What is the range of CIE Lab color model?

[hminle]

Hi,

In this function:

Why Lab output in range of [-100, 100] for a and b channels?
Are they supposed to be in range of [-127, 127] as usual?

Thanks

[KelSolaar]

Hi @hminle,

Those ranges are indicative and not exact: https://colour.readthedocs.io/en/develop/basics.html#domain-range-scales

CIE Lab is not bounded, using ACES2065-1 for example:

>>> colour.RGB_colourspace_limits(colour.models.RGB_COLOURSPACE_ACES2065_1)
array([[ -58.99202084,  102.47216286],
       [-404.18830391,  317.50827994],
       [-274.02976251,  174.47162964]])

Cheers,

Thomas

[hminle]

Hi @KelSolaar , so how can you guys convert CIE Lab from its normal scale (Scale - Reference) to scale 0-1 (Scale - 1) as they are unbounded?

[KelSolaar]

A simple 100 scale, the idea is to bring the data to a scale where things can be compared together.

[hminle]

@KelSolaar so you guys just divide Lab values to 100 ?

[KelSolaar]

That is pretty much it yeah!

In [1]: colour.XYZ_to_Lab(colour.sRGB_to_XYZ([0.1, 0.2, 0.3], apply_cctf_decoding=False))
Out[1]: array([ 50.21115569,  -5.39585823, -17.5358009 ])

In [2]: colour.convert([0.1, 0.2, 0.3], 'Scene-Referred RGB', 'CIE Lab', verbose={'mode': 'long'})
/Users/kelsolaar/Documents/Development/colour-science/colour/colour/utilities/verbose.py:235: ColourUsageWarning: The "Automatic Colour Conversion Graph" is a beta feature, be mindful of this when using it. Please report any unexpected behaviour and do not hesitate to ask any questions should they arise.
This warning can be disabled with the "colour.utilities.suppress_warnings" context manager as follows:
with colour.utilities.suppress_warnings(colour_usage_warnings=True):
    convert(*args, **kwargs)
  warn(*args, **kwargs)
===============================================================================
*                                                                             *
*   [ Conversion Path ]                                                       *
*                                                                             *
*   "RGB_to_RGB" --> "RGB_to_XYZ" --> "XYZ_to_Lab"                            *
*                                                                             *
===============================================================================
===============================================================================
*                                                                             *
*   [ "RGB_to_RGB" ]                                                          *
*                                                                             *
*   [ Signature ]                                                             *
*                                                                             *
*   <Signature (RGB, input_colourspace, output_colourspace,                   *
*   chromatic_adaptation_transform='CAT02', apply_cctf_decoding=False,        *
*   apply_cctf_encoding=False, **kwargs)>                                     *
*                                                                             *
*   [ Conversion Output ]                                                     *
*                                                                             *
*   [ 0.10000612  0.20000785  0.30000349]                                     *
*                                                                             *
===============================================================================
===============================================================================
*                                                                             *
*   [ "RGB_to_XYZ" ]                                                          *
*                                                                             *
*   [ Signature ]                                                             *
*                                                                             *
*   <Signature (RGB, illuminant_RGB, illuminant_XYZ, matrix_RGB_to_XYZ,       *
*   chromatic_adaptation_transform='CAT02', cctf_decoding=None)>              *
*                                                                             *
*   [ Conversion Output ]                                                     *
*                                                                             *
*   [ 0.16691596  0.18596717  0.31092437]                                     *
*                                                                             *
===============================================================================
===============================================================================
*                                                                             *
*   [ "XYZ_to_Lab" ]                                                          *
*                                                                             *
*   [ Signature ]                                                             *
*                                                                             *
*   <Signature (XYZ, illuminant=array([ 0.3127,  0.329 ]))>                   *
*                                                                             *
*   [ Conversion Output ]                                                     *
*                                                                             *
*   [ 0.50212006 -0.05396191 -0.17534951]                                     *
*                                                                             *
===============================================================================
Out[2]: array([ 0.50212006, -0.05396191, -0.17534951])

[hminle]

Thank @KelSolaar