diff --git a/apps/studio/src/orisys_studio/replay_analysis.py b/apps/studio/src/orisys_studio/replay_analysis.py
index 4b4786d..badd464 100644
--- a/apps/studio/src/orisys_studio/replay_analysis.py
+++ b/apps/studio/src/orisys_studio/replay_analysis.py
@@ -294,11 +294,10 @@ class AnalysisSession:
         the pass's own answer — the field is finite only at those cells, and
         :meth:`serve_policy` refuses a display setting that moves the grid — so the grid
         travels from the result's provenance. *Which* candidates are active is a display
-        question for the shared finger painter, and ``magnitude`` is stored precisely so it
-        can be asked again later. A box result on the live painter's grid is different: the
-        2D canvas calls ``orisys.util.draw_arrows``, which does not read the display
-        threshold, so the on/off floors stay at that painter's values (2 / 1.5 px). The 3D
-        overlay uses this same policy so the two figures zero at the same strength.
+        question for the shared painter, which re-selects the recovered field. A box
+        result keeps the box grid's on/off floors (2 / 1.5 px) rather than the operator's
+        finger thresholds, so live and replay zero at the same strength. The 3D overlay
+        uses this same policy.
 
         Without this, a display setting that moves the grid would be refused by
         ``serve_policy`` and then applied to the drawing anyway, re-selecting the recovered
@@ -541,12 +540,7 @@ class AnalysisSession:
         The shared painter re-selects this field, so the inverse divides by that
         product and the cap stays idempotent: the stored offsets come back exactly.
         At ``flow_arrow_scale == 1`` that divisor is ``DISPLACEMENT_PX_PER_FLOW`` times
-        the margin.
-
-        The box painter does not re-select. It colours by the displacement *before*
-        the cap, which ``magnitude`` still holds (``|DISPLACEMENT_PX_PER_FLOW * flow|``).
-        Direction comes from ``dx``/``dy``. Float32 magnitude is not exact enough to
-        reproduce a capped offset, which is why only the box grid uses it.
+        the margin. Box and finger use the same inverse; only the policy numbers differ.
         """
         rows = np.asarray(samples.rows, np.int64)
         cols = np.asarray(samples.cols, np.int64)
@@ -563,25 +557,16 @@ class AnalysisSession:
             # ``NaN`` is the point of the mask, and the selector's tip arithmetic on a
             # discarded cell is defined to be discarded: say so here rather than let
             # numpy warn about an invalid cast on every frame.
-            if uses_sdk_box_grid(self.policy):
-                magnitude = np.asarray(samples.magnitude, np.float64)
-                length = np.sqrt(dx * dx + dy * dy)
-                flow_mag = magnitude / DISPLACEMENT_PX_PER_FLOW
-                safe = np.maximum(length, 1e-12)
-                moving = length > 0.0
-                flow[rows, cols, 0] = np.where(moving, dx / safe * flow_mag, 0.0)
-                flow[rows, cols, 1] = np.where(moving, dy / safe * flow_mag, 0.0)
-            else:
-                scale = float(self.policy.flow_arrow_scale)
-                if scale <= 0.0:
-                    scale = 1.0
-                divisor = (
-                    DISPLACEMENT_PX_PER_FLOW
-                    * float(self.policy.flow_length_margin)
-                    / scale
-                )
-                flow[rows, cols, 0] = dx / divisor
-                flow[rows, cols, 1] = dy / divisor
+            scale = float(self.policy.flow_arrow_scale)
+            if scale <= 0.0:
+                scale = 1.0
+            divisor = (
+                DISPLACEMENT_PX_PER_FLOW
+                * float(self.policy.flow_length_margin)
+                / scale
+            )
+            flow[rows, cols, 0] = dx / divisor
+            flow[rows, cols, 1] = dy / divisor
         return flow
 
