"colour.convert" definition: sRGB should be the default RGB colorspace, but actually is not?

[rsnitsch]

The documentation for colour.convert states that:

Various defaults have been systematically adopted compared to the low-level Colour API: (...)
The default RGB colourspace is the sRGB colourspace. It can also be changed on a per definition basis along the conversion path.

This is my code:

# pip install colour-science networkx
import colour
import numpy as np

np.set_printoptions(suppress=True)

input_rgb = colour.models.eotf_sRGB(np.array([127, 127, 127]) / 255.0)
converted = colour.convert(input_rgb, 'RGB', 'Cie Lab', verbose={'mode': 'Long', 'width': 120})

I assume that this should produce the same output as when changing the last line to

converted = colour.convert(input_rgb, 'sRGB', 'Cie Lab', verbose={'mode': 'Long', 'width': 120})

However, there is a dramatic difference.

This is the output for 'RGB':

========================================================================================================================
*                                                                                                                      *
*   [ Conversion Path ]                                                                                                *
*                                                                                                                      *
*   "RGB_to_XYZ" --> "XYZ_to_Lab"                                                                                      *
*                                                                                                                      *
========================================================================================================================
========================================================================================================================
*                                                                                                                      *
*   [ "RGB_to_XYZ" ]                                                                                                   *
*                                                                                                                      *
*   [ Signature ]                                                                                                      *
*                                                                                                                      *
*   <Signature (RGB, illuminant_RGB, illuminant_XYZ, RGB_to_XYZ_matrix, chromatic_adaptation_transform='CAT02',        *
*   cctf_decoding=None, **kwargs)>                                                                                     *
*                                                                                                                      *
*   [ Conversion Output ]                                                                                              *
*                                                                                                                      *
*   [ 0.20172533  0.21223076  0.23111929]                                                                              *
*                                                                                                                      *
========================================================================================================================
========================================================================================================================
*                                                                                                                      *
*   [ "XYZ_to_Lab" ]                                                                                                   *
*                                                                                                                      *
*   [ Signature ]                                                                                                      *
*                                                                                                                      *
*   <Signature (XYZ, illuminant=array([ 0.3127,  0.329 ]))>                                                            *
*                                                                                                                      *
*   [ Conversion Output ]                                                                                              *
*                                                                                                                      *
*   [ 0.53192777  0.0000461   0.00002109]                                                                              *
*                                                                                                                      *
========================================================================================================================

This is the output for 'sRGB':

========================================================================================================================
*                                                                                                                      *
*   [ Conversion Path ]                                                                                                *
*                                                                                                                      *
*   "sRGB_to_XYZ" --> "XYZ_to_Lab"                                                                                     *
*                                                                                                                      *
========================================================================================================================
========================================================================================================================
*                                                                                                                      *
*   [ "sRGB_to_XYZ" ]                                                                                                  *
*                                                                                                                      *
*   [ Signature ]                                                                                                      *
*                                                                                                                      *
*   <Signature (RGB, illuminant=array([ 0.3127,  0.329 ]), chromatic_adaptation_method='CAT02',                        *
*   apply_cctf_decoding=True, **kwargs)>                                                                               *
*                                                                                                                      *
*   [ Conversion Output ]                                                                                              *
*                                                                                                                      *
*   [ 0.03521062  0.03704431  0.04034126]                                                                              *
*                                                                                                                      *
========================================================================================================================
========================================================================================================================
*                                                                                                                      *
*   [ "XYZ_to_Lab" ]                                                                                                   *
*                                                                                                                      *
*   [ Signature ]                                                                                                      *
*                                                                                                                      *
*   <Signature (XYZ, illuminant=array([ 0.3127,  0.329 ]))>                                                            *
*                                                                                                                      *
*   [ Conversion Output ]                                                                                              *
*                                                                                                                      *
*   [ 0.22669199  0.00002576  0.00001179]                                                                              *
*                                                                                                                      *
========================================================================================================================

EDIT: This is my environment.

