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Re: REBIN needs explaining [message #41928 is a reply to message #41927]

Tue, 30 November 2004 15:56

JD Smith
Messages: 850
Registered: December 1999

Senior Member

On Tue, 30 Nov 2004 16:32:03 -0700, David Fanning wrote:

> Hi Folks,
>
> Can someone (Craig?) explain this to me.
>
> Consider:
>
> v = [0, 10]
>
> I understand this:
>
> Print, rebin(v, 20, /Sample)
> 0 0 0 0 0 0 0 0 0
> 0 10 10 10 10 10 10 10 10
> 10 10
>
> What I don't understand is this:
>
> Print, rebin(v, 20)
> 0 1 2 3 4 5 6 7 8
> 9 10 10 10 10 10 10 10 10
> 10 10
>
> Aren't there an awful lot of 10s in there!? Why doesn't this come
> out like this:
>
> 0 0 1 1 2 2 3 3 4 4 5 5 6 6 7 7 8 8
> 9 9 10 10
>
> I notice REBIN, CONGRID, and even CMCONGRID does this same thing. Why?

From the manual:

When expanding an array, REBIN interpolates, it never
extrapolates. Each of the n-1 intervals in the n-element input array
produces m/n interpolates in the m-element output array. The last m/n
points of the result are obtained by duplicating element n-1 of the
input array because their interpolates would lie outside the input
array.

So here it has just 1 interval producing 10 points, and 10 points are
duplicated from the last value. The manual also recommends the
solution:

If this effect is undesirable, use the INTERPOLATE function.

A good question is why they consider the need to avoid extrapolation
by duplicating elements, rather than just interpolating on all
internal intervals. I'm not sure for what problems the current
behavior would be desirable.

JD

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[Message index]

		Re: REBIN needs explaining By: David Fanning on Wed, 01 December 2004 04:38
		Re: REBIN needs explaining By: Paolo Grigis on Wed, 01 December 2004 00:45
		Re: REBIN needs explaining By: b.hinrichsen on Wed, 01 December 2004 00:38
		Re: REBIN needs explaining By: Chris Lee on Wed, 01 December 2004 00:23
		Re: REBIN needs explaining By: David Fanning on Tue, 30 November 2004 16:53
		Re: REBIN needs explaining By: JD Smith on Tue, 30 November 2004 15:56

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