 
diff --git a/apps/studio/tests/test_bare_video_analysis.py b/apps/studio/tests/test_bare_video_analysis.py
index 084274c..ae1b1bd 100644
--- a/apps/studio/tests/test_bare_video_analysis.py
+++ b/apps/studio/tests/test_bare_video_analysis.py
@@ -17,8 +17,8 @@ Where a failure would be invisible:
   reference frame are read back from the dialog's own widgets;
 * a recomputed force trace and a recorded one are different claims, so the charts' source
   kind and the provenance's ``sources``/``synthetic`` are asserted, not just the pixels;
-* a box result is recorded on the live painter's grid, so the canvas is compared with
-  ``draw_arrows`` on the recovered field once its gaps are filled with zero.
+* a box result is recorded on the box sample grid, so the canvas is compared with
+  ``draw_arrow_display`` on the recovered field.
 
 The fixture clip is the project's recorded box footage under ``dist/``, copied into the test's
 own directory. If it is absent the test **fails** (``_require_clip``) rather than skipping: a
@@ -433,10 +433,10 @@ def test_a_bare_video_is_analysed_and_scrubbed_in_both_directions(
 def test_the_box_arrow_canvas_is_painted_by_the_live_painter(
     bare_clip, window, monkeypatch
 ):
-    """A box pass stores the live painter's grid, so the window draws ``draw_arrows``.
+    """A box pass stores the box sample grid; the panel redraws with that policy.
 
-    The recovered field is still NaN off the recorded cells. Filling those gaps with zero
-    is what the live painter reads as "no motion", and that picture is the panel.
+    The recovered field is NaN off the recorded cells. The shared selector only samples
+    finite cells, so the panel matches ``draw_arrow_display`` on that sparse field.
     """
     open_bare_video(window, bare_clip, monkeypatch)
     settle(window)
@@ -451,7 +451,6 @@ def test_the_box_arrow_canvas_is_painted_by_the_live_painter(
         "draw_style": window._draw_style(),
         "colormap": flow_colormap(window._theme),
         "policy": policy,
-        "shared_painter": False,
     }
 
     set_mode(window, ARROW_PURE)
@@ -460,15 +459,14 @@ def test_the_box_arrow_canvas_is_painted_by_the_live_painter(
     QApplication.processEvents()
     shown = panel_bgr(window)
     assert painted(shown, canvas) > 0, "the operator's arrow canvas is blank"
-    filled = np.nan_to_num(np.asarray(first.flow, dtype=np.float64), nan=0.0)
     expected = draw_arrow_display(
         first.base_bgr,
-        filled,
+        first.flow,
         valid_mask=first.valid_mask,
         **common,
     )
     assert np.array_equal(shown, np.asarray(expected)), (
-        "the panel's canvas is not the live painter's output for this frame"
+        "the panel's canvas is not the shared painter's output for this frame"
     )
 
 
diff --git a/apps/studio/tests/test_replay_analysis.py b/apps/studio/tests/test_replay_analysis.py
index b6fc594..cfb8ed8 100644
--- a/apps/studio/tests/test_replay_analysis.py
+++ b/apps/studio/tests/test_replay_analysis.py
@@ -738,11 +738,11 @@ def test_the_recovered_field_reselects_to_the_stored_samples(session):
 
 
 def test_box_live_and_replay_arrow_canvases_do_not_diverge(box_pass, box_session, monkeypatch):
-    """Live ``draw_arrows`` on the dense field equals the replay canvas, frame for frame.
+    """Live dense field equals the replay canvas, frame for frame.
 
-    This is the discrepancy the box arrow work exists to close: the operator's live view
-    paints the chain's dense flow, and the replay paints a field recovered from the
-    stored samples. Both go through :func:`render_image` with ``draw_style="box"``.
+    The operator's live view paints the chain's dense flow, and the replay paints a
+    field recovered from the stored samples. Both go through :func:`render_image` with
+    ``draw_style="box"`` and the shared painter.
     """
     service, outcome = box_pass
     assert outcome.complete, outcome.reason
diff --git a/shared/src/orisys_app_shared/acquisition/arrows.py b/shared/src/orisys_app_shared/acquisition/arrows.py
index 1b03063..152e033 100644
--- a/shared/src/orisys_app_shared/acquisition/arrows.py
+++ b/shared/src/orisys_app_shared/acquisition/arrows.py
@@ -78,16 +78,16 @@ _ARROW_DRAW_KW = dict(
     below_threshold_color=hex_to_bgr(FLOW_IDLE_HEX),
 )
 