===============================================================================
*                                                                             *
*   Interpreter :                                                             *
*       python : 3.7.8 (tags/v3.7.8:4b47a5b6ba, Jun 28 2020, 08:53:46) [MSC   *
*   v.1916 64 bit (AMD64)]                                                    *
*                                                                             *
*   colour-science.org :                                                      *
*       colour : 0.3.15                                                       *
*                                                                             *
*   Runtime :                                                                 *
*       imageio : 2.9.0                                                       *
*       matplotlib : 3.2.2                                                    *
*       networkx : 2.4                                                        *
*       numpy : 1.19.0                                                        *
*       pandas : 1.0.5                                                        *
*       scipy : 1.5.0                                                         *
*       six : 1.15.0                                                          *
*                                                                             *
===============================================================================

[sobotka]

In this instance, the RGB_to_XYZ assumes a cctf_encoding of None in the first.

In the second, the sRGB_to_XYZ assumes inversely encoded sRGB EOTF values by default.

I don’t believe it makes much sense to assume that the RGB triplet in the first case has a CCTF encoding applied.

[rsnitsch]

In this instance, the RGB_to_XYZ assumes a cctf_encoding of None in the first.

In the second, the sRGB_to_XYZ assumes inversely encoded sRGB EOTF values by default.

I don’t believe it makes much sense to assume that the RGB triplet in the first case has a CCTF encoding applied.

Thank you! You're correct. Indeed, these produce the same output:

input_rgb = colour.models.eotf_sRGB(np.array([127, 127, 127]) / 255.0)
converted = colour.convert(input_rgb, 'RGB', 'Cie Lab', verbose={'mode': 'Long', 'width': 120})
// [ 0.53192777  0.0000461   0.00002109]

input_rgb = np.array([127, 127, 127]) / 255.0
converted = colour.convert(input_rgb, 'sRGB', 'Cie Lab', verbose={'mode': 'Long', 'width': 120})
// [ 0.53192777  0.0000461   0.00002109]

The core issue is: I understand the docs as saying that RGB is assumed to mean sRGB by default. Thus, they should produce the same results with the same inputs IMHO.

[sobotka]

It gets a little ugly at the function level, however.

Assuming a particular transfer function on a particular RGB encoding is problematic, and given the RGB case is an encoding model as opposed to colourspace such as sRGB, it probably would cause more grief?

[rsnitsch]

It gets a little ugly at the function level, however.

Assuming a particular transfer function on a particular RGB encoding is problematic, and given the RGB case is an encoding model as opposed to colourspace such as sRGB, it probably would cause more grief?

I'm new to the colour Python package, so I can't say much with regards to the implementation/uglyness at the function level. I can only tell you what my interpretation of the docs is. The docs say that:

The default RGB colourspace is the sRGB colourspace.

My understanding of that phrase is: For colour.convert, RGB and sRGB are identical. If you want an RGB colourspace other than sRGB, you have to make that explicit. In all other cases, it will implicitly assume that sRGB is meant. And therefore, it should behave exactly as if you specified sRGB explicitly.

[sobotka]

Yeah. I’d disagree. RGB is a colour encoding model, so the understanding as to what state the values are encoded can be quite different.

[KelSolaar]

Hi @rsnitsch,

Thanks! As you saw the main difference pertains to encoding/decoding in that case, we should make the documentation clearer but besides that, the default IS sRGB, whether it is encoded or not is critical obviously but it does change the fact that the illuminant is D65 and the primaries are that of BT.709/sRGB. We will amend the documentation accordingly, that being said there are hints at that in the notes:

    -   The **RGB** colour representation is assumed to be linear and
        representing *scene-referred* imagery. To convert such *RGB* values to
        *output-referred* (*display-referred*) imagery, i.e. encode the *RGB*
        values using an encoding colour component transfer function
        (Encoding CCTF) / opto-electronic transfer function (OETF / OECF), the
        **Output-Referred RGB** representation must be used.

The graph code is still considered experimental as the warning implies so if you have other issues or misunderstanding, please keep them coming!

[KelSolaar]

The documentation has been updated: https://colour.readthedocs.io/en/develop/generated/colour.convert.html#colour.convert