CIELAB values from TIFF image

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Opiuz
Opiuz on 30 May 2012
Commented: Image Analyst on 12 Mar 2022
hello,
I'm trying to read CIELAB values from TIFF image in Matlab with:
A_lab= imread ('A', 'tiff');
what I get is A_lab<1x33x3 unit8> with values [0,255] which are not CIELAB values that I want (positive & negative integer and non-integer).
Have any idea?
thanks

Accepted Answer

Walter Roberson
Walter Roberson on 30 May 2012
Edited: Walter Roberson on 8 Jun 2018
"If a file contains CIELAB color data, imread converts it to ICCLAB before bringing it into the MATLAB workspace because 8- or 16-bit TIFF CIELAB-encoded values use a mixture of signed and unsigned data types that cannot be represented as a single MATLAB array."
See http://www.mathworks.com/help/techdoc/ref/imwrite.html for a description of ICCLAB encoding

More Answers (4)

Thomas
Thomas on 30 May 2012

KAE
KAE on 8 Jun 2018
Edited: KAE on 14 Aug 2018
Here is what I used on a 16-bit CIELab TIFF file based on the 16-bit ICCLAB documentation here. Or there are conversions within encode_color.m which comes with the Image Processing toolbox, though some coefficients are different,
aUint16 = imread(fileData);
% Warning: Converting CIELab-encoded TIFF image to ICCLab encoding.
aDouble = double(aUint16); % Converting to double so can have decimals
Lab(:,:,1) = aDouble(:,:,1)*100/65280; % L
Lab(:,:,2) = aDouble(:,:,2)/256 - 32768; % a
Lab(:,:,3) = aDouble(:,:,3)/256 - 32768 ; % b
% Values are integers in the range [0, 65280]. L* values are multiplied by 65280/100.
% 32768 is added to both the a* and b* values,
% which are represented as integers in the range [0,65535].
  2 Comments
KAE
KAE on 14 Aug 2018
Edited: KAE on 14 Aug 2018
And substitute these 3 lines for 8-bit CIELab TIFF,
Lab(:,:,1) = aDouble(:,:,1)*100/65280; % L
Lab(:,:,2) = aDouble(:,:,2)/256 - 128; % a
Lab(:,:,3) = aDouble(:,:,3)/256 - 128 ; % b
By the way all these constants are powers of 2, for example 65280 = 2^16 - 2^8.
Roger Breton
Roger Breton on 11 Mar 2022
Thanks! Will experiment with this code...

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John
John on 30 May 2012
IMREAD converts CIELAB to ICCLAB (it should also warn you that it is doing this). If you want the raw CIELAB values as stored in the TIFF, try the Tiff class instead.
  5 Comments
KAE
KAE on 15 Aug 2018
I had trouble with trying to do this with the tiff class, possibly because the CIELab tiff files are a mix of signed and unsigned integers, and had to use imread instead.
% The a* and b* values seemed wrong with this approach
t = Tiff(fileScan, 'r');
aUint = read(t);
% This seemed to work better, though Matlab warns:
% "Converting CIELab-encoded TIFF image to ICCLab encoding"
aUint = imread(fileData);
Roger Breton
Roger Breton on 12 Mar 2022
Edited: Walter Roberson on 12 Mar 2022
This code works for me :
aUint8 = imread('LabImage.tif')
aDouble = double(aUint8);
cielab(:,:,1) = aDouble(:,:,1) ./ (255/100);
cielab(:,:,2) = aDouble(:,:,2)-128;
cielab(:,:,3) = aDouble(:,:,3)-128;
The only 'problem', as noted by opiuz, is that, a* and b* lose their fractional values...

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seackone
seackone on 26 Mar 2018
Hi, I want exactly the same! I've got a tiff file and looking for their lab values? Any ideas?
  1 Comment
Image Analyst
Image Analyst on 12 Mar 2022
Try rgb2lab() for arbitrary "book" values. Those will not be the same as the true LAB values you'd get from measuring your sample on a spectrophotometer. If you need that, start a new discussion thread (it's complicated) after reading the attached tutorial.

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