Color Space Conversion: Why lab2rgb(rgb2lab(RGB)) cannot fully recovery a RGB image?

Question

After do a color space conversion from RGB to LAB, I want to recovery the original RGB image by LAB->RGB conversion.

However, When try: RGB --rgb2lab --> LAB -- lab2rgb --> RGB2, the output RGB2 is lighter than input RGB, why?

Matlab code: imshow(lab2rgb(rgb2lab(imread('peppers.png'))))

in detail:

RGB = imread('peppers.png');
LAB = rgb2lab(RGB); %convert RGB to LAB
RGB2 = lab2rgb(LAB); %try to recovery RGB
figure, 
subplot(121); imshow(RGB); 
subplot(122); imshow(RGB2);

Environment: Matlab R2015a

Answer 1

The default function provided by MATLAB will not produce this error.

This error is caused by user defined function rgb2lab overwrite the default function. the overwrite function which may produce that error is as following, see GeometricContext\src\tools\misc

function [L,a,b] =rgb2lab(R,G,B)
% function [L, a, b] = RGB2Lab(R, G, B)
% RGB2Lab takes matrices corresponding to Red, Green, and Blue, and 
% transforms them into CIELab.  This transform is based on ITU-R 
% Recommendation  BT.709 using the D65 white point reference.
% The error in transforming RGB -> Lab -> RGB is approximately
% 10^-5.  RGB values can be either between 0 and 1 or between 0 and 255.  
% By Mark Ruzon from C code by Yossi Rubner, 23 September 1997.
% Updated for MATLAB 5 28 January 1998.

if (nargin == 1)
  B = double(R(:,:,3));
  G = double(R(:,:,2));
  R = double(R(:,:,1));
end

if ((max(max(R)) > 1.0) | (max(max(G)) > 1.0) | (max(max(B)) > 1.0))
  R = R/255;
  G = G/255;
  B = B/255;
end

[M, N] = size(R);
s = M*N;

% Set a threshold
T = 0.008856;

RGB = [reshape(R,1,s); reshape(G,1,s); reshape(B,1,s)];

% RGB to XYZ
MAT = [0.412453 0.357580 0.180423;
       0.212671 0.715160 0.072169;
       0.019334 0.119193 0.950227];
XYZ = MAT * RGB;

X = XYZ(1,:) / 0.950456;
Y = XYZ(2,:);
Z = XYZ(3,:) / 1.088754;

XT = X > T;
YT = Y > T;
ZT = Z > T;

fX = XT .* X.^(1/3) + (~XT) .* (7.787 .* X + 16/116);

% Compute L
Y3 = Y.^(1/3); 
fY = YT .* Y3 + (~YT) .* (7.787 .* Y + 16/116);
L  = YT .* (116 * Y3 - 16.0) + (~YT) .* (903.3 * Y);

fZ = ZT .* Z.^(1/3) + (~ZT) .* (7.787 .* Z + 16/116);

% Compute a and b
a = 500 * (fX - fY);
b = 200 * (fY - fZ);

L = reshape(L, M, N);
a = reshape(a, M, N);
b = reshape(b, M, N);

if ((nargout == 1) | (nargout == 0))
  L = cat(3,L,a,b);
end