if(ITK_USE_GPU)
  find_package(OpenCL REQUIRED)
  if(NOT ${OPENCL_FOUND})
    message(FATAL "Could not find OpenCL")
  endif()

  macro(sourcefile_to_string SOURCE_FILE RESULT_CMAKE_VAR)
    file(STRINGS ${SOURCE_FILE} FileStrings)
    foreach(SourceLine ${FileStrings})
      # replace all \ with \\ to make the c string constant work
      string(
        REGEX
        REPLACE "\\\\"
                "\\\\\\\\"
                TempSourceLine
                "${SourceLine}")
      # replace all " with \" to make the c string constant work
      string(
        REGEX
        REPLACE "\""
                "\\\\\""
                EscapedSourceLine
                "${TempSourceLine}")
      set(${RESULT_CMAKE_VAR} "${${RESULT_CMAKE_VAR}}\n\"${EscapedSourceLine}\\n\"")
    endforeach()
  endmacro()

  macro(
    write_gpu_kernel_to_file
    OPENCL_FILE
    GPUFILTER_NAME
    GPUFILTER_KERNELNAME
    OUTPUT_FILE
    SRC_VAR)
    sourcefile_to_string(${OPENCL_FILE} ${GPUFILTER_KERNELNAME}_SourceString)
    set(${GPUFILTER_KERNELNAME}_KernelString "#include \"itk${GPUFILTER_NAME}.h\"\n\n")
    set(${GPUFILTER_KERNELNAME}_KernelString "${${GPUFILTER_KERNELNAME}_KernelString}namespace itk\n")
    set(${GPUFILTER_KERNELNAME}_KernelString "${${GPUFILTER_KERNELNAME}_KernelString}{\n\n")
    set(${GPUFILTER_KERNELNAME}_KernelString
        "${${GPUFILTER_KERNELNAME}_KernelString}const char* ${GPUFILTER_KERNELNAME}::GetOpenCLSource()\n")
    set(${GPUFILTER_KERNELNAME}_KernelString "${${GPUFILTER_KERNELNAME}_KernelString}{\n")
    set(${GPUFILTER_KERNELNAME}_KernelString
        "${${GPUFILTER_KERNELNAME}_KernelString}  return ${${GPUFILTER_KERNELNAME}_SourceString};\n")
    set(${GPUFILTER_KERNELNAME}_KernelString "${${GPUFILTER_KERNELNAME}_KernelString}}\n\n")
    set(${GPUFILTER_KERNELNAME}_KernelString "${${GPUFILTER_KERNELNAME}_KernelString}}\n")

    file(WRITE ${CMAKE_CURRENT_BINARY_DIR}/${OUTPUT_FILE} "${${GPUFILTER_KERNELNAME}_KernelString}")

    configure_file(${OPENCL_FILE} ${CMAKE_CURRENT_BINARY_DIR}/${OUTPUT_FILE}.cl COPYONLY)
    add_custom_target(
      ${GPUFILTER_KERNELNAME}_Target
      DEPENDS ${CMAKE_CURRENT_BINARY_DIR}/${OUTPUT_FILE} ${CMAKE_CURRENT_SOURCE_DIR}/CMakeLists.txt
              ${CMAKE_CURRENT_BINARY_DIR}/${OUTPUT_FILE}.cl)
    set_source_files_properties(${CMAKE_CURRENT_BINARY_DIR}/${OUTPUT_FILE} PROPERTIES GENERATED ON)
    set(${SRC_VAR} ${${SRC_VAR}} ${CMAKE_CURRENT_BINARY_DIR}/${OUTPUT_FILE})
  endmacro()

  macro(write_gpu_kernels GPUKernels GPU_SRC)
    foreach(GPUKernel ${GPUKernels})
      get_filename_component(FilterName ${GPUKernel} NAME_WE)
      write_gpu_kernel_to_file(
        ${GPUKernel}
        ${FilterName}
        ${FilterName}Kernel
        "${FilterName}Kernel.cxx"
        ${GPU_SRC})
    endforeach()
  endmacro()
endif()