colorwheel

#!/bin/sh

# Given a size in pixels, output a square colorwheel for a Sixel
# capable terminal. Default size 480 pixels.
# Use +dither if you do not want solid, undithered colors.
#
# Mostly based on ImageMagick's documentation on how
# to create a colorwheel.
#
#   "You can generate the separate channel value images, and
#    Combine the images to generate specific types of images
#    which are hard to generate in other ways."
#
# Written by b9 in 2019, donated to the public domain. Failing that,
# this is licensed under the CC-0 (do whatever you want) license.

# Use ./colorwheel +dither to disable dithering.
if [ "$1" = "+dither" ]; then dither=$1; shift; fi

# Use ./colorwheel <size> for a different size square (default 480).
size=${1:-480}			# width and height of output
				
thrice=$((size*3))
half=$((size/2))

# Number of colors the terminal can show.
case "${TERM}" in
    vt340) colors=16  ;;
    vt330) colors=4   ;;		# Actually, vt330 is grayscale.
    vt240) colors=4   ;;
    *)     colors=256 ;;
esac

# Make the pieces
cd /tmp
convert -size ${size}x${thrice} gradient: -rotate 90 \
	-distort Arc '360 -90.1 500' +repage \
	-gravity center -crop ${size}x${size}+0+0 +repage  angular.png
convert -size ${size}x${size} xc:white                     solid.png
convert -size ${size}x${size} radial-gradient: -negate     radial.png

# Stick them together and show them on the sixel terminal
convert angular.png solid.png radial.png \
	-combine -set colorspace HSB \
	-colorspace sRGB ${dither} -colors ${colors} \
	sixel:-

rm angular.png solid.png radial.png