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How do I specify a stride between elements when computing a 3D DCT in terms of 1D DCTs?


I am working on software for acoustics simulation, which requires 3D Discrete Cosine Transforms to be performed. FFTW3 provides the fftwf_plan_r2r_3d function to create plans for just this purpose:

fftwf_plan plan = fftwf_plan_r2r_3d(Nx, Ny, Nz, gInData, gOutData, FFTW_REDFT10, FFTW_REDFT10, FFTW_REDFT10, FFTW_PRESERVE_INPUT);

However, MKL does not support 3D DCTs. So I tried to implement the 3D DCT in terms of multiple 1D DCTs. For some of these 1D DCTs, I need to specify strides between consecutive elements. Again, FFTW3 allows this via the function fftwf_plan_many_r2r, which also doesn't seem to be supported in MKL (the returned plans from MKL's FFTW3 wrappers are always NULL).

fftwf_r2r_kind kind = FFTW_REDFT10;
fftwf_plan planX = fftwf_plan_many_r2r(1, &Nx, 1, gInData, NULL, Ny*Nz, 1, gOutData, NULL, Ny*Nz, 1, &kind, FFTW_PRESERVE_INPUT);
fftwf_plan planY = fftwf_plan_many_r2r(1, &Ny, 1, gInData, NULL, Nz, 1, gOutData, NULL, Nz, 1, &kind, FFTW_PRESERVE_INPUT);
fftwf_plan planZ = fftwf_plan_many_r2r(1, &Nz, 1, gInData, NULL, 1, 1, gOutData, NULL, 1, 1, &kind, FFTW_PRESERVE_INPUT);

for (int x = 0; x < Nx; x++)
    for (int y = 0; y < Ny; y++)
        fftwf_execute_r2r(planZ, &gInData[x*Ny*Nz + y*Nz], &gOutData[x*Ny*Nz + y*Nz]);

for (int x = 0; x < Nx; x++)
    for (int z = 0; z < Nz; z++)
        fftwf_execute_r2r(planY, &gOutData[x*Ny*Nz + z], &gInData[x*Ny*Nz + z]);

for (int y = 0; y < Ny; y++)
    for (int z = 0; z < Nz; z++)
        fftwf_execute_r2r(planX, &gInData[y*Nz + z], &gOutData[y*Nz + z]);

I next tried the raw Trigonometric Transforms functions, s_init_trig_transform and the like. These don't seem to offer any way of setting a stride between elements. Since s_commit_trig_transform returns a DFTI_DESCRIPTOR_HANDLE, I tried using DftiSetValue to specify a stride on the descriptor, followed by a call to DftiCommitDescriptor.


int ir;

int iparX[128];
float* sparX = new float[3*Nx/2 + 2];
s_init_trig_transform(&Nx, &tt_type, iparX, sparX, &ir);
s_commit_trig_transform(gInData, &dftiX, iparX, sparX, &ir);
int xStride[2] = {0, Ny*Nz};
DftiSetValue(dftiX, DFTI_INPUT_STRIDES, xStride);
DftiSetValue(dftiX, DFTI_OUTPUT_STRIDES, xStride);

int iparY[128];
float* sparY = new float[3*Ny/2 + 2];
s_init_trig_transform(&Ny, &tt_type, iparY, sparY, &ir);
s_commit_trig_transform(gInData, &dftiY, iparY, sparY, &ir);
int yStride[2] = {0, Nz};
DftiSetValue(dftiY, DFTI_INPUT_STRIDES, yStride);
DftiSetValue(dftiY, DFTI_OUTPUT_STRIDES, yStride);

int iparZ[128];
float* sparZ = new float[3*Nz/2 + 2];
s_init_trig_transform(&Nz, &tt_type, iparZ, sparZ, &ir);
s_commit_trig_transform(gInData, &dftiZ, iparZ, sparZ, &ir);

iparX[6] = 0;
iparY[6] = 0;
iparZ[6] = 0;

iparX[10] = 1;
iparY[10] = 1;
iparZ[10] = 1;

iparX[14] = Nx;
iparY[14] = Ny;
iparZ[14] = Nz;

memcpy(gOutData, gInData, Nx*Ny*Nz*sizeof(float));

for (int x = 0; x < Nx; x++)
    for (int y = 0; y < Ny; y++)
        s_backward_trig_transform(&gOutData[x*Ny*Nz + y*Nz], &dftiZ, iparZ, sparZ, &ir);

for (int x = 0; x < Nx; x++)
    for (int z = 0; z < Nz; z++)
        s_backward_trig_transform(&gOutData[x*Ny*Nz + z], &dftiY, iparY, sparY, &ir);

for (int y = 0; y < Ny; y++)
    for (int z = 0; z < Nz; z++)
        s_backward_trig_transform(&gOutData[y*Nz + z], &dftiX, iparX, sparX, &ir);


However, this does not produce correct results (i.e., the results don't match at all with the FFTW 3D DCT or the FFTW multiple-1D-DCT results). I am using MKL 10.3 Update 9 on Windows (32-bit).

Have I missed something here? Is there a way to either a) specify strides for 1D DCTs, or b) perform 3D DCTs using MKL? In the absence of a way to use MKL to compute 3D DCTs, I will be forced to switch away from using MKL.

Lakulish Antani. 

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2 Replies

Hi Lakulish, Thank you very much for very detailed description of the problem and the ways you have been trying to solve it. Unfortunately, MKL does not support non-unit stride DCTs. Thanks Dima

Hi Dmitry, Thank you for your reply. It's unfortunate that MKL doesn't support non-unit-stride DCTs. Therefore, I would like to report a feature request to the MKL team, for a) non-unit-stride DCTs, and b) 3D DCTs. Let me know if this forum post suffices for this purpose, or if there's another place for me to report such a feature request. Meanwhile, I will try implementing what I need in terms of non-unit-stride FFTs, which seem to support non-unit strides. Thanks, Lakulish.