core/vil/vil_math.cxx

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// This is core/vil/vil_math.cxx
#include "vil_math.h"
//:
// \file
// \author Amitha Perera.
//
// \verbatim
//  Modifications
// \endverbatim

#include <vcl_iostream.h>
#include <vcl_cstdlib.h>

void vil_math_median_unimplemented()
{
  vcl_cerr << "vil_math_median is currently not implemented for this data type\n";
  vcl_abort();
}

VCL_DEFINE_SPECIALIZATION
void vil_math_median(vxl_byte& median, const vil_image_view<vxl_byte>& im, unsigned p)
{
  unsigned ni = im.ni();
  unsigned nj = im.nj();

  // special case the empty image.
  if ( ni*nj == 0 ) {
    median = 0;
    return;
  }

  unsigned hist[256] = { 0 };
  for (unsigned j=0;j<nj;++j) {
    for (unsigned i=0;i<ni;++i) {
      ++hist[ im(i,j,p) ];
    }
  }

  unsigned tot = ni*nj;
  // Target is ceil(tot/2)
  unsigned tgt = (tot+1) / 2;
  unsigned cnt = 0;
  unsigned idx = 0;
  while ( cnt < tgt ) {
    cnt += hist[idx];
    ++idx;
  }

  // Test for halfway case
  if ( cnt == tgt && tot % 2 == 0 ) {
    // idx is
    unsigned lo = idx-1;
    while ( hist[idx] == 0 ) {
      ++idx;
    }
    median = vxl_byte((lo+idx)/2);
  }
  else {
    median = vxl_byte(idx-1);
  }
}