Set field parameters on blocks. Closes #1407

yt/data_objects/data_containers.py

@@ -1166,9 +1166,12 @@ def blocks(self):
                 # For grids this will be a grid object, and for octrees it will
                 # be an OctreeSubset.  Note that we delegate to the sub-object.
                 o = self._current_chunk.objs[0]
+                cache_fp = o.field_parameters.copy()
+                o.field_parameters.update(self.field_parameters)
                 for b, m in o.select_blocks(self.selector):
                     if m is None: continue
                     yield b, m
+                o.field_parameters = cache_fp
 
 class GenerationInProgress(Exception):
     def __init__(self, fields):

yt/data_objects/tests/test_fluxes.py

@@ -111,3 +111,21 @@ def test_correct_output_unit():
     Nmax = sp1.max('HI_Density')
     sur = ds.surface(sp1,"HI_Density", .5*Nmax)
     sur['x'][0]
+
+@requires_file(ISOGAL)
+def test_radius_surface():
+    # see #1407
+    ds = load(ISOGAL)
+    reg = ds.all_data()
+    sp = ds.sphere(ds.domain_center, (0.5, 'code_length'))
+    for obj in [reg, sp]:
+        for rad in [0.05, .1, .4]:
+            surface = ds.surface(obj, 'radius', (rad, 'code_length'))
+            assert_almost_equal(
+                surface.surface_area.v, 4*np.pi*rad**2, decimal=2)
+            verts = surface.vertices
+            for i in range(3):
+                assert_almost_equal(
+                    verts[i, :].min().v, 0.5-rad, decimal=2)
+                assert_almost_equal(
+                    verts[i, :].max().v, 0.5+rad, decimal=2)