function rho=LSS(B12,sRGB)
% This is the Least Slope Squared (LSS) algorithm for generating
% a "reasonable" reflectance curve from a given sRGB color triplet.
% The reflectance spans the wavelength range 380-730 nm in 10 nm increments.

% It solves min sum(rho_i+1 - rho_i)^2 s.t. T rho = rgb,
% using Lagrangian approach.

% B12 is upper-right 36x3 part of inv([D,T';T,zeros(3)])
% sRGB is a three-element vector of target D65-referenced sRGB values in 0-255 range,
% rho is a 36x1 vector of reflectance values over wavelengths 380-730 nm,

% Written by Scott Allen Burns, 4/25/15.
% Licensed under a Creative Commons Attribution-ShareAlike 4.0 International
% License (http://creativecommons.org/licenses/by-sa/4.0/).
% For more information, see http://www.scottburns.us/subtractive-color-mixture/

% compute target linear rgb values
sRGB=sRGB(:)/255; % convert to 0-1 column vector
rgb=zeros(3,1);
% remove gamma correction to get linear rgb
for i=1:3
    if sRGB(i)<0.04045
        rgb(i)=sRGB(i)/12.92;
    else
        rgb(i)=((sRGB(i)+0.055)/1.055)^2.4;
    end
end

% matrix multiply
rho=B12*rgb;