Hi, Wojtek,
Thanks for your reply . But up to now , I still can not figure out the
exact meaning of the codes there.
May Kim bless us! I hope that he can answer us as soon as possible!
ZhangMi
From: [email protected]
Date: Wed, 3 Jul 2013 14:30:43 +0200
To: [email protected]
Subject: Re: [osg-users] Some Question About OSGOean class :FFTSimulation
One more word. I might have been wrong that it was 0 frequency. Memory does not
serve me well these days ;-). So do not rely on the fact it was 0 frequency or
0 vs N/2 index. It might have been some other frequency but certainly there
were an issue with matching proper indices of FFT input array for frequencies
in generation of PhilipsSpectrum from Tessendorf paper.
Wojtek
2013/7/3 Wojciech Lewandowski <[email protected]>
Hi, Will (Mi ?)
I am also curious what will Kim answer.
I have not studied osgOcean thoroughly and never used it in practice except for
few test runs. But I will want to share some of my observations I made long
time ago when I worked on my own Tessendorf paper implementation. We all are
practical people, so when we see such advanced algorithm based on scientific
paper we often start google and look for some existing code samples. And when I
did that, every piece of FFT ocean wave code I found was doing the same trick
with inverting sign diagonally. See cuda ocean for example (but many other
samples can be found too). I was as puzzled as you are now. I have spent some
time trying to investigate it and learning FFT properties and I have found that
many fwd FFT functions used in these samples expect array of parameters for
frequencies in following order :
// Bourke fwd FFT uses 0-N/2 indices for 0, 1/T, 2/T .. N/2/T frequencies
// and N/2 + 1 to N - 1 indices for -(N/2-1)/T...-2/T, -1/T frequencies
But all those sample implementations of Tessendorf paper seemed to make an
error there. They were assuming 0 frequency factors should be stored in N/2
index while in practice it should be stored on index 0. Because of that,
generation of K param used in Philips spectrum for 0 frequency was often
incorrectly placed. And that 0 frequency was responsible for sign changes in
generated heightfield. I guess people empirically found that sign can be
reverted diagonally and that hack was adopted. But why everybody did the same
mistake, one may ask ? I guess it may mean I was wrong on something (because
complex number FFT can be tricky) or everybody was copying from former
implementations ;-P. But thats just my 2 cent conspiracy theory...
Anyway, it was long ago and I could have made some error while describing it.
But please check generation of PhilipsSpectrum factors and check if they arte
stored on proper indices for particular FFT you use and check how they should
be set according to Tessendorf paper. If they are set correctly that sign
multiplication may be not neccessary.
Wojtek
2013/7/3 WillScott <[email protected]>
Dear Kim,
I have done some research on the paper "Simulating Ocean Water" (the
paper you provide for me several days ago). And the third part -- Practical
Ocean Wave Algorthms ---- is an attriactive algorthms.
However , the corresponding codes in OSGOcean class -- FFTSimualtion--
is not in accord with origianl paper that presented. In the function "void
FFTSimulation::Implementation::computeHeights( osg::FloatArray* waveheights )
const" , the DFT algorithm is done for wave height simulation. But the final
lines in this member function really confused me a lot :
const float signs[2] = { 1.f, -1.f };
for(int y = 0; y < _N; ++y)
{
for(int x = 0; x < _N; ++x )
{
waveheights->at(y*_N+x) = _realData0[x*_N+y][0] * signs[(x
+ y) & 1];
}
}
So , could you please explain to me why "sings[(x+y)&1]" was used
here? In addation , why noly the "_realData0[x*_N+y][0]" used for the wave
height computation? To some extent , the codes here is not that corresponding
to the function(19) presented in the third part of the origianl paper. I have
taken several day to thought about it , but in vain. Therfore , I will feel
appreciate if you can give me a resonable reply or provide some relevant
materials to me.
I am looking forward to your reply.
Sincerely Yours,
Zhang Mi
School of Remote Sensing and Information Engineering , Wuhan Universiy ,
Hubei Provinence , China.
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