GPU version Demons: the size of output is different from that of input image

Beginner Questions
1

I am using GPU Demons for myself project. I find a strange bug: the size of input (fix and moving image) is (174, 256, 256), but the size of output is (87, 128, 128).

The main code is:

	RegistrationFilterType::Pointer DemonsFilter = RegistrationFilterType::New();
	CommandIterationUpdate::Pointer observer = CommandIterationUpdate::New();
	DemonsFilter->AddObserver(itk::IterationEvent(), observer);
	DemonsFilter->SetFixedImage(fixImageCPUToGPUFilter->GetOutput());
	DemonsFilter->SetMovingImage(movingImageCPUToGPUFilter->GetOutput());
	DemonsFilter->GetOutput()->SetRequestedRegionToLargestPossibleRegion();
	auto sizexxx = gpuDisplacementField->GetLargestPossibleRegion().GetSize();
	if (i != 0)
	{
		DemonsFilter->SetInitialDisplacementField(gpuDisplacementField);
	}
	else
	{
		DemonsFilter->SetInitialDisplacementField(nullptr);
	}

	DemonsFilter->SetNumberOfIterations(nIterations[i]);
	DemonsFilter->SetStandardDeviations(1.0);
	DemonsFilter->Update();
	auto fixImageSize = DemonsFilter->GetFixedImage()->GetLargestPossibleRegion().GetSize();
	std::cout << "fix image size: " << fixImageSize << std::endl;
	auto movingImageSize = DemonsFilter->GetMovingImage()->GetLargestPossibleRegion().GetSize();
	std::cout << "moving image size: " << movingImageSize << std::endl;
	auto displacementSize = DemonsFilter->GetOutput()->GetLargestPossibleRegion().GetSize();
	std::cout << "displacement size: " << displacementSize << std::endl;

And the information in the console is:

fix image size: [174, 256, 256]
moving image size: [174, 256, 256]
displacement size: [87, 128, 128]

Why the size of displacement is not (174, 256, 256) ??
Moreover, I have visualized the displacement field, and the result is just random noise. Also, I have checked the input fix and moving image, and they are correct.
How can I make the demons registration correct?

2

I have find the bug.

after I set the initial displacement field by:

DemonsFilter->SetInitialDisplacementField(gpuDisplacementField);

I obtian the size of initial displacement by:

DemonsFilter->GetInitialDisplacementField()->GetLargestPossibleRegion().GetSize();

and I find the result is (174, 256, 256)
actually, the initial displacement field is obtained by:

	cpuToGpuDisplacementFieldFilter->SetInput(cpuDisplacementField);
	cpuToGpuDisplacementFieldFilter->Update();
	gpuDisplacementField = cpuToGpuDisplacementFieldFilter->GetOutput();

If I change:

cpuToGpuDisplacementFieldFilter->Update();

to

cpuToGpuDisplacementFieldFilter->UpdateLargestPossibleRegion();

the result will be correct.
Also, it really make me confuzed what’s the difference between Update and UpdateLargestPossibleRegion.