How to use FFTW on deviceMotion.userAcceleration.x

Question

I am working on an app. that its measuring the motion of a device. (xyz direction )

and now i must use fftw to filter the data.

i don't know how to call the data through fftw. hier below is a part of my code trying to execute the X-data ( i am working on each direction separate so X, Y, and then Z)

// FFTW for X-data

        int SIZE = 97;
        fftw_complex   *dataX, *fft_resultX;
        fftw_plan       plan_X;
        int             i ;
        dataX       = (fftw_complex*) fftw_malloc(sizeof(fftw_complex) * SIZE);
        fft_resultX  = (fftw_complex*) fftw_malloc(sizeof(fftw_complex) * SIZE);
        plan_X  = fftw_plan_dft_1d(SIZE, dataX, fft_resultX,
                                         FFTW_FORWARD, FFTW_ESTIMATE); // FFTW_MEASURE
        for( i = 0 ; i < SIZE ; i++ ) {
            dataX[i][0] = 1.0; // real
            dataX[i][1] = 0.0; // complex 
        for( i = 0 ; i < SIZE ; i++ ) {
            fprintf( stdout, "dataX[%d] = { %2.2f, %2.2f }\n",
                    i, dataX[i][0], dataX[i][1] );
        }
        
        fftw_execute( plan_X);
        
        for( i = 0 ; i < SIZE ; i++ ) {
            fprintf( stdout, "fft_resultX[%d] = { %2.2f, %2.2f }\n",
                    i, fft_resultX[i][0], fft_resultX[i][1] );
        }

and hier is the userAcceleration:

        [[weakSelf.graphViews objectAtIndex:kDeviceMotionGraphTypeUserAcceleration] addX:deviceMotion.userAcceleration.x y:deviceMotion.userAcceleration.y z:deviceMotion.userAcceleration.z];

for example, when i am writing :

dataX= deviceMotion.userAcceleration.x;

i am getting this error:

Assigning to 'fftw_complex *' (aka '_Complex double *') from incompatible type 'double'

any idea how to make fftw work on it ?

thanks for every try

Answer 1

You can't simply convert real data to a real imaginary pair. Each complex number is made up of 2 doubles.

You need to store all your acceleration data into a larger array (256 entries for example) where the x value is assigned to the real part of the complex number and 0 is assigned to the imaginary part.