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Dear Readers,

I'm trying (unsuccessfully) to find some C code that can perform a 2D-DHT on
real-valued data obtained from a 2D image. Does anyone have workable code
they are willing to share? I've been plugging away with the implementation
of fftw_dht
in fftw, but I'm finding it more than challenging to sort through their
code.

Sincerely,

Mark

--
Mark E. Berres
University of Wisconsin - Madison
Genetics and Biotechnology Center
425 Henry Mall
Madison, WI. 53706

http://ravel.zoology.wisc.edu/sgaap
16 T 0303764
     4771858

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Mark E. Berres wrote:
> I'm trying (unsuccessfully) to find some C code that can perform a 2D-DHT on
> real-valued data obtained from a 2D image. Does anyone have workable code
> they are willing to share? I've been plugging away with the implementation
> of fftw_dht
> in fftw, but I'm finding it more than challenging to sort through their
> code.

Note, first of all, that the true 2D DHT is not the separable product of 
1D DHTs along the rows and columns (unlike the 2D DFT).  FFTW's DHT 
would compute the latter if you ask it for a 2d transform, as described 
in the manual...you need an additional post-processing pass for the 
non-separable 2D DHT (see the FFTW manual for references).  I haven't 
seen any other free code that computes a true 2d DHT lying around, 
either; it's all 1d DHTs.

Second, why do you need a 2D DHT?  I'm very curious to know what it is 
good for that a 2D real-input DFT is not as good or better at.

Third, if by "sort through their code" you mean compile it, I'm guessing 
you are a Windows user, in which case see fftw.org/install/windows.html 
for precompiled packages; otherwise, compilation is easy.  If you want 
to read source code for learning the algorithms, though, then FFTW is 
not ideal for beginners.

Cordially,
Steven G. Johnson

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"Mark E. Berres"  wrote...
> Dear Readers,
>
> I'm trying (unsuccessfully) to find some C code that can perform a 2D-DHT on
> real-valued data obtained from a 2D image. Does anyone have workable code
> they are willing to share? I've been plugging away with the implementation
> of fftw_dht
> in fftw, but I'm finding it more than challenging to sort through their
> code.

This page has C code from two different authors.  The
first set appears to have been written for use with BMP
image files.  I haven't actually run this code, so I can't
comment on it's quality.  Good luck.

http://www.jjj.de/fft/fftpage.html

- Brian

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Brian Webb wrote:
> This page has C code from two different authors.  The
> first set appears to have been written for use with BMP
> image files.  I haven't actually run this code, so I can't
> comment on it's quality.  Good luck.
> 
> http://www.jjj.de/fft/fftpage.html

Ah, good, it looks like I mis-spoke; both of these codes implement the 
non-separable transform (by a post-processing pass after computing the 
1D DHTs of the rows and columns).

(I'm still curious to learn why people find this transform useful.)

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On Tue, 03 Feb 2004 22:03:47 -0500, "Steven G. Johnson"
<s...@alum.mit.edu> wrote:

>Brian Webb wrote:
>> This page has C code from two different authors.  The
>> first set appears to have been written for use with BMP
>> image files.  I haven't actually run this code, so I can't
>> comment on it's quality.  Good luck.
>> 
>> http://www.jjj.de/fft/fftpage.html
>
>Ah, good, it looks like I mis-spoke; both of these codes implement the 
>non-separable transform (by a post-processing pass after computing the 
>1D DHTs of the rows and columns).
>
>(I'm still curious to learn why people find this transform useful.)

I don't know if this applies much any more, but FHTs used to be really
good when computational and memory resources were limited.  Many moons
ago I did a lot of stuff with FHTs, primarily because if you're input
data is real (audio in my case at the time), you can save a lot of
computation using FHTs compared to many alternatives.  Easy on
multiplies, etc., etc.