-#: ``draw_arrows`` reads every point of this grid, including those under its threshold
-#: (they become the idle dots). The cap is only a guard: a stitch image does not have
-#: this many points, and a lower cap would drop cells the live painter still draws.
+#: Box/Mini live and analysis walk every point of this grid, including those under
+#: the threshold (they become the idle dots). The cap is only a guard: a stitch image
+#: does not have this many points, and a lower cap would drop cells the painter still draws.
 SDK_BOX_MAX_ARROWS = 100_000
 
 
 def sdk_box_policy():
-    """The sample grid :func:`orisys.util.draw_arrows` walks for a box frame.
+    """The sample grid a box frame is selected and drawn with.
 
-    Live box arrows are not the shared selector's default (spacing 10, 400 strongest).
+    Box arrows are not the finger selector's default (spacing 10, 400 strongest).
     A stored result has to record *this* grid or the replay cannot hand the same cells
     back to the same painter.
     """
@@ -98,8 +98,7 @@ def sdk_box_policy():
         flow_off_px=float(_ARROW_DRAW_KW["threshold"]) * 0.75,
         flow_grid_spacing=int(_ARROW_DRAW_KW["grid_spacing"]),
         flow_arrow_scale=float(_ARROW_DRAW_KW["arrow_scale"]),
-        # ``draw_arrows`` still folds the border cells into its colour range, then
-        # refuses to draw them. Recording them (edge 0) is what keeps that range.
+        # Edge 0 keeps border cells in the colour range the box pass has always used.
         flow_edge_margin=0,
         flow_length_margin=0.8,
         max_arrows=SDK_BOX_MAX_ARROWS,
@@ -107,7 +106,7 @@ def sdk_box_policy():
 
 
 def uses_sdk_box_grid(policy) -> bool:
-    """True when ``policy`` is the box painter's grid, not the shared selector's."""
+    """True when ``policy`` is the box sample grid, not the finger selector's."""
     if policy is None:
         return False
     spacing = getattr(policy, "flow_grid_spacing", None)
@@ -223,8 +222,8 @@ def flow_colormap(theme: str = "dark") -> np.ndarray:
 def arrow_display_scale(draw_style: str = "box") -> float:
     """How much the drawn arrows amplify the measured displacement.
 
-    ``orisys.util.draw_arrows`` divides the displacement by ``arrow_scale``, so
-    the effective display magnification is its reciprocal (box ``0.5`` -> ``2x``,
+    The shared selector divides displacement by ``flow_arrow_scale``, so the
+    effective display magnification is its reciprocal (box ``0.5`` -> ``2x``,
     finger ``1.0`` -> ``1x``). The drawn length is additionally capped at
     ``grid_spacing * 0.8`` pixels, so the returned factor describes the mapping
     only while an arrow has not hit that cap — the UI says so in its tooltip.
@@ -345,9 +344,7 @@ def _policy_from_style(
 
 
 #: The pixels ``cv2.circle(..., radius=1, thickness=-1)`` fills, as ``(dy, dx)`` offsets
-#: from the centre it is given. One marker shape for every sensor type: the SDK's
-#: ``orisys.util.draw_arrows`` paints a below-threshold sample with exactly that call, so
-#: a dot on the finger canvas has to be the same plus-shaped 5-pixel blob a box dot is.
+#: from the centre it is given. One plus-shaped 5-pixel blob for every sensor type.
 #: Painting the offsets in one vectorised write, instead of one ``cv2.circle`` per dot,
 #: is what keeps a full finger grid cheap: on a 276-candidate island the pass costs 47 us
 #: against 223 us for the call loop, and the whole draw gains only ~35 us.
@@ -482,7 +479,6 @@ def draw_arrow_display(
     magnitude_range: tuple[float, float] | None = None,
     per_frame_colour: bool = False,
     idle_dots: bool | None = None,
-    shared_painter: bool = False,
 ) -> np.ndarray:
     """Draw flow arrows on a solid canvas or over the processed frame.
 
