Are the reference swatch colours in the example notebook linear?

[tobias-kirschstein]

Question

In the example notebook, the images are transformed to linear RGB via colour.cctf_decoding(). This means that the colors of the detected SWATCHES are also given in linear RGB coordinates, right?
However, the REFERENCE_SWATCHES are loaded as sRGB color values and, as far as I am concerned, are not further converted to linear RGB. In the subsequent color correction
colour.colour_correction(COLOUR_CHECKER_IMAGES[i], swatches, REFERENCE_SWATCHES) the original image is transformed such that the detected swatches match the REFERENCE_SWATCHES, but isn't this nonsensical as the values are given wrt different color spaces (linear RGB vs sRGB)? Or am I missing something?

Thanks.

[KelSolaar]

Hi @tobias-kirschstein,

The following code from the notebook and specifically the colour.XYZ_to_RGB definition:

REFERENCE_SWATCHES = colour.XYZ_to_RGB(
        colour.xyY_to_XYZ(list(REFERENCE_COLOUR_CHECKER.data.values())),
        REFERENCE_COLOUR_CHECKER.illuminant, D65,
        colour.RGB_COLOURSPACES['sRGB'].matrix_XYZ_to_RGB)

produce linear values, thus REFERENCE_SWATCHES is effectively linear.

Cheers,

Thomas

[tobias-kirschstein]

Hi thanks for the quick reply.

Why does colour.XYZ_to_RGB() produce linear RGB values when used with colour.RGB_COLOURSPACES['sRGB'].matrix_XYZ_to_RGB from the sRGB color space? I though sRGB is non-linear?

Best,
Tobias

[KelSolaar]

Hi @tobias-kirschstein,

Some relevant reading: https://www.colour-science.org/posts/the-importance-of-terminology-and-srgb-uncertainty/

Cheers,

Thomas

[tobias-kirschstein]

Thanks for the link, that is a good reference.
So basically, colour.XYZ_to_RGB() always maps to an additive (=linear) RGB color space per default.
To get the "normal" sRGB color space one can additionally set cctf_encoding="sRGB".
Essentially this means that there is a linear counterpart for every RGB colorspace, right?
I think some clarifying comments in the notebook could have resolved my confusion in the beginning.

Best,
Tobias

[KelSolaar]

To get the "normal" sRGB color space one can additionally set cctf_encoding="sRGB".

You would need to pass a function explicitly, a callable is expected:

Essentially this means that there is a linear counterpart for every RGB colorspace, right?

Correct!

I think some clarifying comments in the notebook could have resolved my confusion in the beginning.

I added a note here:

Cheers,

Thomas

[tobias-kirschstein]

Awesome! Thanks for your time this was very helpful!