Coefficients are shifted after forward transfrom

pierrepaleo / PDWT

A GPU implementation of the Wavelet Transform

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Coefficients are shifted after forward transfrom #2

Open senpai-a opened 4 years ago

senpai-a commented 4 years ago

Hello there. I'm using Wavelets::forward() to perfrom DWT and Wavelets::get_coeff() to get my result.

Here are 2 images I tested: lennagray

The wavelet used here is "db8", and here are the results: coeff lennaCoeff

I compared them with the output from another DWT library (Rice Wavelets Toolbox), who gives these: coeffrwt lennaCoeffrwt

I found that the spatial position of the coefficients seems to be shifted by a small amount.

I'm no an expert at signal processing so I don't know whether this is a bug or a non-critical issue.

pierrepaleo commented 4 years ago

Hi @senpai-a

There are several reasons that might explain why the coefficients are shifted:

The transform is not computed with the same boundary handling methods. PDWT always uses the periodic method, while other software use a symmetric extension by default.
The filters used for "db8" are not exactly the same in both cases (less likely).

PDWT is tested against (somewhat old versions of) Pywavelets, and should be compatible with it, and therefore with the Matlab "wavelets" toolbox. I don't know how the RICE wavelets toolkit does the computations.

senpai-a commented 4 years ago

I see. Thx for the explaination. Closing this issue. BTW, is there a way I could cite this work? I might use it for some research.

senpai-a commented 4 years ago

Hello @pierrepaleo , can you generate a DOI for this repo so that I can cite.

pierrepaleo commented 4 years ago

Hi @senpai-a thanks for the idea, I just generated the DOI and updated README.md.