Calculated xy vs. plotted xy

[frankkjensen]

Hi,

I have calculated the xy color coordinates with colour.XYZ_to_xyY(colour.sd_to_XYZ(sd))
The result was: [0.295, 0.259]
I also plotted the same spectrum with colour.plotting.plot_sds_in_chromaticity_diagram_CIE1931([sd])
However looking at the plot, the color coordinates are closer to [0.273, 0.247]
Why the discrepancy? As far as I can read, they should default to CIE 2 deg. 1931 color matching functions, when nothing is specified.

import colour
sample = {3.80000000e+02: 5.49788032e-03, 3.81000000e+02: 5.60642611e-03, 3.82000000e+02: 6.37441487e-03, 3.83000000e+02: 7.17325622e-03, 3.84000000e+02: 8.38960283e-03, 3.85000000e+02: 1.04755309e-02, 3.86000000e+02: 1.30002308e-02, 3.87000000e+02: 1.54669209e-02, 3.88000000e+02: 1.87322627e-02, 3.89000000e+02: 2.21301022e-02, 3.90000000e+02: 2.72660015e-02, 3.91000000e+02: 3.30015454e-02, 3.92000000e+02: 4.02345247e-02, 3.93000000e+02: 4.68487265e-02, 3.94000000e+02: 5.51627248e-02, 3.95000000e+02: 6.52108873e-02, 3.96000000e+02: 7.55227938e-02, 3.97000000e+02: 8.62633518e-02, 3.98000000e+02: 9.96198380e-02, 3.99000000e+02: 1.10671247e-01, 4.00000000e+02: 1.22690622e-01, 4.01000000e+02: 1.32853451e-01, 4.02000000e+02: 1.41148546e-01, 4.03000000e+02: 1.49121381e-01, 4.04000000e+02: 1.50935485e-01, 4.05000000e+02: 1.52789964e-01, 4.06000000e+02: 1.48045284e-01, 4.07000000e+02: 1.39373474e-01, 4.08000000e+02: 1.34951268e-01, 4.09000000e+02: 1.23201640e-01, 4.10000000e+02: 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sd = colour.SpectralDistribution(sample, name='Sample')
print('xyY:', colour.XYZ_to_xyY(colour.sd_to_XYZ(sd, k=683)))
fig, ax = colour.plotting.plot_sds_in_chromaticity_diagram_CIE1931([sd])

-Frank

PS: sorry for all the questions lately. I am just starting to get into the colour-science module.

[KelSolaar]

@frankkjensen,

It is because the plotting API uses D65 as a default illuminant: https://github.com/colour-science/colour/blob/develop/colour/plotting/diagrams.py#L1009

I'm wondering if we should not change that back to CIE Illuminant E like we did for the automatic colour conversion graph: #1059

[frankkjensen]

I think that would be good!
Being new to colour-science (but not completely new to color science :-)), it can be confusing that there is an extra illuminant involved when converting a spectrum to XYZ. I understand the convenience when looking at transmission and reflection spectra, but if the illuminant is not explicitly specified, it should never use an illuminant, but just do basic spectrum to XYZ conversion.

-Frank

[frankkjensen]

Is there a way to set the illuminant to 'E'?

[KelSolaar]

Yes by passing plot_kwargs={"illuminant": colour.SDS_ILLUMINANTS["E"]}, see

colour/colour/plotting/diagrams.py

Line 856 in ac6949a

plot_kwargs

[frankkjensen]

Thanks!
(I was just passing illuminant= and not plot_kwargs= )

-Frank