@@ -498,9 +494,8 @@ def draw_arrow_display(
 
     ``policy`` is the **shared** :class:`~orisys_app_shared.scene3d.visual_policy.VisualPolicy`
     the 3D surface also draws its overlay from (``visual_policy.policy_for_settings``).
-    Passing it is what makes the two figures trigger at the same strength; the
-    finger-style defaults below are only the fallback for a caller that has no pane
-    (a tool, a test). Box style keeps the SDK path either way.
+    Passing it is what makes the two figures trigger at the same strength. Without it,
+    finger style falls back to the finger defaults and box style to :func:`sdk_box_policy`.
 
     ``magnitude_range`` fixes the colour scale, in the same unit as ``flow_on_px``
     (uv pixels of displacement); the default is :data:`FLOW_MAGNITUDE_RANGE` and a
@@ -508,11 +503,8 @@ def draw_arrow_display(
     back in to normalising every frame against its own maximum, which makes one
     magnitude a different colour from one frame to the next.
 
-    ``idle_dots`` shows or hides the below-threshold markers of the **shared finger
-    canvas**; ``None`` means :data:`DRAW_IDLE_GRID_DOTS`, which is on. Box/Mini draw
-    through the SDK's own ``orisys.util.draw_arrows``, which paints its own below-threshold
-    dots (same tint, same radius-1 marker) and has no such switch: this one governs the
-    canvas that draws here, and Box keeps the appearance it has.
+    ``idle_dots`` shows or hides the below-threshold markers; ``None`` means
+    :data:`DRAW_IDLE_GRID_DOTS`, which is on.
 
     Returns the drawn image, which carries ``frame_diagnostic``: ``None`` when the frame
     was drawn, and otherwise a :class:`~orisys_app_shared.scene3d.contracts.Diagnostic`
@@ -521,22 +513,12 @@ def draw_arrow_display(
     that shows it without reading the attribute reports "nothing measured" for "nothing
     could be drawn". A ``flow`` of ``None`` is a legitimate empty frame and leaves the
     attribute ``None``.
-
-    ``shared_painter`` forces this module's per-sample painter for a field that came from
-    somewhere other than a live chain — a *recovered* field, finite only at the cells a
-    stored analysis result recorded. The SDK painter walks the dense grid, which on such a
-    field is almost entirely ``NaN``; measured on one stored box frame it paints **0 px**
-    where the shared painter paints **1,486 px** (the frame's own candidate cells). It is a
-    statement about the input, not about the sensor: the box *style* still decides the
-    magnification and the idle-dot tint, and a live box frame keeps the SDK path.
     """
     base = _ensure_bgr(processed_frame)
     if flow is None:
         return _flow_result(base)
-    finger = str(draw_style).lower().startswith("f") or bool(shared_painter)
+    finger = str(draw_style).lower().startswith("f")
     draw_kw = dict(_FINGER_ARROW_DRAW_KW if finger else _ARROW_DRAW_KW)
-    if colormap is not None:
-        draw_kw["colormap"] = colormap
     try:
         if background == "pure_color":
             color_bgr = ARROW_BG_BGR if canvas_bgr is None else tuple(int(v) for v in canvas_bgr)
@@ -568,39 +550,33 @@ def draw_arrow_display(
         else:
             canvas = base
 
