% [v] = histogram_streching_threshold(u,alpha,beta,th_a,v_a,th_b,v_b)
%
% This function performs a histogram streching by deviding the input values
% into three regions and applying different strechings to each region.
% Input parameters:
%   u -> Input luminance image with values 0-255 (uint8)
%   alpha -> slope first region
%   beta -> slope second region
%   th_a -> Threshold that separates region 1 and 2
%   v_a -> output value for u=a
%   th_b -> Threshold that separates region 2 and 3  
%   v_b ->output value for u=b
% Output:
%   v -> Enhanced imaged
%
% Programmed by Daniel Garcia-Romero 2-21-2006
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

function [v] = histogram_streching_threshold(u,alpha,beta,th_a,th_b)
[n_row,n_col]=size(u);
v=zeros(n_row,n_col);
u=double(u);

MAX_LUM=255; % Set up the maximum value for Y
MIN_U=min(min(u));
MAX_U=max(max(u));

v_o=-alpha*MIN_U;
v_a=alpha*(th_a)+v_o;
v_b=beta*(th_b-th_a)+v_a;
gamma=(MAX_LUM-v_b)/(MAX_U-th_b); % This is not a free parameter

for i=1:n_row
    for j=1:n_col
        if (u(i,j) < th_a)
            v(i,j)=alpha*(u(i,j))+v_o;
        elseif (u(i,j) >= th_a && u(i,j) < th_b )
            v(i,j)=beta*(u(i,j)-th_a)+v_a;
        elseif (u(i,j) >= th_b && u(i,j) <= MAX_LUM )
            v(i,j)=gamma*(u(i,j)-th_b)+v_b;
        else
            disp('ERROR: The input signal must take values between 0-255');
            return;
        end
    end
end
v=uint8(v);