FIX: Set ROI data before writing array, use uint8

fmriprep/interfaces/gifti.py

@@ -46,17 +46,21 @@ def _run_interface(self, runtime):
         # wb_command -metric-math "abs(var * -1) > 0"
         roi = np.abs(darray.data) > 0
 
+        # Divergence: Set datatype to uint8, since the values are boolean
+        # wb_command sets datatype to float32
         darray = nb.gifti.GiftiDataArray(
             roi,
             intent=darray.intent,
-            datatype=darray.datatype,
+            datatype='uint8',
             encoding=darray.encoding,
             endian=darray.endian,
             coordsys=darray.coordsys,
             ordering=darray.ind_ord,
             meta=meta,
         )
 
+        img.darrays[0] = darray
+
         out_filename = os.path.join(runtime.cwd, f"{subject}.{hemi}.roi.native.shape.gii")
         img.to_filename(out_filename)
         self._results["roi_file"] = out_filename