colors/colors.lua

--[[
	This is a collection of functions for performing color operations,
	designed to work with the LÖVE game engine.

	See `README_COLOR_TOOLS.md` for full documentation.

	LICENSE is BEER-WARE.
]]

local colors = {}

--------------------------------------------------------------------------------
-- Conversion

colors.rgb_to_hsl = function (color)
	-- Error check
	assert(type(color) == 'table' and type(color[1]) == 'number' and type(color[2]) == 'number' and type(color[3]) == 'number')

	-- Do stuff
	local r = (color[1] == 255 and 1 or color[1]/256)
	local g = (color[2] == 255 and 1 or color[2]/256)
	local b = (color[3] == 255 and 1 or color[3]/256)

	local max, min = math.max(r, g, b), math.min(r, g, b)
	if min == max then  return { 0, 0, (max+min)/2 }  end

	local h, s, l = (max+min)/2, (max+min)/2, (max+min)/2

	local d = max - min
	s = l > 0.5 and d / (2 - max - min) or d / (max + min)
	if max == r then
		h = ((g - b) / d + (g < b and 6 or 0)) / 6
	elseif max == g then
		h = ((b - r) / d + 2) / 6
	elseif max == b then
		h = ((r - g) / d + 4) / 6
	end

	return {h, s, l}
end

local function hue_to_rgb (p, q, t)
	assert(type(p) == 'number')
	assert(type(q) == 'number')
	assert(type(t) == 'number')

	if t < 0 then
		t = t + 1
	end
	if t > 1 then
		t = t - 1
	end
	if t < 1/6 then
		return p + (q - p) * 6 * t
	elseif t < 1/2 then
		return q
	elseif t < 2/3 then
		return p + (q - p) * 6 * (2/3 - t)
	end
	return p
end

colors.hsl_to_rgb = function (color)
	-- Error check
	assert(type(color) == 'table' and type(color[1]) == 'number' and type(color[2]) == 'number' and type(color[3]) == 'number')

	-- Do stuff
	local h, s, l = color[1], color[2], color[3]
	local r, g, b
	if s == 0 then
		r, g, b = l, l, l
	else
		local q = l < 0.5 and l * (1+s) or l+s - l*s
		local p = 2 * l - q
		r, g, b = hue_to_rgb(p,q,h+1/3), hue_to_rgb(p,q,h), hue_to_rgb(p,q,h-1/3)
	end
	r, g, b = math.floor(math.min(255,r*256)+0.5), math.floor(math.min(255,g*256)+0.5), math.floor(math.min(255,b*256)+0.5)
	return { r, g, b }
end

--------------------------------------------------------------------------------
-- Interpolation

colors.interpolate_rgb = function (c1, c2, t)
	assert(type(c1) == 'table' and type(c1[1]) == 'number' and type(c1[2]) == 'number' and type(c1[3]) == 'number' and (type(c1[4]) == 'number' or c1[4] == nil))
	assert(type(c2) == 'table' and type(c2[1]) == 'number' and type(c2[2]) == 'number' and type(c2[3]) == 'number' and (type(c2[4]) == 'number' or c2[4] == nil))
	assert(type(t) == 'number')
	--
	local alpha = nil
	if c1[4] or c2[4] then  alpha = (c1[4] or 255) + ((c2[4] or 255)-(c1[4] or 255))*t  end
	return { c1[1]+(c2[1]-c1[1])*t, c1[2]+(c2[2]-c1[2])*t, c1[3]+(c2[3]-c1[3])*t, alpha }
end

local function angle_delta (a1, a2)
	assert(type(a1) == 'number')
	assert(type(a2) == 'number')
	local r1, r2 = (a1-a2)%1, (a2-a1)%1
	return r1 < r2 and -r1 or r2
end

colors.interpolate_hsl = function (c1, c2, t)
	-- Error handling
	assert(type(c1) == 'table' and type(c1[1]) == 'number' and type(c1[2]) == 'number' and type(c1[3]) == 'number')
	assert(type(c2) == 'table' and type(c2[1]) == 'number' and type(c2[2]) == 'number' and type(c2[3]) == 'number')
	assert(type(t) == 'number')
	-- Do stuff
	local h = c1[1] + angle_delta(c1[1], c2[1]) * t
	local s = c1[2] + (c2[2] - c1[2]) * t
	local l = c1[3] + (c2[3] - c1[3]) * t
	return { h, s, l }
end

colors.interpolate_rgb_by_way_of_hsl = function (c1, c2, t)
	-- Error handling
	assert(type(c1) == 'table' and type(c1[1]) == 'number' and type(c1[2]) == 'number' and type(c1[3]) == 'number')
	assert(type(c2) == 'table' and type(c2[1]) == 'number' and type(c2[2]) == 'number' and type(c2[3]) == 'number')
	assert(type(t) == 'number')
	-- Do stuff
	local c1_hsl = colors.rgb_to_hsl(c1)
	local c2_hsl = colors.rgb_to_hsl(c2)
	local c3_hsl = colors.interpolate_hsl(c1_hsl, c2_hsl, t)
	return colors.hsl_to_rgb(c3_hsl)
end

--------------------------------------------------------------------------------
-- Inversion

colors.invert_rgb = function (color)
	assert(type(color) == 'table' and type(color[1]) == 'number' and type(color[2]) == 'number' and type(color[3]) == 'number')
	return { 255 - color[1], 255 - color[2], 255 - color[3] }
end

--------------------------------------------------------------------------------
-- Parsing

colors.parse_rgb = function (str)
	-- Error handling
	assert(type(str) == 'string')
	-- Do stuff
	local r, g, b
	local o  = (str:sub(1,1) == '#') and 1 or 0 -- Offset
	if #str == 3 or #str == 4 then
		r, g, b = str:sub(o+1,o+1):rep(2), str:sub(o+2,o+2):rep(2), str:sub(o+3,o+3):rep(2)
	else
		r, g, b = str:sub(o+1,o+2), str:sub(o+3,o+4), str:sub(o+5,o+6)
	end
	return {tonumber('0x'..r), tonumber('0x'..g), tonumber('0x'..b)}
end

--------------------------------------------------------------------------------

return colors