petermac-
9/5/2016 - 9:38 AM

## Print a 256-colour test pattern in the terminal

Print a 256-colour test pattern in the terminal

``````#!/bin/bash

# Tom Hale, 2016. MIT Licence.
# Print out 256 colours, with each number printed in its corresponding colour

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 {
local r g b luminance
colour="\$1"

if (( colour < 16 )); then # Initial 16 ANSI colours
(( colour == 0 )) && 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[48;5;%sm" "\$colour"                # Start block of colour
printf "\e[38;5;%sm%3d" "\$contrast" "\$colour" # In contrast, print number
printf "\e[0m "                               # Reset 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_run 0 16 # 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
``````