But nowadays all this newfangled technology makes computing power and
memory pretty cheap, so I think the utility of the DHT/FHT is not
quite what it used to be.  Still, though, if resources are tight you
can get a small advantage with them, especially if your input signal
is real-valued.
Eric Jacobsen
Minister of Algorithms, Intel Corp.
My opinions may not be Intel's opinions.
http://www.ericjacobsen.org

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The following article might be of interest to the readers of this thread:

O Alshibami & S Boussakta
Signal Processing 82, pp 121-126 (2002)


It describes a method of directly calculating multidimensional DHTs, which
is 40% more efficient than using multiple iterations of the 1D DHT.

~Glynne



"Eric Jacobsen" <e...@delete.ieee.org> wrote in message
news:4...@news.west.earthlink.net...
> On Tue, 03 Feb 2004 22:03:47 -0500, "Steven G. Johnson"
> <s...@alum.mit.edu> wrote:
>
> >Brian Webb wrote:
> >> This page has C code from two different authors.  The
> >> first set appears to have been written for use with BMP
> >> image files.  I haven't actually run this code, so I can't
> >> comment on it's quality.  Good luck.
> >>
> >> http://www.jjj.de/fft/fftpage.html
> >
> >Ah, good, it looks like I mis-spoke; both of these codes implement the
> >non-separable transform (by a post-processing pass after computing the
> >1D DHTs of the rows and columns).
> >
> >(I'm still curious to learn why people find this transform useful.)
>
> I don't know if this applies much any more, but FHTs used to be really
> good when computational and memory resources were limited.  Many moons
> ago I did a lot of stuff with FHTs, primarily because if you're input
> data is real (audio in my case at the time), you can save a lot of
> computation using FHTs compared to many alternatives.  Easy on
> multiplies, etc., etc.
>
> But nowadays all this newfangled technology makes computing power and
> memory pretty cheap, so I think the utility of the DHT/FHT is not
> quite what it used to be.  Still, though, if resources are tight you
> can get a small advantage with them, especially if your input signal
> is real-valued.
> Eric Jacobsen
> Minister of Algorithms, Intel Corp.
> My opinions may not be Intel's opinions.
> http://www.ericjacobsen.org

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Eric Jacobsen wrote:
> I don't know if this applies much any more, but FHTs used to be really
> good when computational and memory resources were limited.  Many moons
> ago I did a lot of stuff with FHTs, primarily because if you're input
> data is real (audio in my case at the time), you can save a lot of
> computation using FHTs compared to many alternatives.  Easy on
> multiplies, etc., etc.

I think you've fallen victem to the persistent myth that an FHT requires 
half as as much computation and/or memory compared to the FFT, but this 
has never been true.  Even before the FHT was introduced, specialized 
variants of the FFT for real inputs were known that require ~half as 
much computation/memory as for complex data.

In fact, it was argued by Sorensen in 1987 that the best 
(fewest-operation) known FFT algorithm for real data actually required 
fewer operations than the best known FHT algorithm.  See also:

     http://en.wikipedia.org/wiki/Discrete_Hartley_transform

Of course, these days the speed of such a transform depends more on 
quality of implementation than on flop counts per se, but since the FHT 
and real-data FFT have similar structures I'm not aware of any intrinsic 
advantage of the former from an implementation standpoint, either.

What I'm curious to know is, given the fact that the Discrete Hartley 
Transform is not intrinsically any more efficient to compute than the 
Discrete Fourier Transform, are there applications in which the former 
is still somehow more appropriate?

Cordially,
Steven G. Johnson

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Glynne wrote:
> The following article might be of interest to the readers of this thread:
> 
> O Alshibami & S Boussakta
> Signal Processing 82, pp 121-126 (2002)
> 
> It describes a method of directly calculating multidimensional DHTs, which
> is 40% more efficient than using multiple iterations of the 1D DHT.

Actually, the total asymptotic arithmetic savings (asymptotic 
adds+mults) claimed in that paper are about 25% (the 40% is just 
multiplies).   For comparison, you can save 20% just by switching to 
split-radix 1D FHTs instead of the radix-2 1D FHT algorithm I think they 
consider.  They get additional savings because they use vector-radix 
(radix 2x2x2); vector-radix is also known to improve arithmetic 
complexity for multi-dimensional DFTs as well.  So, none of this really 
impacts the DFT vs. DHT question.

