Spc fit poly below on highwavelength range with high polynomial order can not solve vander monde matrix

[ADorosz5]

spectra_CHstretch <- spectra[,, 2600 ~ 3150]
plot (spectra_CHstretch)
baseline <- spc.fit.poly.below(spectra_CHstretch, poly.order = 6)

[GegznaV]

@cbeleites, is it enough information for you to reproduce the error? Could a reproducible example be added to this thread?

[cbeleites]

@GegznaV yes, it's good for me. I think I know the reason of the problem, and I'll be able to construct also unit test data.

We found it as part of a course, and ran into the other issue #68 about dput() when trying to add a reproducible example - otherwise there would have been one....

[GegznaV]

@cbeleites, this issue has the status "Urgent": would you have time to try solving this issue ASAP?
Otherwise wouldn't it be better to use a lower level of priority?

[GegznaV]

1. This issue is not solved for 3 years, so it is not urgent.
2. This issue lacks a reprex, so I'll try to create it myself. @cbeleites and @ADorosz5, please, confirm that the reprex shows the essence of the issue:

suppressPackageStartupMessages({
  library(hyperSpec)
})

spectra_CHstretch <- flu[,, 450 ~ 495]
baseline <- spc_fit_poly_below(spectra_CHstretch, poly.order = 6)
#> Error in qr.solve(vdm[use, ], y[use, i]): singular matrix 'a' in solve

^{Created on 2024-05-02 with reprex v2.1.0}