end of user options

  esac
  shift   # next option
done

[ $# -lt 2 ] && Usage "Too few arguments."
[ $# -gt 2 ] && Usage "Too many arguments."

size="$1"
output="$2"

# clean up the arguments
while   expr "$angle" \< 0 >/dev/null;   do
  angle=`expr $angle + 60`
done
while expr "$angle" \> 60 >/dev/null; do
  angle=`expr $angle - 60`
done

a1=$angle
a2=`expr $a1 + 60`
a3=`expr $a1 + 120`

b1=`expr $a1 + 180`
b2=`expr $a2 + 180`
b3=`expr $a3 + 180`

bright=`( echo 'scale=2'; echo "$bright" / 100; ) | bc`


# Do the task (slow)...

magick -size "$size" xc: -fx 'rand()' -virtual-pixel tile \
            -colorspace Gray -sigmoidal-contrast 120x0% -negate -blur 0x.3 \
            \( -clone 0  -motion-blur 0x10+$a1  -motion-blur 0x10+$b1 \) \
            \( -clone 0  -motion-blur 0x10+$a2  -motion-blur 0x10+$b2 \) \
            \( -clone 0  -motion-blur 0x10+$a3  -motion-blur 0x10+$b3 \) \
            -compose screen -background black -flatten  -normalize \
            -evaluate multiply $bright  "$output"