1#!/usr/bin/env bash
 2
 3# Tom Hale, 2016. MIT Licence.
 4# Print out 256 colours, with each number printed in its corresponding colour
 5# See http://askubuntu.com/questions/821157/print-a-256-color-test-pattern-in-the-terminal/821163#821163
 6
 7set -eu # Fail on errors or undeclared variables
 8
 9printable_colours=256
10
11# Return a colour that contrasts with the given colour
12# Bash only does integer division, so keep it integral
13function contrast_colour {
14    local r g b luminance
15    colour="$1"
16
17    if (( colour < 16 )); then # Initial 16 ANSI colours
18        (( colour == 0 )) && printf "15" || printf "0"
19        return
20    fi
21
22    # Greyscale # rgb_R = rgb_G = rgb_B = (number - 232) * 10 + 8
23    if (( colour > 231 )); then # Greyscale ramp
24        (( colour < 244 )) && printf "15" || printf "0"
25        return
26    fi
27
28    # All other colours:
29    # 6x6x6 colour cube = 16 + 36*R + 6*G + B  # Where RGB are [0..5]
30    # See http://stackoverflow.com/a/27165165/5353461
31
32    # r=$(( (colour-16) / 36 ))
33    g=$(( ((colour-16) % 36) / 6 ))
34    # b=$(( (colour-16) % 6 ))
35
36    # If luminance is bright, print number in black, white otherwise.
37    # Green contributes 587/1000 to human perceived luminance - ITU R-REC-BT.601
38    (( g > 2)) && printf "0" || printf "15"
39    return
40
41    # Uncomment the below for more precise luminance calculations
42
43    # # Calculate perceived brightness
44    # # See https://www.w3.org/TR/AERT#color-contrast
45    # # and http://www.itu.int/rec/R-REC-BT.601
46    # # Luminance is in range 0..5000 as each value is 0..5
47    # luminance=$(( (r * 299) + (g * 587) + (b * 114) ))
48    # (( $luminance > 2500 )) && printf "0" || printf "15"
49}
50
51# Print a coloured block with the number of that colour
52function print_colour {
53    local colour="$1" contrast
54    contrast=$(contrast_colour "$1")
55    printf "\e[48;5;%sm" "$colour"                # Start block of colour
56    printf "\e[38;5;%sm%3d" "$contrast" "$colour" # In contrast, print number
57    printf "\e[0m "                               # Reset colour
58}
59
60# Starting at $1, print a run of $2 colours
61function print_run {
62    local i
63    for (( i = "$1"; i < "$1" + "$2" && i < printable_colours; i++ )) do
64        print_colour "$i"
65    done
66    printf "  "
67}
68
69# Print blocks of colours
70function print_blocks {
71    local start="$1" i
72    local end="$2" # inclusive
73    local block_cols="$3"
74    local block_rows="$4"
75    local blocks_per_line="$5"
76    local block_length=$((block_cols * block_rows))
77
78    # Print sets of blocks
79    for (( i = start; i <= end; i += (blocks_per_line-1) * block_length )) do
80        printf "\n" # Space before each set of blocks
81        # For each block row
82        for (( row = 0; row < block_rows; row++ )) do
83            # Print block columns for all blocks on the line
84            for (( block = 0; block < blocks_per_line; block++ )) do
85                print_run $(( i + (block * block_length) )) "$block_cols"
86            done
87            (( i += block_cols )) # Prepare to print the next row
88            printf "\n"
89        done
90    done
91}
92
93print_run 0 16 # The first 16 colours are spread over the whole spectrum
94printf "\n"
95print_blocks 16 231 6 6 3 # 6x6x6 colour cube between 16 and 231 inclusive
96print_blocks 232 255 12 2 1 # Not 50, but 24 Shades of Grey