The usage of "colorspace( )"

I have used the function “colorspace( )” which can be found in the page of " http://www.mathworks.com/matlabcentral/fileexchange/28790-colorspace-transformations " as follows:

rgb = imread('peppers.png');
C = makecform('srgb2lab');
lab = applycform(rgb,C);

lab_2 = colorspace('lab<-rgb', rgb);   
lab_3 = lab2uint8(lab_2);

I was surprised that the values in lab and lab_3 were quite different. I want to know the mistake in my code or my logical.

Thanks a lot.

Answer 1

[Image Analyst]

I don't have that File Exchange file and didn't try it. Maybe it wants double. Try this code:

clc;
rgb = imread('peppers.png');
C = makecform('srgb2lab');
lab = applycform(double(rgb)/255,C);
lChannel = lab(:,:,1);
aChannel = lab(:,:,2);
bChannel = lab(:,:,3);

lab_2_uint8 = colorspace('lab<-rgb', rgb);   
lChannel2u = lab_2(:,:,1);
aChannel2u = lab_2(:,:,2);
bChannel2u = lab_2(:,:,3);

lab_2_double = colorspace('lab<-rgb', double(rgb)/255);   
lChannel2d = lab_2(:,:,1);
aChannel2d = lab_2(:,:,2);
bChannel2d = lab_2(:,:,3);

% Now compute differences between them
lDiffu = lChannel2u - lChannel;
aDiffu = aChannel2u - aChannel;
bDiffu = bChannel2u - bChannel;

lDiffd = lChannel2d - lChannel;
aDiffd = aChannel2d - aChannel;
bDiffd = bChannel2d - bChannel;

% lab_3 = lab2uint8(lab_2);

Let me know if that reveals anything.

[x.d]

Thanks for your attention!

I have tested the above codes, but there is not essential differences in

lab_2_double = colorspace('lab<-rgb', double(rgb)/255);

lab_2_uint8 = colorspace('lab<-rgb', rgb);

as the "colorspace()" has Input parsing codes as follows:

%%%%%%%%%%% Input parsing %%%%%%%%%%%

if nargin < 2, error('Not enough input arguments.'); end

[SrcSpace,DestSpace] = parse(Conversion);

if nargin == 2

Image = varargin{1};

elseif nargin >= 3

Image = cat(3,varargin{:});

else

error('Invalid number of input arguments.');

end

FlipDims = (size(Image,3) == 1);

if FlipDims, Image = permute(Image,[1,3,2]); end

if ~isa(Image,'double'), Image = double(Image)/255; end

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

What's more, lDiffu is not an all-zero matrix at last.

max(max(aDiffd)) = 3.4091;

min(min(aDiffd)) = -4.8193;

[Image Analyst]

Since my demo code worked and colorspace() isn't, I suggest you contact the author of colorspace().