raster auto-center: allow extra grid padding at the bottom of the y scan

Make the first-grid y padding asymmetric: the bottom (far end of the n_y scan)
can be padded more than the top via daq.auto_raster.grid_padding_fraction_y_
bottom (defaults to grid_padding_fraction_y, so symmetric unless set higher).
The top padding still shifts smargon_top_left; the extra bottom padding only
grows n_y.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/aare/daq/operations/common/ml_bounding_box.py

@@ -198,21 +198,25 @@ def _box_to_raster_request(
     if grid_padding:
         # Pad the grid by a fraction of its size on each side (at least one cell),
         # extending *before* cell 0 as well, so the top-left moves outward and
-        # smargon_top_left shifts with it.
+        # smargon_top_left shifts with it. Y is asymmetric: the bottom (far end
+        # of the n_y scan) can be padded more than the top.
         frac_x = float(cfg_get("daq.auto_raster.grid_padding_fraction_x", 0.15))
-        frac_y = float(cfg_get("daq.auto_raster.grid_padding_fraction_y", 0.15))
+        frac_y_top = float(cfg_get("daq.auto_raster.grid_padding_fraction_y", 0.15))
+        frac_y_bottom = float(cfg_get("daq.auto_raster.grid_padding_fraction_y_bottom", frac_y_top))
         pad_x = max(1, int(ceil(frac_x * n_x)))
-        pad_y = max(1, int(ceil(frac_y * n_y)))
+        pad_y_top = max(1, int(ceil(frac_y_top * n_y)))
+        pad_y_bottom = max(1, int(ceil(frac_y_bottom * n_y)))
         x1 = x1 - pad_x * grid_size.x / geom.pixel_in_mm
-        y1 = y1 - pad_y * grid_size.y / geom.pixel_in_mm
+        y1 = y1 - pad_y_top * grid_size.y / geom.pixel_in_mm
         n_x = n_x + 2 * pad_x
-        n_y = n_y + 2 * pad_y
+        n_y = n_y + pad_y_top + pad_y_bottom
         logger.info(
             "Padded auto-center raster grid",
             extra=merge_log_context(
                 sample_log_context(sample),
                 {"sample_id": sample_id, "ml_image_name": filename,
-                 "pad_cells_x": pad_x, "pad_cells_y": pad_y, "n_x": n_x, "n_y": n_y},
+                 "pad_cells_x": pad_x, "pad_cells_y_top": pad_y_top,
+                 "pad_cells_y_bottom": pad_y_bottom, "n_x": n_x, "n_y": n_y},
             ),
         )
 

tests/unit/daq/operations/test_ml_raster_plan.py

@@ -105,6 +105,26 @@ def test_grid_padding_grows_and_shifts_top_left(monkeypatch):
     assert pad.smargon_top_left.sh_mm.z != nopad.smargon_top_left.sh_mm.z
 
 
+def test_grid_padding_y_bottom_asymmetric(monkeypatch):
+    box = (150, 150, 400, 450)  # tall box
+
+    def cfg(y_bottom):
+        return lambda k, d=None: (
+            y_bottom if "grid_padding_fraction_y_bottom" in k
+            else (0.0 if "grid_padding_fraction" in k else d)
+        )
+
+    monkeypatch.setattr(mlb, "cfg_get", cfg(0.0))   # bottom == top (min 1 cell each)
+    sym = _grid(box, grid_padding=True)
+    monkeypatch.setattr(mlb, "cfg_get", cfg(0.6))   # much more padding at the bottom
+    bottom = _grid(box, grid_padding=True)
+
+    assert bottom.n_y > sym.n_y                      # extra cells added at the bottom
+    assert bottom.n_x == sym.n_x                      # x unaffected
+    # top padding identical -> smargon_top_left (cell 0) unchanged
+    assert bottom.smargon_top_left == sym.smargon_top_left
+
+
 def test_grid_padding_fraction_scales(monkeypatch):
     box = (150, 150, 520, 420)
     monkeypatch.setattr(mlb, "cfg_get", lambda k, d=None: 0.0 if "grid_padding_fraction" in k else d)