-        if finger:
-            selected = (
-                policy
-                if policy is not None
-                else _policy_from_style(
-                    draw_style,
-                    flow_on_px=flow_on_px,
-                    flow_off_px=flow_off_px,
-                    flow_grid_spacing=flow_grid_spacing,
-                    max_arrows=max_arrows,
-                )
+        if policy is not None:
+            selected = policy
+        elif finger:
+            selected = _policy_from_style(
+                draw_style,
+                flow_on_px=flow_on_px,
+                flow_off_px=flow_off_px,
+                flow_grid_spacing=flow_grid_spacing,
+                max_arrows=max_arrows,
             )
-            return _flow_result(
-                _paint_shared_flow_arrows(
-                    canvas,
-                    np.asarray(flow),
-                    valid_mask=valid_mask,
-                    policy=selected,
-                    colormap=colormap,
-                    below_threshold_color=tuple(draw_kw["below_threshold_color"]),
-                    magnitude_range=magnitude_range,
-                    per_frame_colour=per_frame_colour,
-                    # Read here rather than as a parameter default: the module constant is
-                    # the display decision, and a caller that flips it (a build, a test)
-                    # must be obeyed without every call site having to pass it.
-                    idle_dots=DRAW_IDLE_GRID_DOTS if idle_dots is None else bool(idle_dots),
-                )
-            )
-
-        import orisys
-
+        else:
+            selected = sdk_box_policy()
         return _flow_result(
-            orisys.util.draw_arrows(canvas, flow, valid_mask=valid_mask, **draw_kw)
+            _paint_shared_flow_arrows(
+                canvas,
+                np.asarray(flow),
+                valid_mask=valid_mask,
+                policy=selected,
+                colormap=colormap,
+                below_threshold_color=tuple(draw_kw["below_threshold_color"]),
+                magnitude_range=magnitude_range,
+                per_frame_colour=per_frame_colour,
+                # Read here rather than as a parameter default: the module constant is
+                # the display decision, and a caller that flips it (a build, a test)
+                # must be obeyed without every call site having to pass it.
+                idle_dots=DRAW_IDLE_GRID_DOTS if idle_dots is None else bool(idle_dots),
+            )
         )
     except Exception as exc:
         # The frame is *not* idle: it failed to draw. The image alone cannot say so, so
diff --git a/shared/src/orisys_app_shared/acquisition/image_backend.py b/shared/src/orisys_app_shared/acquisition/image_backend.py
index 04051db..14ad1ec 100644
--- a/shared/src/orisys_app_shared/acquisition/image_backend.py
+++ b/shared/src/orisys_app_shared/acquisition/image_backend.py
@@ -33,7 +33,6 @@ from orisys_app_shared.acquisition.arrows import (
     depth_map_to_bgr,
     draw_arrow_display,
     flow_colormap,
-    uses_sdk_box_grid,
 )
 from orisys_app_shared.acquisition.display_orientation import (
     ORIENTATION_IDENTITY,
@@ -396,26 +395,14 @@ def _draw_flow(
     domain_mask: np.ndarray | None,
     arrow_policy,
     canvas_bgr: Sequence[int] | None = None,
-    sdk_result: bool | None = None,
 ) -> np.ndarray:
     """Arrows on ``base``, using the same painter the pure-arrow views use."""
-    if sdk_result is None:
-        box_style = not str(draw_style).lower().startswith("f")
-        sdk_result = (
-            box_style
-            and str(frame.source or "") in RESULT_SOURCES
-            and uses_sdk_box_grid(arrow_policy)
-        )
-    flow = frame.flow
-    if sdk_result and flow is not None:
-        flow = np.nan_to_num(np.asarray(flow, dtype=np.float64), nan=0.0)
     return draw_arrow_display(
         base,
-        flow,
+        frame.flow,
         background=background,
         canvas_bgr=canvas_bgr,
         draw_style=draw_style,
-        shared_painter=str(frame.source or "") in RESULT_SOURCES and not sdk_result,
         valid_mask=frame.valid_mask,
         domain_mask=domain_mask,
         colormap=flow_colormap(theme),
diff --git a/shared/src/orisys_app_shared/analysis/service.py b/shared/src/orisys_app_shared/analysis/service.py
index 98bdadd..d18b34e 100644
--- a/shared/src/orisys_app_shared/analysis/service.py
+++ b/shared/src/orisys_app_shared/analysis/service.py
@@ -534,9 +534,8 @@ class AnalysisService:
         )
         #: The policy the samples are selected with. Recorded, never keyed (see
         #: ``model.py``): the shipped defaults make a stored result independent of the
-        #: operator's display preferences. A box chain records the live painter's own
-        #: grid; the shared defaults are the finger painter's and would drop every cell
-        #: ``draw_arrows`` still draws.
+        #: operator's display preferences. A box chain records the box sample grid;
+        #: the finger defaults would drop cells that grid still draws.
         if policy is None and chain.sensor_type == "box":
             from orisys_app_shared.acquisition.arrows import sdk_box_policy
 
