[Insight-developers] RE: [Insight-users] random number generator
warfield at bwh.harvard.edu
Wed Mar 16 15:37:14 EST 2005
I would vote for using the Mersenne Twister. I have been using that on
and off for a couple of years, with good experiences.
Lorensen, William E (Research) wrote:
> Maybe we should add a proposal on the itk Wiki.
> -----Original Message-----
> *From:* insight-developers-bounces at itk.org
> [mailto:insight-developers-bounces at itk.org]*On Behalf Of *Blezek,
> Daniel J (Research)
> *Sent:* Wednesday, March 16, 2005 10:05 AM
> *To:* Stefan Klein; Insight-users at itk.org; insight-developers at itk.org
> *Subject:* [Insight-developers] RE: [Insight-users] random number
> Stefan, I started a thread on this a while ago relating to
> non-deterministic behavior across platforms. If the ITK design
> committee approves the move, it would be great to standardize the
> random number generator to be sufficiently random, fast and
> generate the same sequences (from a given seed) across platforms.
> The discussion culminated with Brad's post:
> I'm not sure any action happened on the suggestions.
> As you point out, many registration algorithms depend on random
> sampling, exactly where I came across the problem.
> -----Original Message-----
> *From:* insight-users-bounces at itk.org
> [mailto:insight-users-bounces at itk.org]*On Behalf Of *Stefan Klein
> *Sent:* Wednesday, March 16, 2005 9:16 AM
> *To:* Insight-users at itk.org
> *Subject:* [Insight-users] random number generator
> Dear itk-users,
> I did some tests with the underlying random generator of the
> itkImageRandomIteratorWithIndex and it seems that, in Windows,
> it is 'not very random'.
> The itkImageRandomIterator uses the following random-generator:
> In Linux the drand48 random-generator is used, which is a good
> In Windows however a "simple congruential random number
> generator" is implemented, since drand48 is not available in
> Windows. This gives inferior results, in my experience.
> To get a feel for the result look at the image1.<mhd/raw>,
> which is in the file: randomtestresults.zip, which you can
> download from: http://www.isi.uu.nl/People/Stefan/
> The gray-values in this image show how many times a pixel was
> sampled. The sampling
> process works was defined as follows:
> "An ImageRegionIterator walks N=200 times through the image
> and tests
> at each voxel whether to sample it or not. The test is
> performed by
> drawing a number between 0 and 1 using the random generator
> defined in
> vnl_sample.h; a value >=0.5 means that the voxel is sampled"
> As you can see in the image the pixels are not really selected
> at random...
> If we use the itkImageRandomIteratorWithIndex to sample the
> image, the result (image3.<mhd,raw>) may look better at first
> sight, but the histogram looks terrible.
> On the internet I found the following link:
> It describes the files vnl_random.<h,cxx>; those files are not
> included in the vxl-version that comes with ITK. I tried to
> use these as a random generator. The results are now much
> better. Image2, which is generated in the same was as image1,
> but with the random generator defined in vnl_random, does not
> show any structure anymore. Image4, which is generated using
> an ImageRandomIterator that uses the vnl_random as its
> underlying random number generator, has the histogram that you
> would expect.
> For more details please look at the source of the test
> program, which you can also download from
> http://www.isi.uu.nl/People/Stefan/ : itkrandomtestsource.zip.
> You may reproduce the results with this program. Note that
> only in Windows bad results will be obtained. In Linux there
> is no problem.
> Algorithms that rely on a good random generator may fail if
> you use the itkRandomImageIteratorWithIndex under Windows. For
> example, in my research on nonrigid registration I tried to
> use a stochastic gradient descent optimisation method for
> minimising the MattesMutualInformation in combination with a
> B-spline transform (it may speed up your registration
> algorithms; if you are interested:
> ). When I used the itkImageRandomConstIteratorWithIndex for
> selecting spatial samples the registration results got
> significantly worse than when I used the
> itkImageMoreRandomConstIteratorWithIndex (which uses the
> vnl_random as underlying random number generator).
> Any comments on this would be appreciated!
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Simon K. Warfield, Ph.D. warfield at bwh.harvard.edu Phone:617-732-7090
http://www.spl.harvard.edu/~warfield FAX: 617-582-6033
Associate Professor of Radiology, Harvard Medical School
Director, Computational Radiology Laboratory
Thorn 329, Dept Radiology, Brigham and Women's Hospital
75 Francis St, Boston, MA, 02115
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