BUG: Support serialization of an empty itk.Image

If an itk.Image's memory has not been allocated return None from
itk.array_view_from_image.

Pickle and unpickles an unallocated itk.Image to support sending the
image's metadata with Dask. InsightSoftwareConsortium#4267.

Modules/Bridge/NumPy/wrapping/PyBuffer.i.in

@@ -9,6 +9,10 @@
         i.e. k,j,i versus i,j,k. However C-order indexing is expected by most
         algorithms in NumPy / SciPy.
         """
+
+        if image.GetBufferPointer() is None:
+            return None
+
         if update:
             # Ensure the image regions and image pixel buffer have been updated
             # correctly
@@ -46,6 +50,8 @@
 
         This is a deep copy of the image buffer and is completely safe and without potential side effects.
         """
+        if image.GetBufferPointer() is None:
+            return None
 
         arrayView = itkPyBuffer@PyBufferTypes@.GetArrayViewFromImage(image, keep_axes, update)
 

Modules/Bridge/NumPy/wrapping/test/itkPyBufferTest.py

@@ -70,6 +70,38 @@ def test_NDArrayITKBase_pickle(self):
             equal = (recon_obj == ndarray_itk_base).all()
             assert equal, "Different results before and after pickle"
 
+    def test_EmptyImage_pickle(self):
+        """
+        Test the serialization of an empty itk.Image
+        """
+        Dimension = 3
+        ImageType = itk.Image[itk.UC, Dimension]
+        RegionType = itk.ImageRegion[Dimension]
+
+        region = RegionType()
+        region.SetSize(0, 6)
+        region.SetSize(1, 6)
+        region.SetSize(2, 6)
+
+        image = ImageType.New()
+        image.SetRegions(region)
+        spacing = [3.0, 4.0, 5.0]
+        image.SetSpacing(spacing)
+        # Before allocation
+        # scalarImage.Allocate(True)
+
+        import pickle
+        pickled = pickle.dumps(image)
+        reloaded = pickle.loads(pickled)
+        reloaded_spacing = reloaded.GetSpacing()
+        spacing = reloaded.GetSpacing()
+        assert spacing[0] == reloaded_spacing[0]
+        assert spacing[1] == reloaded_spacing[1]
+        assert spacing[2] == reloaded_spacing[2]
+
+        ndarray_itk_base = itk.array_view_from_image(image)
+        assert ndarray_itk_base == None
+
     def test_NumPyBridge_itkScalarImage(self):
         "Try to convert all pixel types to NumPy array view"
 

Wrapping/Generators/Python/itk/support/extras.py

@@ -805,12 +805,17 @@ def image_from_dict(image_dict: Dict) -> "itkt.Image":
     import itk
 
     ImageType = image_type_from_wasm_type(image_dict["imageType"])
-    image = itk.PyBuffer[ImageType].GetImageViewFromArray(image_dict["data"])
+    if image_dict["data"] is None:
+        image = ImageType.New()
+        image.SetRegions(image_dict["size"])
+        image.Allocate(True)
+    else:
+        image = itk.PyBuffer[ImageType].GetImageViewFromArray(image_dict["data"])
+        image.SetRegions(image_dict["size"])
     image.SetOrigin(image_dict["origin"])
     image.SetSpacing(image_dict["spacing"])
     image.SetDirection(image_dict["direction"])
     image.SetObjectName(image_dict["name"])
-    image.SetRegions(image_dict["size"])
     return image