diff --git a/shared/tests/test_image_backend.py b/shared/tests/test_image_backend.py
index 713f85a..49c40df 100644
--- a/shared/tests/test_image_backend.py
+++ b/shared/tests/test_image_backend.py
@@ -582,28 +582,25 @@ def test_the_idle_dot_switch_hides_the_dots_and_leaves_the_arrows_alone():
 
 
 def test_the_idle_marker_is_the_marker_a_box_sample_gets():
-    """One marker shape and one tint across sensor types.
+    """One marker shape and one tint across sensor styles.
 
-    Box and Mini draw through the SDK's ``orisys.util.draw_arrows``, which paints a
-    below-threshold sample with ``cv2.circle(radius=1, thickness=-1)`` in the same
-    ``below_threshold_color`` this module passes it. The finger canvas has to put the same
-    blob on the same kind of location, or a quiet sample would look like two different
-    things depending on the sensor; this compares the two paths' pixels for one
-    below-threshold sample.
+    Box and finger both use the shared per-sample painter. With the same policy they
+    must put the same plus-shaped idle blob on the same quiet sample.
     """
-    pytest.importorskip("orisys", reason="the SDK path needs the SDK")
     from orisys_app_shared.acquisition.arrows import FLOW_IDLE_HEX, hex_to_bgr
     from orisys_app_shared.scene3d.visual_policy import VisualPolicy
 
-    # One candidate location, shared by both paths: the box draw's own grid is 20 px with
-    # a 15 px edge margin, so a 60x60 frame has exactly the (30, 30) sample.
+    # One candidate location on a 20 px grid with a 15 px edge: a 60x60 frame has
+    # exactly the (30, 30) sample inside the margin.
     size = 60
     flow = np.zeros((size, size, 2), dtype=np.float64)
-    flow[30, 30] = (0.1, 0.0)  # 0.2 px: under both paths' thresholds
+    flow[30, 30] = (0.1, 0.0)  # 0.2 px: under the threshold
     canvas = _flat_canvas(size, size)
     policy = VisualPolicy(flow_on_px=0.4, flow_grid_spacing=20, flow_edge_margin=15)
 
-    box = draw_arrow_display(canvas, flow, background="captured", draw_style="box")
+    box = draw_arrow_display(
+        canvas, flow, background="captured", draw_style="box", policy=policy
+    )
     finger = draw_arrow_display(
         canvas, flow, background="captured", draw_style="finger", policy=policy
     )
@@ -970,19 +967,17 @@ def _painted(image, canvas=(0x16, 0x13, 0x12)) -> int:
 
 
 def test_a_recovered_field_is_drawn_by_the_shared_painter_whatever_the_style_is():
-    """Where a frame came from decides the painter, not which sensor's style is selected.
+    """Live and analysis sources use the same painter on a sparse recovered field.
 
-    The SDK painter walks the dense sampling grid, so on a field that is finite only at a
-    stored result's candidate cells it finds no sample at all and paints nothing: measured
-    on a real box recording, **0 px** against the shared painter's **1,486 px** on the same
-    frame. So the routing is by ``ImageFrame.source`` — the frame came from a result — and
-    the box *style* still owns the magnification and the idle-dot tint.
+    The field is finite only at the stored candidate cells. The shared selector walks
+    those cells, so both sources paint the same picture under the same policy.
     """
     from orisys_app_shared.scene3d.visual_policy import select_flow_samples
 
     sparse, samples = _recovered_flow()
     base = np.zeros((60, 80, 3), dtype=np.uint8)
     base[:, :] = (0x16, 0x13, 0x12)
+    policy = _policy()
 
