Wrong results cufft 3D in-place

Question

I write because I'm facing problems with the cufft 3D transform in-place, while I have no problems for the out-of-place version. I tried to follow Robert Crovella's answer here but I'm not obtaining the correct results when I make a FFT+IFT. This is my code:

#include <stdio.h>
#include <stdlib.h>
#include <cuda_runtime.h>
#include <complex.h>
#include <cuComplex.h>
#include <cufft.h>

// Main function
int main(int argc, char **argv){
    int N = 4; 
    double *in = NULL, *d_in = NULL;
    cuDoubleComplex *out = NULL, *d_out = NULL;
    cufftHandle plan_r2c, plan_c2r;

    unsigned int out_mem_size = sizeof(cuDoubleComplex) * N*N*(N/2 + 1);
    unsigned int in_mem_size = out_mem_size;

    in  = (double *) malloc (in_mem_size);
    out  = (cuDoubleComplex *)in;

    cudaMalloc((void **)&d_in, in_mem_size);
    d_out = (cuDoubleComplex *)d_in;

    cufftPlan3d(&plan_r2c, N, N, N, CUFFT_D2Z);
    cufftPlan3d(&plan_c2r, N, N, N, CUFFT_Z2D);

    memset(in, 0, in_mem_size);
    unsigned int idx;
    for (int z = 0; z < N; z++){
        for (int y = 0; y < N; y++){
            for (int x = 0; x < N; x++){
                idx = z + N * ( y + x * N);
                in[idx] = idx;
            }
        }
    }
    printf("\nStart: \n");
    for (int z = 0; z < N; z++){
        printf("plane = %d ----------------------------\n", z);
        for (int x = 0; x < N; x++){
            for (int y = 0; y < N; y++){
                idx = z + N * ( y + x * N);
                printf("%.3f \t", in[idx]);
            }
            printf("\n");
        }
    }
    cudaMemcpy(d_in, in, in_mem_size, cudaMemcpyHostToDevice);

    cufftExecD2Z(plan_r2c, (cufftDoubleReal *)d_in, (cufftDoubleComplex *)d_out);
    cufftExecZ2D(plan_c2r, (cufftDoubleComplex *)d_out, (cufftDoubleReal *)d_in);

    memset(in, 0, in_mem_size);
    CU_ERR_CHECK( cudaMemcpy(in, d_in, in_mem_size, cudaMemcpyDeviceToHost) );

    printf("\nAfter FFT+IFT: \n");
    for (int z = 0; z < N; z++){
        printf("plane = %d ----------------------------\n", z);
        for (int x = 0; x < N; x++){
            for (int y = 0; y < N; y++){
                idx = z + N * ( y + x * N);
                // Normalisation
                in[idx] /= (N*N*N);
                printf("%.3f \t", in[idx]);
            }
            printf("\n");
        }
    }

    return 0;
}

The program outputs the following data:

Starting file

plane = 0 ----------------------------

0.000 4.000 8.000 12.000
16.000 20.000 24.000 28.000
32.000 36.000 40.000 44.000
48.000 52.000 56.000 60.000

plane = 1 ----------------------------

1.000 5.000 9.000 13.000
17.000 21.000 25.000 29.000
33.000 37.000 41.000 45.000
49.000 53.000 57.000 61.000

plane = 2 ----------------------------

2.000 6.000 10.000 14.000
18.000 22.000 26.000 30.000
34.000 38.000 42.000 46.000
50.000 54.000 58.000 62.000

plane = 3 ----------------------------

3.000 7.000 11.000 15.000
19.000 23.000 27.000 31.000
35.000 39.000 43.000 47.000
51.000 55.000 59.000 63.000

After FFT+IFT

plane = 0 ----------------------------

-0.000 -0.344 8.000 12.000
-0.031 20.000 24.000 -0.031
32.000 36.000 0.031 44.000
48.000 -0.094 56.000 60.000

plane = 1 ----------------------------

1.000 -0.000 9.000 13.000
-0.000 21.000 25.000 0.125
33.000 37.000 0.000 45.000
49.000 0.000 57.000 61.000

plane = 2 ----------------------------

2.000 6.000 -0.000 14.000
18.000 0.000 26.000 30.000
0.000 38.000 42.000 -0.000
50.000 54.000 -0.000 62.000

plane = 3 ----------------------------

3.000 7.000 0.031 15.000
19.000 -0.031 27.000 31.000
-0.031 39.000 43.000 0.031
51.000 55.000 0.031 63.000

I even tried to pad the data this way:

// With padding
    unsigned int idx;
    for (int x = 0; x < N; x++){
        for (int y = 0; y < N; y++){
            for (int z = 0; z < 2*(N/2+1); z++){
                idx = z + N * ( y + x * N);
                if (z < 4) in[idx] = idx;
                else in[idx] = 0;
            }
        }
    }

What am I doing wrong?

havogt havogt · Accepted Answer · 2015-10-27T15:06:43

As you already found out, you need padding if you use the CUFFT_COMPATIBILITY_FFTW_PADDINGcompatibility mode which is default. For your code to work you could use cufftSetCompatibilityMode() to set CUFFT_COMPATIBILITY_NATIVE. However, this mode is marked as deprecated in the current version of CUDA.

Therefore, I recommend to use the default compatibility mode and use padding. Your try to implement padding is wrong. The formula to calculate a linear index for 3 dimension x, y, z where z is the fastest running index is idx = z + Nz*(y + Ny*x). The size Nz of the z dimension including padding is Nz = (N/2+1)*2. Then, the correct initialization of the array is:

unsigned int idx;
for (int z = 0; z < N; z++){
    for (int y = 0; y < N; y++){
        for (int x = 0; x < N; x++){
            idx = z + (N/2+1)*2 * ( y + x * N);
            in[idx] = idx;
        }
    }
}

Accordingly for the print loops.

Wrong results cufft 3D in-place

1 Answers