ENH: Add support for multiple metrics and regularization

[Leirbag-gabrieL]

Hi, apparently it was not possible to do a multi-image registration with a transform metric like this:

(Metric "AdvancedMattesMutualInformation" "AdvancedMattesMutualInformation" "AdvancedMattesMutualInformation" "AdvancedMattesMutualInformation" "TransformBendingEnergyPenalty")
(Metric0Weight 1.0)
(Metric1Weight 1.0)
(Metric2Weight 1.0)
(Metric3Weight 1.0)
(Metric4Weight 10.0)

(NumberOfHistogramBins0 32)
(NumberOfHistogramBins1 32)
(NumberOfHistogramBins2 32)
(NumberOfHistogramBins3 32)

Now it is possible :)

[N-Dekker]

Thank you very much @Leirbag-gabrieL

Can you possibly make a minimal example of a registration that is made possible by your proposed fix? Ideally in the form of a library unit test, like this one:

elastix/Core/Main/GTesting/itkElastixRegistrationMethodGTest.cxx

Lines 3006 to 3048 in 0c0b19f

	GTEST_TEST(itkElastixRegistrationMethod, EuclideanDistancePointMetric)
	{
	static constexpr auto ImageDimension = 2U;
	using PixelType = float;
	using ImageType = itk::Image<PixelType, ImageDimension>;
	using SizeType = itk::Size<ImageDimension>;
	using OffsetType = itk::Offset<ImageDimension>;
	const OffsetType translationOffset{ { 1, -2 } };
	const SizeType imageSize{ { 5, 6 } };
	const auto fixedImage = CreateImage<PixelType>(imageSize);
	const auto movingImage = CreateImage<PixelType>(imageSize);
	elx::DefaultConstruct<ElastixRegistrationMethodType<ImageType>> registration{};
	using PointType = itk::Point<float, ImageDimension>;
	const PointType fixedPoint{};
	PointType movingPoint = fixedPoint;
	for (unsigned int i{}; i < ImageDimension; ++i)
	{
	movingPoint[i] += translationOffset[i];
	}
	registration.SetFixedImage(fixedImage);
	registration.SetMovingImage(movingImage);
	registration.SetFixedPoints(::MakeVectorContainer(std::vector<PointType>{ fixedPoint }));
	registration.SetMovingPoints(::MakeVectorContainer(std::vector<PointType>{ movingPoint }));
	registration.SetParameterObject(CreateParameterObject(
	ParameterMapType{ // Parameters in alphabetic order:
	{ "ImageSampler", { "Full" } },
	{ "MaximumNumberOfIterations", { "2" } },
	{ "Metric", { "AdvancedNormalizedCorrelation", "CorrespondingPointsEuclideanDistanceMetric" } },
	{ "Optimizer", { "AdaptiveStochasticGradientDescent" } },
	{ "Registration", { "MultiMetricMultiResolutionRegistration" } },
	{ "Transform", { "TranslationTransform" } } }));
	registration.Update();
	const auto transformParameters = GetTransformParametersFromFilter(registration);
	EXPECT_EQ(ConvertToOffset<ImageDimension>(transformParameters), translationOffset);
	}

But then, having something like { "Metric", { "AdvancedMattesMutualInformation", "TransformBendingEnergyPenalty" } } instead.

Otherwise, a complete ("dummy") test set, including elastix parameter file, would also be of help to us. Again, preferably "minimal", so that it won't take much time to process.

[N-Dekker]

Nowadays, we are using using, instead of typedef. But a coding style issue like that is not a show-stopper. I can adjust it afterwards. Also, I think I would move the definition of TransformMetricType into the function body of CombinationImageToImageMetric::SetMetric, as that seems to be the only place where it is needed.

[N-Dekker]

TransformMetricPointer is never used, so I think I would remove it. (No show-stopper for now.)

[N-Dekker]

I would make it:

if( dynamic_cast<PointSetMetricType *>(metric) )

And remove the local testPtr1 variable.

Similar for testPtr2, oldTestPtr1, oldTestPtr2. No show-stopper to me, though. Just a style thing.