Errors when using OpenCLFixedGenericImagePyramid

[Jakob Guldberg Aaes]

I have finally been able to compile elastix with the OpenCL features after compiling the  right ITK (5.1.1) and OpenCL. But when I run elastix with my images and masks with OpenCL features enabled in the config file it fails with the following error

```
Initialization of AdvancedMattesMutualInformation metric took: 0 ms.
Time spent in resolution 0 (ITK initialization and iterating): 0.001 s.
Stopping condition: Error in metric.

itk::ExceptionObject (0x7fcc68001540)
Location: "ElastixTemplate - Run()"
File: /home/fuzie/Documents/Speciale/code/elastix-build/InsightToolkit-5.1.1/Modules/Core/Common/include/itkImageConstIterator.h
Line: 210
Description: itk::ERROR: Region ImageRegion (0x7ffdc32ad8b0)
  Dimension: 2
  Index: [-9223372036854775808, 29]
  Size: [4, 2]
 is outside of buffered region ImageRegion (0x55794bfad4d0)
  Dimension: 2
  Index: [0, 0]
  Size: [32, 32]


Error occurred during actual registration.


Errors occurred!
```

When I run it purely on CPU it works just fine.
The parameters I use are the following
```
// ********** Image Types

(FixedInternalImagePixelType "float")
(FixedImageDimension 3)
(MovingInternalImagePixelType "float")
(MovingImageDimension 3)
(UseBinaryFormatForTransformationParameters "false")

// ********** Components

(Registration "MultiResolutionRegistration")
// // CPU - pyramids
// (FixedImagePyramid "FixedSmoothingImagePyramid")
// (MovingImagePyramid "MovingSmoothingImagePyramid")
// GPU pyramids
(FixedImagePyramid "OpenCLFixedGenericImagePyramid")
(MovingImagePyramid "OpenCLMovingGenericImagePyramid")

(OpenCLFixedGenericImagePyramidUseOpenCL "true")
(OpenCLMovingGenericImagePyramidUseOpenCL "true")
(Interpolator "BSplineInterpolator")
(Metric "AdvancedMattesMutualInformation")
(Optimizer "StandardGradientDescent")
(ResampleInterpolator "FinalBSplineInterpolator")
(Transform "BSplineTransform")
(UseDirectionCosines "false")

// ********** Pyramid

// Total number of resolutions
(NumberOfResolutions 8)
// default schedule: isotropic upsampling with factor 2

// ********** Transform

(FinalGridSpacingInPhysicalUnits 12.0 12.0 12.0)
(GridSpacingSchedule 16.0 16.0 16.0 16.0 8.0 4.0 2.0 1.0)
(HowToCombineTransforms "Compose")


// ********** Optimizer

// Maximum number of iterations in each resolution level:
(MaximumNumberOfIterations 1000)

//SP: Param_a in each resolution level. a_k = a/(A+k+1)^alpha
(SP_a 10000.0)

//SP: Param_alpha in each resolution level. a_k = a/(A+k+1)^alpha
(SP_alpha 0.602)

//SP: Param_A in each resolution level. a_k = a/(A+k+1)^alpha
(SP_A 50.0)


// ********** Metric

//Number of grey level bins in each resolution level:
(NumberOfHistogramBins 32)
(FixedLimitRangeRatio 0.0)
(MovingLimitRangeRatio 0.0)
(FixedKernelBSplineOrder 1)
(MovingKernelBSplineOrder 3)


// ********** Several

(WriteTransformParametersEachIteration "false")
(WriteTransformParametersEachResolution "false")
(WriteResultImageAfterEachResolution "false")
(WriteResultImage "false")
(ShowExactMetricValue "false")
(ErodeMask "false")


// ********** ImageSampler

//Number of spatial samples used to compute the mutual information in each resolution level:
(ImageSampler "RandomSparseMask")
(NumberOfSpatialSamples 2000)
(NewSamplesEveryIteration "true")
(UseRandomSampleRegion "false")


// ********** Interpolator and Resampler

//Order of B-Spline interpolation used in each resolution level:
(BSplineInterpolationOrder 1)

//Order of B-Spline interpolation used for applying the final deformation:
(FinalBSplineInterpolationOrder 3)

//Default pixel value for pixels that come from outside the picture:
(DefaultPixelValue -1000)

// Resampler specific
// (Resampler "DefaultResampler")
(Resampler "OpenCLResampler")
(OpenCLResamplerUseOpenCL "true")
(ResultImageFormat "nii")
(ResultImagePixelType "float")
(CompressResultImage "true")
```

Any idea how this can be solved?

Best regards
Jakob