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Re: Non-monotonic Abscissa values for IDL function SPLINE_P [message #66702] Wed, 03 June 2009 11:31 Go to previous message
Jeremy Bailin is currently offline  Jeremy Bailin
Messages: 618
Registered: April 2008
Senior Member
On Jun 3, 11:50 am, Xavier Ceamanos García <xavier.ceama...@gmail.com>
wrote:
> Thank you so much Jeremy for the answer!
>
> You are very right. I use splines to interpolate spectra. Then, the
> main goal is to re-sample these spectra. Each value of a spectrum
> corresponds to a given wavelength and in my case all points are
> separated by a constant wavelength distance. Hence, it is crucial to
> know which wavelengths corresponds to each point of the over-sampled
> spectra. A good re-sampling is not possible otherwise.
>
> Then, are you telling me that the number of points of the output
> vector depends on the input data? That would mean that it would be
> different for each spectrum interpolation...
>
> So far, I am using the "SPLINE" function which produces a monotonic
> output. In this case, it is easy to know the output wavelengths. It is
> slower though...
>
> I was just wandering if there is any way to get the same results I get
> with SPLINE but using SPLINE_P...
>
> Thanks again!
>
> Xavi

Xavi,

Yes, the number of points in the output vector depends on the input
data. For example:

IDL> x = findgen(20)
IDL> y = sin(0.5*x)
IDL> spline_p, x, y, xr, yr, interval=0.25
IDL> help, xr, yr
XR FLOAT = Array[96]
YR FLOAT = Array[96]
IDL> y = sin(0.5*x)+x
IDL> spline_p, x, y, xr, yr, interval=0.25
IDL> help, xr, yr
XR FLOAT = Array[120]
YR FLOAT = Array[120]

So it will indeed be different for each spectrum. But your code should
be general enough to deal with that using n_elements(xr).

You might want to directly use SPL_INIT and SPL_INTERP... I suspect
that may do what you want.

-Jeremy.

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