[Insight-users] an interesting problem

[Guorong Wu]

hello all:
	I have compared the performance of non-rigid registration algorithms provided by ITK these few days. They are DeformableRegistration2.cxx, DeformableRegistration3.cxx, and DeformableRegistration5.cxx, which are the implementation of original demons, symmetric force, and levelset motion respectively. Before I notice the problem, I found symmetric force demons-like algorithm gave nearly good performance as levelset motion based algorithm did except that it takes much more time than the latter one.  
	As you known, the performance is evaluated by the GetMetric function which returns the value of m_SumOfSquaredDifference. The variable is updated during each iterations. I found in DemonsRegistrationFunction and LevelSetMotionRegistrationFunction this variable is update as follows:
    globalData->m_SumOfSquaredDifference+=vnl_math_sqr(fixedImageValue-movingImageValue);
But in SymmtricForcesDemonsRegistrationFunction the update is in another way (at line 318):
    globlaData->m_SumOfSquaredDifference+=vnl_math_sqr(fixedImageValue-NewmovingImageValue);
which NewmovingImageValue the output of interpolator which is evaluated at the coordinate of deformation before iterations plus the updated value in this iteration(at line 289-322)      	
	So in order to make fair comparision, I modify the code of SymmetricForcesDemonsRegistrationFunction making SSD is the square of fixedImageValue minusing movingImageValue, not the NewmovingImageValue. After done this, I found the SSD of symmetricforce algorightm is much bigger than before. But I think the modification will not cause the problem because the NewMovingImageValue is just the value of movingImageValue computed at next iteration. Would you like to give me an answer? Thanks.
	
	   	

　　　　　　　　Guorong Wu
　　　　　　　　grwu at sjtu.edu.cn
　　　　　　　　　　2004-07-28

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Guorong Wu	
The Department of Computer Science and Technology
Shanghai Jiao Tong University
1954 Huashan RD
Shanghai China    
200030
Tel:  +86 21 3226 1236  , 6293 2089
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