What algorithm is used to approximate colors?

[swg2you 0]

Hello.

When output to the monitor, the color 0xRRGGBB is approximated to the palette set in the monitor.  I need to know exactly which color I will get through gpu.get when outputting color 0xRRGGBB.

Can you answer (or show example on the Lua): What algorithm is used to approximate colors?

Sorry for the bad english.

-=-

Below, just a small program that I use to test colors. Maybe it will be useful to someone.

sicot.lua   "SimpleColorTester",  "MC 1.12.2", "OpenComputers  version 1.7.0.4", "Lua 5.3"

local gpu=require('component').gpu
local ev=require('event')

local function setColor(f,b)
  gpu.setForeground(f)
  gpu.setBackground(b)
end

local w=0xFFFFFF
require('term').clear()
gpu.fill(1,1,80,12," ")
gpu.set(27,1,'← click →')
gpu.set(69,1,' RMB to exit')
gpu.set(69,3,'◄ Tier 2')
gpu.set(69,5,'◄ Tier 3')
gpu.set(69,6,'       ▼')

local function showcolor(c)
  for i=0,24 do
    local b= (c & 2^i) >> i 
    setColor((1-b)*w, b*w)   
    gpu.set(24-i,1,tostring(b))
  end
  setColor(c ~ w, c)  
  gpu.fill(1,3,24,3," ")
  gpu.set(7,4,string.format('Set: %06X',c))
  local _,_,bg=gpu.get(1,3)
  gpu.set(7,5,string.format('Get: %06X',bg))
  setColor(w,0)
  gpu.set(1,2,'└──┴┴──┘└──┴┴──┘└──┴┴──┘')
end

local color=math.random(w)
showcolor(color)

local s='SimpleColorTest'
for i=1,#s do
  setColor(color ~ w, color) 
  gpu.set(36+i*2,1,string.sub(s,i,i)..' ')
  color=math.random(w)
end

--[[ https://en.wikipedia.org/wiki/List_of_software_palettes#6-8-5_levels_RGB
    Tier3 palette is constructed with six levels for red, eight levels for green and five levels for the blue primaries, 
	giving 6×8×5 = 240 combinations. The index can be addressed by (40×R)+(5×G)+B, with R ranging from 0 to 5, G from 
	0 to 7 and B from 0 to 4. Levels are chosen in function of sensibility of the normal human eye to every primary color.]]


--Tier2: safe colors, use this if you need that your program look the same on tier2 and tier3 monitors
palTier2={0x000000,0x333333,0x333399,0x336600,0x336699,0x33CC33,0x663300,0x6699FF,0x9933CC,0xCC66CC,0xCCCCCC,0xFF3333,0xFF6699,0xFFCC33,0xFFFF33,0xFFFFFF}
--alternative palettes
palCGA={0x000000,0x0000AA,0x00AA00,0x00AAAA,0xAA0000,0xAA00AA,0xAA5500,0xAAAAAA,0x555555,0x5555FF,0x55FF55,0x55FFFF,0xFF5555,0xFF55FF,0xFFFF55,0xFFFFFF}
palRGBI={0x000000,0x0000AA,0x00AA00,0x00AAAA,0xAA0000,0xAA00AA,0xAAAA00,0xAAAAAA,0x555555,0x5555FF,0x55FF55,0x55FFFF,0xFF5555,0xFF55FF,0xFFFF55,0xFFFFFF}
palZX={0x000000,0x0000CD,0xCD0000,0xCD00CD,0x00CD00,0x00CDCD,0xCDCD00,0xCDCDCD,0x000000,0x0000FF,0xFF0000,0xFF00FF,0x00FF00,0x00FFFF,0xFFFF00,0xFFFFFF}
pal7f={0x000000,0x000080,0x008000,0x008080,0x800000,0x800080,0x808000,0x808080,0x1f1f1f,0x1f1fFF,0x1fFF1f,0x1fFFFF,0xFF1f1f,0xFF1fFF,0xFFFF1f,0xFFFFFF}

--show getPaletteColor(0..15)
for x=0,7 do
  for y=0,1 do
    local c=gpu.getPaletteColor(x+y*8)
    setColor(w,c)
    gpu.set(1+x*10, 14+y, '   ')
    setColor(w,0)
    gpu.set(1+x*10+3, 14+y, string.format('%06X',c))
  end
end

--show Tier2 safe colors
for x=0,15 do
  setColor(w,palTier2[x+1]) 
  gpu.set(36+2*x, 3, '  ') 
end

--show Tier3, by default 16 shades of gray
for y=0,0xF do 
  setColor(w,(y+1)*0x0f0f0f) 
  gpu.set(36+y*2, 5, '  ')
end

--Tier3: big palette 6r*8g*5b=240 fixed colors (approximately) are generated by the following algorithm
for r=0,5 do
  for g=0,7 do
    for b=0,4 do
      setColor(w, r*0x330000 + g*0x2400 + b*0x3F)
      gpu.set(1+2*(b+g*5), 7+r, '  ')
    end 
  end
end
-- for greater accuracy it is necessary to use tables
--  RR = {0x00, 0x33, 0x66, 0x99, 0xCC, 0xFF}
--  GG = {0x00, 0x24, 0x49, 0x6D, 0x92, 0xB6, 0xDB, 0xFF}
--  BB = {0x00, 0x40, 0x80, 0xC0, 0xFF}
-- and formula color = RR[r] << 16  +  GG[g] << 8  +  BB[b]
-- when r = 1 .. 6;    g = 1 .. 8;    b = 1 .. 5

local t=ev.timer(1,function() setColor(w,0) gpu.set(69,1,'→RMB to exit') os.sleep(.05) gpu.set(69,1,' RMB to exit') end, math.huge)
repeat
  local _,_,x,y,k=ev.pull('touch')
  local s,fg,bg=gpu.get(x,y)
  if y==1 and x<=24 then
    local bit=24-x
    color = color ~ 2^bit 
    showcolor(color)
  else
    color=bg
    showcolor(color)
  end
  --temp test compute approximation
  r = (color & 0xFF0000) >> 16
  g = (color & 0x00FF00) >> 8
  b = color & 0x0000FF
  r = r*6//256
  g = g*8//256
  b = b*5//256
  RR={0x00,0x33,0x66,0x99,0xCC,0xFF}
  GG={0x00,0x24,0x49,0x6D,0x92,0xB6,0xDB,0xFF}
  BB={0x00,0x40,0x80,0xC0,0xFF}
  --temp show test
  gpu.set(1+00, 16, string.format('%02X',RR[r+1]))
  gpu.set(1+10, 16, string.format('%02X',GG[g+1]))
  gpu.set(1+20, 16, string.format('%02X',BB[b+1]))
  
until k==1
ev.cancel(t)
setColor(w,0)
require('term').clear()

 

[Fingercomp 37]

This page, though in Russian, may be useful to you. The code there contains functions to deflate a color into its palette index and vice versa. Generate a color palette and use palette:inflate(palette:deflate(color)).

[swg2you 0]

2 hours ago, Fingercomp said:

This page, though in Russian, may be useful to you. The code there contains functions to deflate a color into its palette index and vice versa. Generate a color palette and use palette:inflate(palette:deflate(color)).

Thanks a lot.

I correctly understood? "хитромудрый код" is taken directly from the code of OpenComputers?

P.S Say hello to computercraft.ru