Cordially,
Steven G. Johnson

PS. I'm not sure what the "best" (in terms of arithmetic counts) known 
multi-dimensional DHT algorithm is.  There are some other papers 
claiming the best thing is to compute it with the help of DFTs (FFTs), 
actually.

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Steven G. Johnson wrote:
> I think you've fallen victem to the persistent myth that an FHT requires 

s/victem/victim/, grrr

> Of course, these days the speed of such a transform depends more on 
> quality of implementation than on flop counts per se, but since the FHT 
> and real-data FFT have similar structures I'm not aware of any intrinsic 
> advantage of the former from an implementation standpoint, either.

I should amend this to say that, in the multi-dimensional case, the DFT 
*does* have an intrinsic advantage over the DHT, in that the former is 
separable and the latter is not.  Meaning: you can use separable 
algorithms in the DFT case (e.g. to leverage highly-optimized 1d code), 
but not in the DHT case (which requires post-processing passes if you 
use 1d transforms).

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In article <4026f5cb$0$560$b...@senator-bedfellow.mit.edu>,
"Steven G. Johnson" <s...@alum.mit.edu> wrote:

>Subject: Re: 2D discrete hartley transform
>From: "Steven G. Johnson" <s...@alum.mit.edu>
>Reply-To: <4...@news.west.earthlink.net>
>Date: Sun, 08 Feb 2004 21:51:55 -0500
>Newsgroups: sci.math.num-analysis,comp.dsp
>
>Eric Jacobsen wrote:
>> I don't know if this applies much any more, but FHTs used to be really
>> good when computational and memory resources were limited.  Many moons
>> ago I did a lot of stuff with FHTs, primarily because if you're input
>> data is real (audio in my case at the time), you can save a lot of
>> computation using FHTs compared to many alternatives.  Easy on
>> multiplies, etc., etc.
>
>I think you've fallen victem to the persistent myth that an FHT requires 
>half as as much computation and/or memory compared to the FFT, but this 
>has never been true.  Even before the FHT was introduced, specialized 
>variants of the FFT for real inputs were known that require ~half as 
>much computation/memory as for complex data.

This myth is probably based on comparing the 30 line FFT from the back
of the book with whatever is being proposed by someone who is rather 
uncomfortable with the use of complex analysis. The DHT also avoids
the mysteries of negative frequencies which seem to cause as much
discomfort as complex analysis. No need for "specialized variants"
that are of interest to the specialists.

This is not unlike the question of whether frequency is measured in
cycles or radians. As a freshman I had a summer job where I measured
frequency in cycles per second (actually it was intersting as the range
was below the usual instruments and I had to learn first priciples
of the task at hand fairly quickly). Then in Advanced Calculus I learned
the joys of radian measure. But in graduate school I discovered Harmonic
Analysis with kernels on groups and was back to cycles. In Advanced
Calculus the important property was that sin(x) had a derivative of 1
at 0 (and had some period) but in Harmonic Analysis the important property
is that it has period 1 (and some derivative at 0). Of course the magic
constant in both cases is 2 pi and it is an issue of convenience where it 
appears. In numerical practice the ability to avoid the question of
what value to use for 2 pi is often more than just a convenience.

>In fact, it was argued by Sorensen in 1987 that the best 
>(fewest-operation) known FFT algorithm for real data actually required 
>fewer operations than the best known FHT algorithm.  See also:
>
>     http://en.wikipedia.org/wiki/Discrete_Hartley_transform
>
>Of course, these days the speed of such a transform depends more on 
>quality of implementation than on flop counts per se, but since the FHT 
>and real-data FFT have similar structures I'm not aware of any intrinsic 
>advantage of the former from an implementation standpoint, either.
>
>What I'm curious to know is, given the fact that the Discrete Hartley 
>Transform is not intrinsically any more efficient to compute than the 
>Discrete Fourier Transform, are there applications in which the former 
>is still somehow more appropriate?
>
>Cordially,
>Steven G. Johnson

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