bugGNU Octave - Bugs: bug #50429, [octave forge] (image) xyz2lab...


bug #50429: [octave forge] (image) xyz2lab changed slightly in Matlab (and other color conversion functions)

Submitter:  Hartmut <hardy>
Submitted:  Wed 01 Mar 2017 07:31:14 PM UTC
Category:  Octave Package Severity:  1 - Wish
Priority:  3 - Low Item Group:  Matlab Compatibility
Status:  None Assigned to:  None
Originator Name:  Open/Closed:  * Open
Release:  * dev Operating System:  * Any
Fixed Release:  None Planned Release:  None
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Wed 01 Mar 2017 07:43:06 PM UTC, comment #1: 

my personal comments:

  • So the remark about Matlab's D65 values in comment #0 might well be correct. So this is a slight Matlab incompatiblity.

  • Nevertheless, my intention in writing those color conversion functions in the Octave image package was never to be perfectly Matlab compatible. The current results are mostly Matlab compatible (maybe only to Matlab version before V2016a), but the have a better "round trip accuracy" than Matlab had at that time. See "help rgb2lab" for some more detail on this.

I have no objections to making the Octave color conversion functions even more Matlab compatible, also to the newest Matlab version. But I myself won't find the time to do this in the forseeable future.

Hartmut <hardy>
Wed 01 Mar 2017 07:31:14 PM UTC, original submission:  

quoting the original issue reporter:

Octave - image - xyz2lab

For a research project, I was comparing the results between the built-in Matlab function and the implementation on http://www.brucelindbloom.com/index.html?Eqn_XYZ_to_Lab.html . In these calculations it turns out that Matlab does use "D65 = [0.95047, 1, 1.08883];", despite of the truncation in the whitepoint function.This means that the comment on line 58 can be changed.

I also have another remark: the sourcing on Wikipedia for the white point is pretty poor. I found an Excel file on the website of the CIE, from which it should be possible to calculate the D65. I haven't implemented this, as that is way more precision than I need, but it would improve the rigor. I might write something on Wikipedia, but I won't promise. I copied the Excel file download link to the way back machine, so it can safely be cited: https://web.archive.org/web/20170131100357/http://files.cie.co.at/204.xls

Best regard,
R. W.

Hartmut <hardy>


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    Follow 4 latest changes.

    Date Changed by Updated Field Previous Value => Replaced by
    2019-02-26 mtmiller Severity3 - Normal 1 - Wish
        Priority5 - Normal 3 - Low
        Release4.2.1 dev
    2017-08-12 jwe Summaryimage package: xyz2lab changed slightly in Matlab (and other color conversion functions) [octave forge] (image) xyz2lab changed slightly in Matlab (and other color conversion functions)

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