[Insight-users] random number generator
stefan at isi.uu.nl
Wed Mar 16 09:15:54 EST 2005
I did some tests with the underlying random generator of the
itkImageRandomIteratorWithIndex and it seems that, in Windows, it is 'not
The itkImageRandomIterator uses the following random-generator:
In Linux the drand48 random-generator is used, which is a good choice.
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:
The gray-values in this image show how many times a pixel was sampled. The
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
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
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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