How to extrapolate a spectrum linearly?

[frankkjensen]

Hi,

I am trying to extrapolate some spectra. As default they will be extrapolated using 'constant', i.e. will just continue with the last value. I try to set the extrapolator method to 'Linear', but it doesn't work. What am I doing wrong?

sd.extrapolator_kwargs['method'] = 'Linear'
sd_reshaped = colour.colorimetry.reshape_sd(sd, colour.SpectralShape(300, 900, 1))

Best regards,
Frank

[KelSolaar]

Hi @frankkjensen,

This one is confusing and I think we should document it better! There are two schools for interpolating and extrapolating the spectral data in Colour:

• The Old School - CIE Compliant way that uses the interpolate, extrapolate, align (and trim) methods of colour.SpectralDistribution, those will follow CIE recommendations.
• The New School - Ignore CIE that leverages the underlying continuous function that allows you to do things like that: sd[colour.SpectralShape(300, 900, 1).wavelengths] and directly sample it.

What you did by changing directly the extrapolator_kwargs would have worked with the latter. Unfortunately for the colour.colorimetry.reshape_sd definition that uses the Old School - CIE Compliant way, you need to pass kwargs as follows:
colour.colorimetry.reshape_sd(sd, colour.SpectralShape(300, 900, 1), extrapolator_kwargs={"method": "Linear"}). The reason is that the CIE 15:2004 and CIE 167:2005 recommend constant extrapolation so it is hardcoded but can still be overridden.

Cheers,

Thomas

[frankkjensen]

Ok, thanks for the explanation :-)

Best regards,
Frank