egd_colorscheme.erl

%% Copyright (C) 2011 Björn-Egil Dahlberg
%%
%% File:    egd_colorscheme.erl
%% Author:  Björn-Egil Dahlberg
%% Created: 2011-09-02

-module(egd_colorscheme).

-export([
	hsl2rgb/1, 
	rgb2hsl/1,
	select/2
    ]).

% L - low dark, high bright
% S

select(core1, I) -> select(I, 17, 0.3, 0.3, 200);
select(_Default, I) -> select(I + 6, 31, 0.8, 0.4, 210).
select(I, Hm, S, L, A) -> egd:color(hsl2rgb({I*Hm rem 360, S, L, A})).

%% color conversions
%% H, hue has the range of [0, 360]
%% S, saturation has the range of [0,1]
%% L, lightness has the range of [0,1]

-define(float_error, 0.000000000001).

hsl2rgb({H,S,L}) -> hsl2rgb({H,S,L,255});
hsl2rgb({H,S,L,A}) ->
    Q  = if
	L < 0.5 -> L * (1 + S);
	true    -> L + S - (L * S)
    end,
    P  = 2 * L - Q,
    Hk = H/360,
    Rt = Hk + 1/3,
    Gt = Hk,
    Bt = Hk - 1/3,

    Cts = lists:map(fun
	(Tc) when Tc < 0.0 -> Tc + 1.0;
	(Tc) when Tc > 1.0 -> Tc - 1.0;
	(Tc) ->  Tc
    end, [Rt, Gt, Bt]),
    [R,G,B] = lists:map(fun
	(Tc) when Tc < 1/6 -> P + ((Q - P) * 6 * Tc);
	(Tc) when Tc < 1/2, Tc >= 1/6 -> Q;
	(Tc) when Tc < 2/3, Tc >= 1/2 -> P + ((Q - P) * 6 * (2/3 - Tc));
	(_ ) -> P
    end, Cts),
    {trunc(R*255),trunc(G*255),trunc(B*255),A}.

byte2float(B) -> envelope(B/255, 0, 1).

envelope(V, Min, Max) when V >= Min andalso V =< Max -> V;
envelope(V, Min, Max) when V < Min andalso V < Max -> Min;
envelope(V, Min, Max) when V > Min andalso V > Max -> Max.

rgb2hsl({R,G,B}) -> rgb2hsl({R,G,B,255});
rgb2hsl({R,G,B,A}) ->
    Rf  = byte2float(R),
    Gf  = byte2float(G),
    Bf  = byte2float(B),

    Max = lists:max([Rf,Gf,Bf]),
    Min = lists:min([Rf,Gf,Bf]),
    H   = if
	    abs(Max - Min) < ?float_error ->
		0.0;
	    abs(Max - Rf)  < ?float_error ->
		D  = 60 * (Gf - Bf)/(Max - Min),
		Dt = trunc(D),
		case {((Dt + 360) rem 360), (D - Dt)} of
		    {0, Frac} when Frac < 0 -> 360.0 + Frac;
		    {Degs, Frac} -> Degs + Frac
		end;
	    abs(Max - Gf) < ?float_error ->
		60 * (Bf - Rf)/(Max - Min) + 120;
	    abs(Max - Bf) < ?float_error ->
		60 * (Rf - Gf)/(Max - Min) + 240;
	    true -> 
		0.0
	end,
    L   = (Max + Min)/2,
    S   = if
	    abs(Max - Min) < ?float_error ->
		0;
	    L > 0.5 ->
		(Max - Min)/(2 - (Max + Min));
	    true ->
		(Max - Min)/(Max + Min)
	end,
    {H, envelope(S, 0.0, 1.0), envelope(L, 0.0, 1.0), A}.