MantisBT - ITK
I was producing inconsistent results in my medical image processing software when using powell optimization in my code that prompted me to run valgrind (See report at end of this message).
The least invasive way to avoid this bug is to initialize the variable “bx=0” on lines 87 and 124. This, however, is not the best solution in my opinion. Currently the the powell optimizer uses vn_brent.h which states in it’s header
==============================
//: Brent 1D minimizer (deprecated)
//
// Please use vnl_brent_minimizer instead.
//
// This routine used to contain copyrighted code, and is deprecated.
// It is now simply a wrapper around vnl_brent_minimizer.
==============================
I’ve made a minimally modified version of vnl_powell.cxx that uses vnl_brent_minimizer directly (saving 3 levels of indirect function calls) that makes it much easier to read and removes the errors.
http://www.nitrc.org/plugins/scmsvn/viewcvs.php/trunk/lib/vnl_powell_fixed.cxx?revision=172&root=art&sortdir=down&view=markup [^]
In my program, this fix reliably runs to completion in 24 seconds where it had been occasionally segmentation faulting or running somewhere between 120 seconds to 300 seconds depending on what value was being filled in.
Please consider implementing this bug fix in the next release of vxl.
Thanks,
Hans J. Johnson
hans-johnson@uiowa.edu
## Valgrind is our friend!
==15048== Conditional jump or move depends on uninitialised value(s)
==15048== at 0x890BE9: vnl_bracket_minimum(vnl_cost_function&, double&, double&, double&, double&, double&, double&) (vnl_bracket_minimum.cxx:46)
==15048== by 0x890938: vnl_brent::bracket_minimum(double*, double*, double*, double*, double*, double*) (vnl_brent.cxx:58)
==15048== by 0x8908E0: vnl_brent::bracket_minimum(double*, double*, double*) (vnl_brent.cxx:52)
==15048== by 0x88F876: vnl_powell::minimize(vnl_vector<double>&) (vnl_powell.cxx:85)
==15048== by 0x676E4D: PowellOptimizeMSP(itk::SmartPointer<itk::OrientedImage<short, 3> >&, itk::Point<double, 3> const&, itk::Point<double, 3> const&, itk::Point<double, 3> const&, PlaneObject const&, itk::Point<double, 3>&, Plan
eObject&, double) (acpcdetect_itk.cxx:361)
==15048== by 0x678079: ComputeMSP(itk::SmartPointer<itk::OrientedImage<short, 3> >, PlaneObject&, itk::Point<double, 3>&, itk::Matrix<double, 3, 3>&, unsigned) (acpcdetect_itk.cxx:679)
==15048== by 0x678571: computeTmsp(itk::SmartPointer<itk::OrientedImage<short, 3> >&, PlaneObject&, itk::Point<double, 3>&) (acpcdetect_itk.cxx:739)
==15048== by 0x56D4C8: main (acpcResampleMSP.cxx:74)
vnl_powell.cxx
====================
lines 82-85 && lines 122-125
double ax = 0.0;
double xx = initial_step_;
double bx; /**NOTE bx is not initialized!*/
brent.bracket_minimum(&ax, &xx, &bx);
vnl_brent.cxx
====================
lines 49-59
void vnl_brent::bracket_minimum(double *ax, double *bx, double *cx) //**NOTE cx is not initialized
{
double fa, fb, fc;
bracket_minimum(ax,bx,cx,&fa,&fb,&fc);
}
void vnl_brent::bracket_minimum(double *ax, double *bx, double *cx, //**NOTE cx is not initialized
double *fa, double *fb, double *fc)
{
vnl_bracket_minimum( *f_, *cx, *bx, *ax, *fc, *fb, *fa );
/*^----------- NOTE: cx is not initialized */
}
vnl_bracket_minimum.cxx
====================
lines 38-48
void vnl_bracket_minimum(vnl_cost_function& fn,
double& a, double& b, double& c, //** Note a is not initialized
double& fa, double& fb, double& fc)
{
// Set up object to evaluate function
// Note that fn takes a vector input - f converts a scalar to a vector
vnl_bm_func f(fn);
if (b==a) b=a+1.0;
/*^-------------- Note comparison of value where a is not initialized. In my case, a=324e+124 sometimes
fa = f(a);
/*^-------A not initialized here, and should not be used (this is bx in the original calling function.*/
fb = f(b);
vnl_powell_fixed.cxx (4,379) 2008-10-30 20:38https://public.kitware.com/Bug/file/1815/vnl_powell_fixed.cxx
vnl_powell_fixed.h (1,459) 2008-10-30 20:39https://public.kitware.com/Bug/file/1816/vnl_powell_fixed.h