[Insight-users] Inaccuracy in fftw calculation

[Antonio Gómez Barquero]

The code is here:
		
	// FFT CALCULATION
	// Inizialization of the neccesary elemnets for calculating the FFT.


	fftw_plan p1;  //variable for storing the FFT 
	int N_fft= ancho*alto;  //number of points of the image
	fftw_complex *F1 =(fftw_complex*)
fftw_malloc(sizeof(fftw_complex)*alto*((ancho/2)+1));  // result pointer
	p1 = fftw_plan_dft_r2c_2d(alto,ancho, f1, F1, FFTW_ESTIMATE);  //
FFT planning
	fftw_execute(p1);  // FFT calculation
	fftw_destroy_plan(p1); // plan is destroyed 

I want to do the FFT to double pointer called f1, it is the force vector
calculated from the gradient of an image (dT1):

	f1 = new double[ancho*alto]; //double *f1 = (double*)
malloc(sizeof(double)*ancho*alto);
	for ( int x= 0; x<alto; x++){
	for (int y = 0; y<ancho; y++){
		*(f1+ancho*x+y) =
gamma*(R[ancho*x+y]-T[ancho*x+y])*dT1[ancho*x+y]; // f1=gamma*L.*dT1;
	}
	}

Everything is also done for the second dimension.

Thanks Cory!

Antonio

-----Mensaje original-----
De: Cory Quammen [mailto:cquammen at cs.unc.edu] 
Enviado el: miércoles, 05 de octubre de 2011 16:53
Para: Antonio Gómez Barquero
CC: insight-users at itk.org
Asunto: Re: [Insight-users] Inaccuracy in fftw calculation

Please post your code that is exhibiting the problem and state what version
of ITK you are using.

Thanks,
Cory

2011/10/5 Antonio Gómez Barquero <agb1 at alu.upct.es>:
> Inaccuracy in fftw calculation
>
>
>
> I am calculating the fft of an image which pixels are double with the 
> FFTW library, for that purpose i am using the library 
> fftw(http://www.fftw.org/) library in Visual Studio 2008 and using ITK.
>
>
>
> When I calculate the fft from a double pointer, with the method 
> ‘fftw_plan_dft_c2r_2d’, it makes the calculation but introducing 
> inaccuracy, it is around +- 0.5

>
>
>
> I don’t really know what is wrong, maybe the spacing between pixels? 
> The type of the data used? Did someone have this problem or something 
> similar before?
>
>
>
> Many thanks in advanced.
>
>
>
> Antonio Gómez Barquero
>
>
>
> Ingeniero de Telecomunicaciones -Becario Investigador asociado a 
> Actividades de I+D+I
>
> GTTS ( Grupo de Tratamiento y Teoría de la Señal)[ 
> http://gtts.upct.es/]
>
> UPCT (Universidad Politécnica de Cartagena)[ http://www.upct.es/]
>
>
>
> _____________________________________
> Powered by www.kitware.com
>
> Visit other Kitware open-source projects at 
> http://www.kitware.com/opensource/opensource.html
>
> Kitware offers ITK Training Courses, for more information visit:
> http://www.kitware.com/products/protraining.html
>
> Please keep messages on-topic and check the ITK FAQ at:
> http://www.itk.org/Wiki/ITK_FAQ
>
> Follow this link to subscribe/unsubscribe:
> http://www.itk.org/mailman/listinfo/insight-users
>
>



--
Cory Quammen
Research Associate
Department of Computer Science
The University of North Carolina at Chapel Hill