[Insight-users] Patch for bug in itkCannyEdgeDetectionImageFilter.txx

[Joshua Cates]

Hi Zachary,

Thanks for the bug report, analysis, and solution.  I have filed a bug and 
will fix this in the ITK CVS repository soon.

Josh.

______________________________
 Josh Cates			
 Scientific Computing and Imaging Institute
 University of Utah
 Email: cates at sci.utah.edu
 Phone: (801) 587-7697
 URL:   http://www.sci.utah.edu/~cates


On Mon, 9 Feb 2004, Zachary Pincus wrote:

> Hello,
> 
> There is a subtle but problematic bug in  
> itkCannyEdgeDetectionImageFilter.txx (in Code/BasicFilters).
> 
> Basically, Hysteresis Thresholding (the third or fourth step of Canny  
> edge finding, depending on how you count it) fails in this  
> implementation.
> 
> Here's the important loop of FollowEdge (line 431) as it is. This loop  
> is supposed to follow edges until they dip below a lower bound. The  
> problem is that every time a new node is pushed onto the stack, it is  
> immediately popped off, so no actual following goes on.
> 
>   while(!m_NodeList->Empty())
>      {
>      node = m_NodeList->Front();
>      cIndex = node->m_Value;
>      oit.SetLocation(cIndex);
>      uit.SetIndex(cIndex);
>      uit.Value() = 1;
> 
>      for(int i = 0; i < nSize; i++)
>        {
>        nIndex = oit.GetIndex(i);
>        uit.SetIndex(nIndex);
>        if(InBounds(nIndex))
>          if(oit.GetPixel(i) > m_LowerThreshold && uit.Value() != 1  )
>            {
>            node = m_NodeStore->Borrow();
>            node->m_Value = nIndex;
>            m_NodeList->PushFront(node); // NEW NODE PUSHED HERE
> 
>            uit.SetIndex(nIndex);
>            uit.Value() = 1;
>            }
>        }
> 
>      m_NodeList->PopFront(); // AND IMMEDIATELY POPPED!
>      m_NodeStore->Return(node);
> 
>      }
> 
> That PopFront() command at the second-to-last line is the problem.  
> Every time a new node is pushed to the search stack in the "if" block  
> above, it is immediately popped off. (This pop is meant to simply free  
> the original node grabbed at the top of the while loop.)
> My solution is to do the PopFront command at the top of the loop, and  
> *then* push the new node (stored in a temp variable to avoid problems  
> with the Return(node) statement at the end).
> Here's the diff:
> %diff  
> ITK-CVS-src/Insight/Code/BasicFilters/ 
> itkCannyEdgeDetectionImageFilter.txx  
> NEWCannyEdgeDetectionImageFilter.txx
> 
> 437c437
> <   ListNodeType * node;
> ---
>  >   ListNodeType * node, *tempNode;
> 454a455
>  >     m_NodeList->PopFront();
> 467,469c468,470
> <           node = m_NodeStore->Borrow();
> <           node->m_Value = nIndex;
> <           m_NodeList->PushFront(node);
> ---
>  >           tempNode = m_NodeStore->Borrow();
>  >           tempNode->m_Value = nIndex;
>  >           m_NodeList->PushFront(tempNode);
> 476d476
> <     m_NodeList->PopFront();
> 
> 
> 
> Zach Pincus
> 
> Department of Biochemistry and Program in Biomedical Informatics
> Stanford University School of Medicine
> 
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