3d c2c fft with fftw library

I am trying to do 3D FFT with the FFTW library, but I have some difficulties with the inverse transform.

First, I will transform the preface through:

fftwf_plan_dft_3d(_dimensions[0], _dimensions[1], _dimensions[2], (fftwf_complex*)_inputBuffer, (fftwf_complex*)_outputBuffer, FFTW_FORWARD, FFTW_ESTIMATE);

Although my data is real data, I use the complex for complex transformations, because I want to replace it later with opencl fft, which only supports complex complex transformations.

In three-dimensional Fourier space, I make a very simple low-pass filter:

for all x, y, z:

// global position of the current bin
int gid = (y * w + x) + (z * w * h);

// position of the symmetric bin
vec3 conPos(M - x - 1, N - y - 1, L - z - 1);

// global position of the symmetric element
int conGid = (conPos.y * w + conPos.x) + (conPos.z * w * h);

if (sqrt(x * x + y * y + z * z) > 500)
{
    complex[gid].real = 0.0f;
    complex[gid].imag = 0.0f;
    complex[conGid].real = 0.0f;
    complex[conGid].imag = 0.0f;
}

Finally, the inverse transform is:

fftwf_plan_dft_3d(_dimensions[0], _dimensions[1], _dimensions[2], (fftwf_complex*)_inputBuffer, (fftwf_complex*)_outputBuffer, FFTW_BACKWARD, FFTW_ESTIMATE);
// normalization ...

The result is not what I would expect. After the inverse transformation, the imaginary parts are not all equal to zero, since they should be.

, , . (. c2c ). , , fftw, , .

2D- :

int w = 4;
int h = 4;
int size  = w * h;

cl_float rawImage[16] = ...; // loading image

fftwf_complex *complexImage = (fftwf_complex*) fftwf_malloc(sizeof(fftwf_complex) * size);
fftwf_complex *freqBuffer = (fftwf_complex*) fftwf_malloc(sizeof(fftwf_complex) * size);

for (int i = 0; i < size; i++)
{
    complexImage[i][0] = rawImage[i]; complexImage[i][1] = 0.0f;
}

fftwf_plan forward = fftwf_plan_dft_2d(w, h, complexImage, freqBuffer, FFTW_FORWARD, FFTW_ESTIMATE);

fftwf_execute(forward);

for (int y = 0; y < h; y++)
{
    for (int x = 0; x < w; x++)
    {
        int gid = y * w + x;
        qDebug() << gid << "real:" << freqBuffer[gid][0] << "imag:" << freqBuffer[gid][1];
    }
}

:

gid
0    real 3060 imag 0 
1    real 510 imag 510 
2    real 0 imag 0 
3    real 510 imag -510 
4    real 510 imag 510 
5    real 0 imag -510 
6    real 0 imag 0 
7    real -510 imag 0 
8    real 0 imag 0 
9    real 0 imag 0 
10   real 0 imag 0 
11   real 0 imag 0 
12   real 510 imag -510 
13   real -510 imag 0 
14   real 0 imag 0 
15   real 0 imag 510 

, . ?

, - .

Wolf