module Foundation::Palette

Overview

xterm 256 color palette: index -> sRGB. The target set for downampling truecolor to ANSI256/ANSI16 by nearest ΔE76 perceptual distance.

See reference xterm 256 color chart: https://jonasjacek.github.io/colors/

• Indices 0-15: conventional xterm system colors
• Indices 16-231: a 6x6x6 color cube where each channel increment follows {0, 95, 135, 175, 215, 255} and index = 16 + 36r + 6g + b. So for example, the color at cube position (r=1, g=2, b=3) → RGB (95, 135, 175) → palette index 16 + 36 + 12 + 3 = 67.
• Indices 232-255: 24-step grayscale ramp, where value = 8 + 10*i

Defined in:

foundation/palette.cr

Constant Summary

ANSI256 = begin colors = Array(RGB).new(256) colors.concat(BASE_16) CUBE_LEVELS.each do |red| CUBE_LEVELS.each do |green| CUBE_LEVELS.each do |blue| colors << (RGB.new(red, green, blue)) end end end 24.times do |i| v = (8 + (i * 10)).to_u8 colors << (RGB.new(v, v, v)) end colors end

All 256 palette colors index 0-255 in an array.

BASE_16 = ["#000000", "#800000", "#008000", "#808000", "#000080", "#800080", "#008080", "#c0c0c0", "#808080", "#ff0000", "#00ff00", "#ffff00", "#0000ff", "#ff00ff", "#00ffff", "#ffffff"].map do |hex| RGB.parse(hex) end

Indices 0-15 ANSI system colors

CUBE_LEVELS = {0_u8, 95_u8, 135_u8, 175_u8, 215_u8, 255_u8}

The six color cube levels