Store slice overlap indices in cell ID field

openmc/lib/plot.py

@@ -265,22 +265,40 @@ def property_map(plot):
 
 
 # Python wrappings for overlap functions
-def slice_data_overlap_count():
+def slice_data_overlap_count() -> int:
+    """Return the number of unique overlaps from the last slice plot.
+
+    Returns
+    -------
+    int
+        Number of unique overlapping cell pairs detected by the most recent
+        :func:`slice_data` call with overlap checking enabled.
+    """
     count = c_size_t()
     _dll.openmc_slice_data_overlap_count(count)
     return count.value
 
 
-def slice_data_overlap_info():
+def slice_data_overlap_info() -> np.ndarray:
+    """Return identifying information for overlaps from the last slice plot.
+
+    Returns
+    -------
+    numpy.ndarray
+        Array of shape ``(n, 3)`` with int32 dtype, where ``n`` is the number
+        of unique overlaps detected by the most recent :func:`slice_data` call
+        with overlap checking enabled. Each row contains ``[universe_id,
+        cell1_id, cell2_id]``.
+    """
     n = slice_data_overlap_count()
-    overlap_info = np.empty(n * 3, dtype=np.int32)
+    overlap_info = np.empty((n, 3), dtype=np.int32)
 
     if n > 0:
         _dll.openmc_slice_data_overlap_info(
             n,
             overlap_info.ctypes.data_as(POINTER(c_int32)),
         )
-    return overlap_info, n
+    return overlap_info
 
 
 _dll.openmc_get_plot_index.argtypes = [c_int32, POINTER(c_int32)]

src/plot.cpp

@@ -156,12 +156,12 @@ void RasterData::set_value(size_t y, size_t x, const Particle& p, int level,
 
 void RasterData::set_overlap(size_t y, size_t x, int overlap_idx)
 {
-  // Set cell, instance, and material to OVERLAP, but preserve filter bin
-  id_data_(y, x, 0) = OVERLAP;
+  // Set cell, instance, and material to OVERLAP, but preserve filter bin for
+  // tally plotting. Cell encodes the overlap index as a negative number so that
+  // it can be used to look up overlap information in the plotter.
+  id_data_(y, x, 0) = OVERLAP - overlap_idx - 1;
   id_data_(y, x, 1) = OVERLAP;
-  id_data_(y, x, 2) = OVERLAP - overlap_idx - 1;
-  // Note: id_data_(y, x, 3) is NOT overwritten - preserves filter bin for tally
-  // plotting
+  id_data_(y, x, 2) = OVERLAP;
 
   property_data_(y, x, 0) = OVERLAP;
   property_data_(y, x, 1) = OVERLAP;

tests/unit_tests/test_slice_data_overlap_info.py

@@ -55,22 +55,23 @@ def test_overlaps_enabled(overlap_model):
     # Run a single slice with overlap detection enabled and check all
     # expected properties in one pass.
     geom_data = run_slice()
-    overlap_info, n = openmc.lib.slice_data_overlap_info()
-    mat_ids = geom_data[:, :, 2]
+    overlap_info = openmc.lib.slice_data_overlap_info()
+    n = overlap_info.shape[0]
+    cell_ids = geom_data[:, :, 0]
 
-    # mat_ids should contain values more negative than _OVERLAP; RasterData
+    # cell_ids should contain values more negative than _OVERLAP; RasterData
     # encodes each unique overlap as OVERLAP - overlap_idx - 1 into slot 2.
-    assert np.any(mat_ids < _OVERLAP)
+    assert np.any(cell_ids < _OVERLAP)
 
     # overlap_keys should have 2 entries for the two distinct overlapping
     # cylinder pairs in this model.
     assert n == 2, f"Expected exactly 2 overlap entries, got {n}"
 
     # Each entry is a (universe_id, cell1_id, cell2_id) triple; verify values.
     for i in range(n):
-        universe_id = int(overlap_info[i * 3])
-        cell1_id = int(overlap_info[i * 3 + 1])
-        cell2_id = int(overlap_info[i * 3 + 2])
+        universe_id = int(overlap_info[i, 0])
+        cell1_id = int(overlap_info[i, 1])
+        cell2_id = int(overlap_info[i, 2])
         assert universe_id == 1
         assert cell1_id in {1, 2, 3}
         assert cell2_id in {1, 2, 3}
@@ -81,7 +82,8 @@ def test_overlaps_disabled(overlap_model):
     # With show_overlaps=False, set_overlap is never called and overlap_keys
     # is never written to, so the image and map should both be clean.
     geom_data = run_slice(show_overlaps=False)
-    _, n = openmc.lib.slice_data_overlap_info()
+    overlap_info = openmc.lib.slice_data_overlap_info()
+    n = overlap_info.shape[0]
 
     assert not np.any(geom_data[:, :, 2] < _OVERLAP)
     assert n == 0