Bash 24-bit HSV Color & Gray Scale

24-bit-color.sh

#!/bin/bash
#----------------------------------------------------------------------
#	Author:		EABASE
#	Date:		2022-01-16
#	Version:	1.1
#----------------------------------------------------------------------
#	History:
#		The original file was taken from iterm2 [1], and later modified
#		by EABASE in [2]. 
#
#	Description:
#
#		This file echoes a bunch of 24-bit color codes
#		to the terminal to demonstrate its functionality.
#		The foreground escape sequence is:	^[38;2;<r>;<g>;<b>m
#		The background escape sequence is:	^[48;2;<r>;<g>;<b>m
#		<r> <g> <b> range from 0 to 255 inclusive.
#		The escape sequence ^[0m resets the terminal to default.
# 
#	Usage:	Run with unbuffered output using:
#			stdbuf -i0 -o0 -e0 ./24-bit-color.sh
#
#	References:
#		[1] https://github.com/gnachman/iTerm2/blob/master/tests/24-bit-color.sh
#		[2] https://gist.github.com/eabase/4e0444f790c45b2732aba49d61199e3a
#		[3] 
#		[4] https://en.wikipedia.org/wiki/HSL_and_HSV
#		[5] https://en.wikipedia.org/wiki/List_of_software_palettes
#		[6] https://en.wikipedia.org/wiki/Shades_of_gray
#		[7] https://www.geeksforgeeks.org/how-to-convert-three-channels-of-colored-image-into-grayscale-image-in-matlab/
#	Notes:
#	
#  rgb2gray()
#		0.2989 *R + 0.5870 *G + 0.1140 *B	# [1] NTSC
#		0.2126 *R + 0.7152 *G + 0.0722 *B	# [2] luminance signal EY
#		0.2627 *R + 0.6780 *G + 0.0593 *B	# [3] UHDTV
#
#	[1] BT.601	 https://www.itu.int/rec/R-REC-BT.601-7-201103-I/en
#	[2] BT.709	 https://www.itu.int/rec/R-REC-BT.709-6-201506-I/en
#	[3] BT.2020	 https://www.itu.int/rec/R-REC-BT.2020-2-201510-I/en
#----------------------------------------------------------------------

setBackgroundColor() {
	#printf '\x1bPtmux;\x1b\x1b[48;2;%s;%s;%sm' $1 $2 $3
	printf '\x1b[48;2;%s;%s;%sm' $1 $2 $3
}

resetOutput() {
	echo -en "\x1b[0m\n"
}

#----------------------------------------------------------------------
# Gives a color $1/255 % along HSV
# Who knows what happens when $1 is outside 0-255 ?
# Echoes "$red $green $blue" where
# $red $green and $blue are [0..255]
#----------------------------------------------------------------------
rainbowColor() {

	let h=$1/43
	let f=$1-43*$h
	let t=$f*255/43
	let q=255-$t

	#echo -e "\n($h $f $t $q)\n" 2>&1
	
	if [ $h -eq 0 ]
	then
		echo "255 $t 0"
	elif [ $h -eq 1 ]
	then
		echo "$q 255 0"
	elif [ $h -eq 2 ]
	then
		echo "0 255 $t"
	elif [ $h -eq 3 ]
	then
		echo "0 $q 255"
	elif [ $h -eq 4 ]
	then
		echo "$t 0 255"
	elif [ $h -eq 5 ]
	then
		echo "255 0 $q"
	else
		# execution should never reach here
		echo "0 0 0"
	fi
}

#------------------------------
# Red
#------------------------------
showRed() {
	for i in `seq 0 127`; do
		setBackgroundColor $i 0 0
		echo -en " "
	done
	resetOutput
	for i in `seq 255 -1 128`; do
		setBackgroundColor $i 0 0
		echo -en " "
	done
	resetOutput
}

#------------------------------
# Green
#------------------------------
showGreen() {
	for i in `seq 0 127`; do
		setBackgroundColor 0 $i 0
		echo -n " "
	done
	resetOutput
	for i in `seq 255 -1 128`; do
		setBackgroundColor 0 $i 0
		echo -n " "
	done
	resetOutput
}

#------------------------------
# Blue
#------------------------------
showBlue() {
	for i in `seq 0 127`; do
		setBackgroundColor 0 0 $i
		echo -n " "
	done
	resetOutput
	for i in `seq 255 -1 128`; do
		setBackgroundColor 0 0 $i
		echo -n " "
	done
	resetOutput
}

#------------------------------
# Rainbow Scale
#-----------------------------
showRainbow() {
	echo -e "\nThe HSV/HSL Rainbow\n"
	for i in `seq 0 127`; do
		setBackgroundColor `rainbowColor $i`
		echo -n " "
	done
	resetOutput
	for i in `seq 255 -1 128`; do
		setBackgroundColor `rainbowColor $i`
		echo -n " "
	done
	resetOutput
}

showLongBow() {
	echo -e "\nTesting in one line...\n" 
	for i in `seq 0 255`; do setBackgroundColor `rainbowColor $i`; echo -n " "; done
	resetOutput
}

#------------------------------
# Gray Scale
#------------------------------
# Grayscale = R/3 + G/3 + B/3			# average
# Grayscale = 0.299R + 0.587G + 0.114B	# weighted

grayScale() {
	# Average
	let r=$1/3
	let g=$2/3
	let b=$3/3
	echo "$r $g $b"
}

grayScale2() {
	# NOT WORKING!
	# weighted
	let r=$1/3				#$1*0.299
	let g=$2*6/10			#$2*0.587
	let b=$3/10				#$3*0.114
	echo "$r $g $b"
}

showGray() {
	echo -e "\nThe Average Grayscale\n"
	for i in `seq 0 6 768`; do
		#rgb=`rainbowColor $i`
		#gry=`grayScale $rgb`
		gry=`grayScale $i $i $i`
		#echo "($rgb) : ($gry)"		
		setBackgroundColor $gry
		echo -n " "
	done
	resetOutput
}

#----------------------------------------------------------------------
# Main
#----------------------------------------------------------------------
echo -e "\nThe 24-bit RGB scales (reduced to 8-bit display).\n"

showRed
showGreen
showBlue

showRainbow
showGray

echo -e "\n\nok"
exit 0

#----------------------------------------------------------------------
#  EOF
#----------------------------------------------------------------------