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Re: Image warping in IDL [message #51269 is a reply to message #51188]

Fri, 10 November 2006 08:38

James Kuyper
Messages: 425
Registered: March 2000

Senior Member

Wox wrote:
> On Fri, 10 Nov 2006 10:05:25 +0100, Wox <nomail@hotmail.com> wrote:
>
>> 2. Is there a "magic operation" for converting CP's and uvec/vvec so
>> that xmap and ymap describe distortion of output pixels?
>
> Maybe some simple analog example to make this clear:
>
> Take a 1D polynomial for which you know the coeff. and degree:
> y(t)=a+b.t+c.t^2+d.t^3
>
> Is there a "magic operation" that can estimate/calculate a',b',c' and
> d' in the following :
> t(y)=a'+b'.y+c'.y^2+d'.y^3
>
> The 2D splines are a little bit more complicated, but they are just
> piecewise polynomial surfaces.

The solution is straightforward, in one dimension. Choose a set of t
values that is sufficiently large, and use your splines for the forward
transformation to calculate corresponding y values. Then fit a spline
in y to your t values. This will provide a spline approximation to the
inverse of your first spline. It will be exactly correct at the points
you chose, and less accurate as you move away from those points.

However, this approach doesn't generalize to 2-D data very well. I hope
someone else can help you, but the only efficient algorithms I know of
for fitting 2-D splines require that the function being fitted, z(x,y),
is tabulated on the outer product of a set of x values and a set of y
values. When the only way to determine the x and y values is by
evaluating a spline interpolant, I don't see any easy way to arrange
that they form an outer-product set.

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[Message index]

		Image warping in IDL By: Wox on Wed, 08 November 2006 04:53
		Re: Image warping in IDL By: David Fanning on Fri, 10 November 2006 15:11
		Re: Image warping in IDL By: JD Smith on Fri, 10 November 2006 14:51
		Re: Image warping in IDL By: Wox on Tue, 14 November 2006 02:20
		Re: Image warping in IDL By: JD Smith on Mon, 20 November 2006 12:17
		Re: Image warping in IDL By: James Kuyper on Fri, 10 November 2006 08:38
		Re: Image warping in IDL By: Wox on Fri, 10 November 2006 01:12
		Re: Image warping in IDL By: Wox on Fri, 10 November 2006 01:05
		Re: Image warping in IDL By: JD Smith on Thu, 09 November 2006 11:50
		Re: Image warping in IDL By: Wox on Wed, 15 November 2006 04:18
		Re: Image warping in IDL By: Wox on Tue, 14 November 2006 08:20
		Re: Image warping in IDL By: Wox on Tue, 21 November 2006 01:18
		Re: Image warping in IDL By: JD Smith on Tue, 21 November 2006 09:10
		Re: Image warping in IDL By: Jeff Hester on Mon, 20 November 2006 09:00
		Re: Image warping in IDL By: Wox on Mon, 20 November 2006 01:18
		Re: Image warping in IDL By: JD Smith on Wed, 22 November 2006 08:22
		Re: Image warping in IDL By: Wox on Wed, 22 November 2006 00:21

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