AdvancedCombinationTransform::GetNumberOfTransforms() returns zero when initial transform is not a combination!

[N-Dekker]

It appears that AdvancedCombinationTransform::GetNumberOfTransforms() returns zero when the initial transform is not a combination transform. Looking at the original commit (2fc2c49, November 14, 2013):

elastix/src/Common/Transforms/itkAdvancedCombinationTransform.hxx

Lines 105 to 132 in 2fc2c49

	template <typename TScalarType, unsigned int NDimensions>
	SizeValueType
	AdvancedCombinationTransform<TScalarType, NDimensions>
	::GetNumberOfTransforms( void ) const
	{
	SizeValueType num = 0;
	CurrentTransformConstPointer currentTransform = GetCurrentTransform();
	if( currentTransform.IsNotNull() )
	{
	InitialTransformConstPointer initialTransform = GetInitialTransform();
	if( initialTransform.IsNotNull() )
	{
	const Self *initialTransformCasted =
	dynamic_cast< const Self * >( initialTransform.GetPointer() );
	if ( initialTransformCasted )
	{
	num += initialTransformCasted->GetNumberOfTransforms() + 1;
	}
	}
	else
	{
	num++;
	}
	}
	return num;
	} // end GetNumberOfTransforms()

Simplified as follows (by the first commit of pull request #925):

 if (GetCurrentTransform()) 
 { 
   if (const InitialTransformConstPointer initialTransform = GetInitialTransform()) 
   { 
     if (const auto initialTransformCasted = dynamic_cast<const Self *>(initialTransform.GetPointer())) 
     { 
       return initialTransformCasted->GetNumberOfTransforms() + 1; 
     } 
   } 
   else 
   { 
     return 1; 
   } 
 } 
  
 return 0; 

You see, if the dynamic_cast<const Self *> fails, the function falls back to just return 0. Is that OK? Because it seems to me that it should return 2 instead.

Example unit test:

  const auto Dimension = 2U;

  using TranslationTransformElastixType = elx::TranslationTransformElastix<ElastixType<Dimension>>;
  using AdvancedCombinationTransformType = TranslationTransformElastixType::Superclass1;
  using AdvancedTransformType = AdvancedCombinationTransformType::Superclass;
  const auto TranslationTransformElastix = TranslationTransformElastixType::New();
  const auto initialTranslationTransformElastix = TranslationTransformElastixType::New();

  // The following expectation passes as well, no problem.
  EXPECT_EQ(TranslationTransformElastix->GetNumberOfTransforms(), 1);
  TranslationTransformElastix->AdvancedCombinationTransformType::SetInitialTransform(
    initialTranslationTransformElastix);
  // The following expectation passes, no problem.
  EXPECT_EQ(TranslationTransformElastix->GetNumberOfTransforms(), 2);

  const auto advancedTranslationTransform = itk::AdvancedTranslationTransform<double, Dimension>::New();
  TranslationTransformElastix->AdvancedCombinationTransformType::SetInitialTransform(advancedTranslationTransform);
  // The following expectation fails, because GetNumberOfTransforms() returns zero!!!
  EXPECT_EQ(TranslationTransformElastix->GetNumberOfTransforms(), 2);

[N-Dekker]

@mstaring @stefanklein Could it be that in practice the initial transform is always also a combination transform? (Meaning that the dynamic_cast<const Self *>(initialTransform.GetPointer()) always returns a non-null pointer anyway?)