4.1: cufftPlan2d(): X & Y params reversed?

The X & Y params for the cufftPlan2d() call seem to be reversed.

Both my app and the ‘convolutionFFT2D’ sample only work correctly if nx = height and ny = width. From the sample:

cufftSafeCall( cufftPlan2d(&fftPlanFwd, fftH, fftW, CUFFT_R2C) );

Note nx = ‘fftH’

The docs (CUFFT_Library.pdf) show the same confusion:

[i]“nx The transform size in the X‐dimension (number of rows)”

“ny The transform size in the Y‐dimension (number of columns)[/i]”

The nomenclature used by CUFFT for the dimensions is consistent with FFTW:

“…nx and ny in fftw2d_create_plan are positive integers specifying the dimensions of the rank 2 array to be transformed. i.e. they specify that the transform will operate on nx x ny arrays in row-major order, where nx is the number of rows and ny is the number of columns.”

I am not familiar with the convolutionFFT2D example (I assume it’s from the CUDA SDK). If it does not work correctly as shipped with CUDA 4.1 RC1, I would suggest filing a bug.

I see, maybe you could add this explanation to the docs somewhere - they shouldn’t assume that someone is already familiar with FFTW, and this convention is very counter-intuitive to anyone with a graphics background.

The SDK sample works fine as it already inputs the dimensions correctly - it was just to illustrate the strange x/y ordering.

Best I can tell the CUFFT documentation describes the behavior correctly and cleary, so I see no need to augment it. I mentioned FFTW as another data point that shows that the naming convention used by CUFFT is not unusual in the context of FFTs. Why FFT folks prefer that convention, I do not know.

I see you are correct - I missed this in the docs (from “CUFFT Transform Types”):

“For higher‐dimensional transforms (2D and 3D), CUFFT performs FFTs in row‐major or C order. For example, if the user requests a 3D transform plan for sizes X, Y, and Z, CUFFT transforms along Z, Y, and then X. The user can configure column‐major FFTs by simply changing the order of the size parameters to the plan creation API functions.”

Thanks njuffa.