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#!/bin/bash
# Tom Hale, 2016. MIT Licence.
# Print out 256 colours, with each number printed in its corresponding colour
# See http://askubuntu.com/questions/821157/print-a-256-color-test-pattern-in-the-terminal/821163#821163
set -eu # Fail on errors or undeclared variables
printable_colours=256
# Return a colour that contrasts with the given colour
# Bash only does integer division, so keep it integral
function contrast_colour {
colour="$1"
if (( colour < 16 )); then # Initial 16 ANSI colours
(( colour == 0 || colour == 8 )) && printf "15" || printf "0"
return
fi
# Greyscale # rgb_R = rgb_G = rgb_B = (number - 232) * 10 + 8
if (( colour > 231 )); then # Greyscale ramp
(( colour < 244 )) && printf "15" || printf "0"
return
fi
# All other colours:
# 6x6x6 colour cube = 16 + 36*R + 6*G + B # Where RGB are [0..5]
# See http://stackoverflow.com/a/27165165/5353461
# r=$(( (colour-16) / 36 ))
g=$(( ((colour-16) % 36) / 6 ))
# b=$(( (colour-16) % 6 ))
# If luminance is bright, print number in black, white otherwise.
# Green contributes 587/1000 to human perceived luminance - ITU R-REC-BT.601
(( g > 2)) && printf "0" || printf "15"
return
# Uncomment the below for more precise luminance calculations
# # Calculate percieved brightness
# # See https://www.w3.org/TR/AERT#color-contrast
# # and http://www.itu.int/rec/R-REC-BT.601
# # Luminance is in range 0..5000 as each value is 0..5
# luminance=$(( (r * 299) + (g * 587) + (b * 114) ))
# (( $luminance > 2500 )) && printf "0" || printf "15"
}
# Print a coloured block with the number of that colour
function print_colour {
local colour="$1" contrast
contrast=$(contrast_colour "$1")
printf "\\e[38;5;%s;48;5;%sm%3d\\e[0m " "$contrast" "$colour" "$colour"
}
# Starting at $1, print a run of $2 colours
function print_run {
local i
for (( i = "$1"; i < "$1" + "$2" && i < printable_colours; i++ )) do
print_colour "$i"
done
printf " "
}
# Print blocks of colours
function print_blocks {
local start="$1" i
local end="$2" # inclusive
local block_cols="$3"
local block_rows="$4"
local blocks_per_line="$5"
local block_length=$((block_cols * block_rows))
# Print sets of blocks
for (( i = start; i <= end; i += (blocks_per_line-1) * block_length )) do
printf "\\n" # Space before each set of blocks
# For each block row
for (( row = 0; row < block_rows; row++ )) do
# Print block columns for all blocks on the line
for (( block = 0; block < blocks_per_line; block++ )) do
print_run $(( i + (block * block_length) )) "$block_cols"
done
(( i += block_cols )) # Prepare to print the next row
printf "\\n"
done
done
}
print_blocks 0 15 8 2 1 # The first 16 colours are spread over the whole spectrum
printf "\\n"
print_blocks 16 231 6 6 3 # 6x6x6 colour cube between 16 and 231 inclusive
print_blocks 232 255 12 2 1 # Not 50, but 24 Shades of Grey
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