how to convert rgb to lab

can any1 tell me how excatly this works

function [L,a,b] = RGB2Lab(R,G,B)
if nargin == 1
B = double(R(:,:,3)); .........why double
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......... why max(max())
  R = double(R) / 255; ...................why 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

[Image Analyst]

No. Not really interested in explaining the formulas especially when there is a built in function to do it: rgb2lab(). Just use the built in function instead and don't worry about what the code does or if it's right, because it will be. Well, as "right" as a "book formula" can be, which of course will never give you the actual "TRUE" lab values you'd get from measuring your object with a spectrophotometer.

[Image Analyst]

By the way, 255 is the max for a uint8 image and it needs to divide by that to get a "normalized" image. Why is normalized in quotes? Because it's not really normalized. I can get different "normalized" values using that method of normalization just by changing my exposure. The only way to get calibrated lab values out of a system is to show it a calibrated standard such as the X-rite Color Checker Chart and develop a transform to map RGB into LAB.