endless.sh

#!/bin/bash

# sixel endless mode
# Should print an endless sine curve, abort with Ctrl-C.
# A genuine VT340 terminal updates this continuously. 

# Always send ST to end sixel mode when script exits.
# If smooth scrolling was off before, reset it.
ST=$'\e\\'
trap 'echo -n "${ST}${resetsmooth}"'	EXIT

period=200
amplitude=50

sixels=(@$ A$ C$ G$ O$ _\$-)
pi=3.141592653589793238

# Number of samples (m*period) must be divisible by both $period and 6. 
declare -i m=1
if ! factor $period 2>&- | cut -d: -f2- | grep -q -w 2; then
    m=m*2
fi
if ! factor $period 2>&- | cut -d: -f2- | grep -q -w 3; then
    m=m*3
fi

# Enable smooth scrolling. Note that, on a genuine VT340,
# Sixel Graphics Newline always jump scrolls.
IFS=";$" read -a REPLY -t${timeout:-1} -s -p $'\e[?4$p' -d y
if [[ ${REPLY[1]} == 2 ]]; then		# "2" means disabled, but changeable.
    echo -n $'\e[?4h'			# Set smooth scrolling to high.
    resetsmooth=$'\e[?4l'		# When script exits, reset to low.
fi

# Start sixels graphics string
echo -ne "\x1bP0;0;0q\"1;1#1;2;100;0;0#1"

# Cache the calculations in an array for speed
for (( y=0; y < m*period; y++ ))
do
  x=$(echo "x=s(2*${pi}*${y}/${period})*${amplitude}+2*${amplitude}+0.5; scale=0; x/1" | bc -l)
  echo -ne "!${x}?${sixels[$((y%6))]}"		   	# 1.8 KBps (uncached)
  f[y]=$(echo -ne "!${x}?${sixels[$((y%6))]}")		# 1.8 MBps (cached elt)
  g+=$(echo -ne "!${x}?${sixels[$((y%6))]}")		# 88 MBps (string)
done

while :
do
    for y in ${!f[@]}; do echo -n "${f[y]}"; done 	# 1.8 MBps (cached elt)
#    echo -n "${f[*]}"					# 48 MBps (all elts)
#    echo -n "$g"					# 88 MBps (string)
done


# NOTES
#
# Simple way to measure the speed is `./endless.sh | pv >/dev/null`