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Re: 3D congrid without interpolation [message #53411 is a reply to message #53410]

Fri, 13 April 2007 15:57

JD Smith
Messages: 850
Registered: December 1999

Senior Member

On Thu, 12 Apr 2007 15:40:07 -0700, mgalloy@gmail.com wrote:

> On Apr 12, 3:19 pm, David Fanning <n...@dfanning.com> wrote:
>> Humm. Hard for me to imagine what you are using to
>> do this that is interpolating anything for you.
>> CONGRID is normally used, but that won't interpolate
>> unless you explicitly tell it to.
>
> CONGRID interpolates 3-dimensional arrays by default. From the online
> help for the INTERP keyword for CONGRID:
>
> INTERP
> Set this keyword to force CONGRID to use linear interpolation when
> resizing a 1- or 2-dimensional array. CONGRID automatically uses
> linear interpolation if the input array is 3-dimensional. When the
> input array is 1- or 2-dimensional, the default is to employ nearest-
> neighbor sampling.

How is "nearest neighbor sampling" not interpolation? Does it
explicitly avoid knowledge of how the new array cell is positioned
w.r.t. the old one, and simply grab averages of nearby neighbors? Why
would this ever be preferable to a linear interpolation?

BTW, there has be a good deal of progress on interpolators, especially
for image data, which IDL hasn't taken advantage of. For example,
when downsizing, you need to take care to avoid moire artifacts and
aliasing.

Here's a classic comparison of various interpolators:

http://www.all-in-one.ee/~dersch/interpolator/interpolator.h tml

JD

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[Message index]

		Re: 3D congrid without interpolation By: James Kuyper on Sat, 14 April 2007 10:59
		Re: 3D congrid without interpolation By: JD Smith on Fri, 13 April 2007 18:00
		Re: 3D congrid without interpolation By: David Fanning on Fri, 13 April 2007 17:33
		Re: 3D congrid without interpolation By: JD Smith on Fri, 13 April 2007 15:57
		Re: 3D congrid without interpolation By: Mike[2] on Fri, 13 April 2007 08:21
		Re: 3D congrid without interpolation By: Qing on Thu, 12 April 2007 19:45
		Re: 3D congrid without interpolation By: David Fanning on Thu, 12 April 2007 17:30
		Re: 3D congrid without interpolation By: Michael Galloy on Thu, 12 April 2007 15:40
		Re: 3D congrid without interpolation By: David Fanning on Thu, 12 April 2007 14:19
		Re: 3D congrid without interpolation By: mxhamidi on Mon, 16 April 2007 11:43
		Re: 3D congrid without interpolation By: JD Smith on Mon, 23 April 2007 09:53

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