diff --git a/OverLua/docs/rgb2hsl.lua b/OverLua/docs/rgb2hsl.lua
new file mode 100644
index 000000000..dbe8b301f
--- /dev/null
+++ b/OverLua/docs/rgb2hsl.lua
@@ -0,0 +1,55 @@
+--- Convert RGB colour to HSL (hue/saturation/luminance)
+-- The output image will have H instead of R,
+-- S instead of G and L instead of B.
+
+function render_frame(f, t)
+	local w, h = f.width, f.height
+	for y = 0, h-1 do
+		for x = 0, w-1 do
+			local r, g, b = f(x, y)
+			local h, s, l = RGB2HSL(r, g, b)
+			f[y*w+x] = {h, s, l}
+		end
+	end
+end
+
+function clip_colorval(v)
+	if v < 0 then
+		return 0
+	elseif v > 255 then
+		return 255
+	else
+		return math.floor(v)
+	end
+end
+
+function RGB2HSL(R, G, B)
+	local r, g, b = R/255, G/255, B/255
+	local h, s, l
+	
+	local minrgb, maxrgb = math.min(r, math.min(g, b)), math.max(r, math.max(g, b))
+	
+	l = (minrgb + maxrgb) / 2
+	
+	if minrgb == maxrgb then
+		h, s = 0, 0
+	else
+		if l < 0.5 then
+			s = (maxrgb - minrgb) / (maxrgb + minrgb)
+		else
+			s = (maxrgb - minrgb) / (2 - maxrgb - minrgb)
+		end
+		if r == maxrgb then
+			h = (g - b) / (maxrgb - minrgb) + 0
+		elseif g == maxrgb then
+			h = (b - r) / (maxrgb - minrgb) + 2
+		else
+			h = (r - g) / (maxrgb - minrgb) + 4
+		end
+	end
+	
+	if h < 0 then h = h + 6 end
+	if h >= 6 then h = h - 6 end
+	
+	return clip_colorval(h*255/6), clip_colorval(s*255), clip_colorval(l*255)
+end