CIELAB values from TIFF image

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Opiuz
Opiuz 2012-5-30
评论： Image Analyst 2022-3-12
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

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Walter Roberson
Walter Roberson 2012-5-30
编辑：Walter Roberson 2018-6-8
http://www.mathworks.com/help/techdoc/ref/imread.html says,
"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

更多回答（4 个）

Thomas
Thomas 2012-5-30
KAE
KAE 2018-6-8
编辑：KAE 2018-8-14
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 个评论
KAE
KAE 2018-8-14
编辑：KAE 2018-8-14
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 2022-3-11
Thanks! Will experiment with this code...

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John
John 2012-5-30
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 个评论
显示 3更早的评论
KAE
KAE 2018-8-15
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 2022-3-12
编辑：Walter Roberson 2022-3-12
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 2018-3-26
Hi, I want exactly the same! I've got a tiff file and looking for their lab values? Any ideas?
  1 个评论
Image Analyst
Image Analyst 2022-3-12
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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