how to convert color image into lab color model ? its showing error ?

clear all;
close all;
lab = rgb2lab(rgb);

%RGB2LAB Convert an image from RGB to CIELAB
%
% function [L, a, b] = RGB2Lab(R, G, B)
% function [L, a, b] = RGB2Lab(I)
% function I = RGB2Lab(...)
%
% RGB2Lab takes red, green, and blue matrices, or a single M x N x 3 image, 
% and returns an image in the CIELAB color space.  RGB values can be
% either between 0 and 1 or between 0 and 255.  Values for L are in the
% range [0,100] while a and b are roughly in the range [-110,110].  The
% output is of type double.
%
% 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.  
%
% See also LAB2RGB.

% By Mark Ruzon from C code by Yossi Rubner, 23 September 1997.
% Updated for MATLAB 5 28 January 1998.
% Updated for MATLAB 7 30 March 2009.

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 = double(R) / 255;
  G = double(G) / 255;
  B = double(B) / 255;
end

% Set a threshold
T = 0.008856;

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

% Normalize for D65 white point
X = XYZ(1,:) / 0.950456;
Y = XYZ(2,:);
Z = XYZ(3,:) / 1.088754;

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

Y3 = Y.^(1/3); 

fX = XT .* X.^(1/3) + (~XT) .* (7.787 .* X + 16/116);
fY = YT .* Y3 + (~YT) .* (7.787 .* Y + 16/116);
fZ = ZT .* Z.^(1/3) + (~ZT) .* (7.787 .* Z + 16/116);

L = reshape(YT .* (116 * Y3 - 16.0) + (~YT) .* (903.3 * Y), M, N);
a = reshape(500 * (fX - fY), M, N);
b = reshape(200 * (fY - fZ), M, N);

if nargout < 2
  L = cat(3,L,a,b);
end

Answer 1

[Walter Roberson]

You appear to have taken a routine named RGB2Lab and removed the 'function' line from it and put it on the end of your code. And then you appear to be calling rgb2lab() which you are not showing any definition for.

You should have downloaded RGB2Lab complete with its function line, and you should leave it in its own RGB2Lab.m file. And you should call it with the name RGB2Lab -- MATLAB is case sensitive.

But before you call it, you need to define your variable "rgb". We know for sure that it has no definition because you have "clear all" which would delete any existing variables you had. "clear all" should never be used except at the command line in my opinion.

[venmal devi]

how to define the variable in the code ?

[Walter Roberson]

rgb = imread('SomeImageFileNameHere');