HSV colour transitions

[luminosity]

Something I did for my game recently was a small colour module to allow me to calculate transitions between two colours using the hsv colour space. This means I can make a gradient from one colour to another, without getting muddy-looking colours in the middle, which transitioning through RGB gets you. For instance, I use it to calculate what colour to draw a hit point bar, from red at 20% or below, to green at 80% or above.

The code is fairly game neutral so I thought people here might potentially find it useful, and it's nice to give back. Consider it public domain.

Code: Select all

local function smallColorRange(largeColor)
	return largeColor / 255
end

local function largeColorRange(smallColor)
	return math.floor(smallColor * 255 + 0.5)
end

local function pack(...)
	return arg
end

local function rgbToHsv(r, g, b)
	-- convert rgb from 0-255, to 0-1
	r = smallColorRange(r)
	g = smallColorRange(g)
	b = smallColorRange(b)
	local h, s, v
	local min = math.min(r, g, b)
	local max  = math.max(r, g, b)
	local delta = max - min
	-- value
	v = max
	-- saturation
	if delta ~= 0 then -- we know max won't be zero, as min can't be less than zero and the difference is not 0
		s = delta / max
	else
		h = -1
		s = 0
		return h, s, v
	end
	-- hue
	if r == max then -- yellow <-> magenta
		h = (g - b) / delta
	elseif g == max then -- cyan <-> yellow
		h = 2 + (b - r) / delta
	else -- magenta <-> cyan
		h = 4 + (r - g) / delta
	end
	h = h * 60 -- 60 degrees
	if h < 0 then
		h = h + 360
	end
	return h, s, v
end

local function hsvToRgb(h, s, v)
	local r, g, b
	if s == 0 then -- monochromatic
		-- restore colors from 0-1, to 0-255
		r = largeColorRange(v)
		g = largeColorRange(v)
		b = largeColorRange(v)
		return r, g, b
	end
	
	h = h / 60 -- sector of wheel
	local i = math.floor(h)
	local f = h - i -- factorial part of h
	local p = v * (1 - s)
	local q = v * (1 - s * f)
	local t = v * (1 - s * (1 - f))
	
	if i == 0 then
		r = v
		g = t
		b = p
	elseif i == 1 then
		r = q
		g = v
		b = p
	elseif i == 2 then
		r = p
		g = v
		b = t
	elseif i == 3 then
		r = p
		g = q
		b = v
	elseif i == 4 then
		r = t
		g = p
		b = v
	else
		r = v
		g = p
		b = q
	end
	
	r = largeColorRange(r)
	g = largeColorRange(g)
	b = largeColorRange(b)
	
	return r, g, b
end

local function gradientColor(color1, color2, proportion)
	if proportion < 0 or proportion > 1 then 
		error("Gradient proportion must be between 0 and 1.") 
	end
	
	local h1, s1, v1 = rgbToHsv(unpack(color1))
	local h2, s2, v2 = rgbToHsv(unpack(color2))
	-- find difference between two values, and advance proportion of that difference from value 1
	local hDiff = (h2 - h1) * proportion
	local sDiff = (s2 - s1) * proportion
	local vDiff = (v2 - v1) * proportion
	
	local gradH = hDiff + h1
	local gradS = sDiff + s1
	local gradV = vDiff + v1
	
	return pack(hsvToRgb(gradH, gradS, gradV))
end

return {
	rgbToHsv=rgbToHsv,
	hsvToRgb=hsvToRgb,
	gradientColor=gradientColor,
}

So to use it I'd say

Code: Select all

local Color = require('color')

-- randomly picked probably ugly colours
local red = {200, 20, 20}
local green = {30, 200, 10}
local hpBarColor = Color.gradientColor(red, green, curHP / totalHP)

[TechnoCat]

Oh my, HSV is very yummy.

Colorful Cake.

800px-HSV_color_solid_cylinder_alpha_lowgamma.png (253.63 KiB) Viewed 1547 times

The hue saturation approach to color is very useful indeed. Thank you for the contribution.

[Taehl]

I, personally, prefer HSL, Photoshop be damned. I've put my conversion function on the wiki, in case anyone wants it.