FIX: Allow tr=0 in mghformat

Author: larsoner

nibabel/freesurfer/mghformat.py

@@ -276,10 +276,12 @@ def set_zooms(self, zooms):
         ndims = self._ndims()
         if len(zooms) > ndims:
             raise HeaderDataError('Expecting %d zoom values' % ndims)
-        if np.any(zooms <= 0):
-            raise HeaderDataError('zooms must be positive')
+        if np.any(zooms[:3] <= 0):
+            raise HeaderDataError('zooms[:3] must be positive')
         hdr['delta'] = zooms[:3]
         if len(zooms) == 4:
+            if zooms[3] < 0:
+                raise HeaderDataError('zooms[3] must be non-negative')
             hdr['tr'] = zooms[3]
 
     def get_data_shape(self):

nibabel/freesurfer/tests/test_mghformat.py

@@ -159,6 +159,8 @@ def test_set_zooms():
                   (1, 1, 1, 1, 5)):
         with assert_raises(HeaderDataError):
             h.set_zooms(zooms)
+    # smoke test for tr=0
+    h.set_zooms((1, 1, 1, 0))
 
 
 def bad_dtype_mgh():