     result_frame = backend.ImageFrame(
         base, flow=sparse, source=backend.SOURCE_ANALYSIS, flow_valid=True
@@ -991,7 +986,7 @@ def test_a_recovered_field_is_drawn_by_the_shared_painter_whatever_the_style_is(
         base, flow=sparse, source=backend.SOURCE_LIVE, flow_valid=True
     )
     drawn = backend.render_image(
-        result_frame, mode=backend.ARROW_PURE, draw_style="box", arrow_policy=_policy()
+        result_frame, mode=backend.ARROW_PURE, draw_style="box", arrow_policy=policy
     )
     expected = draw_arrow_display(
         base,
@@ -1001,30 +996,27 @@ def test_a_recovered_field_is_drawn_by_the_shared_painter_whatever_the_style_is(
         # The backend hands the theme's ramp to the painter, so the reference call has to
         # as well; the default is ``cv2.COLORMAP_JET``, which is a different picture.
         colormap=flow_colormap("dark"),
-        policy=_policy(),
-        shared_painter=True,
+        policy=policy,
     )
     assert np.array_equal(np.asarray(drawn), np.asarray(expected)), (
         "a result's canvas is not the shared painter's output"
     )
     assert _painted(drawn) > 0, "the recovered field painted nothing"
 
-    # The same array handed over as a *live* field keeps the SDK path — which is what the
-    # sensor family's draw style has always meant, and which paints nothing here.
-    sdk = backend.render_image(
-        live_frame, mode=backend.ARROW_PURE, draw_style="box", arrow_policy=_policy()
+    live = backend.render_image(
+        live_frame, mode=backend.ARROW_PURE, draw_style="box", arrow_policy=policy
+    )
+    assert np.array_equal(np.asarray(drawn), np.asarray(live)), (
+        "live and analysis disagree on the same sparse field"
     )
-    assert _painted(sdk) == 0, "the SDK painter drew a recovered field"
-    assert not np.array_equal(np.asarray(drawn), np.asarray(sdk))
 
     # One marker per recorded cell at most: the recovered field carries no other value, so
     # nothing outside those cells can become a sample.
-    assert select_flow_samples(sparse, None, _policy()).rows.size == samples.rows.size
+    assert select_flow_samples(sparse, None, policy).rows.size == samples.rows.size
 
 
 def test_a_box_result_on_the_sdk_grid_matches_the_live_painter():
-    """A box pass stores the live painter's grid, so filling the gaps and calling it
-    back draws the same picture as the dense field."""
+    """A box pass stores the box grid; the idempotent inverse redraws the dense field."""
     from orisys_app_shared.acquisition.arrows import sdk_box_policy
     from orisys_app_shared.scene3d.visual_policy import (
         DISPLACEMENT_PX_PER_FLOW,
@@ -1039,13 +1031,13 @@ def test_a_box_result_on_the_sdk_grid_matches_the_live_painter():
     assert samples.rows.size > 0
     dx = np.asarray(samples.dx, np.float64)
     dy = np.asarray(samples.dy, np.float64)
-    length = np.sqrt(dx * dx + dy * dy)
-    flow_mag = np.asarray(samples.magnitude, np.float64) / DISPLACEMENT_PX_PER_FLOW
-    safe = np.maximum(length, 1e-12)
-    moving = length > 0.0
+    scale = float(policy.flow_arrow_scale) or 1.0
+    divisor = (
+        DISPLACEMENT_PX_PER_FLOW * float(policy.flow_length_margin) / scale
+    )
     sparse = np.full(dense.shape, np.nan, dtype=np.float64)
-    sparse[samples.rows, samples.cols, 0] = np.where(moving, dx / safe * flow_mag, 0.0)
-    sparse[samples.rows, samples.cols, 1] = np.where(moving, dy / safe * flow_mag, 0.0)
+    sparse[samples.rows, samples.cols, 0] = dx / divisor
+    sparse[samples.rows, samples.cols, 1] = dy / divisor
     base = np.zeros(dense.shape[:2] + (3,), np.uint8)
     base[:] = (0x16, 0x13, 0x12)
     cmap = flow_colormap("dark")
@@ -1056,6 +1048,7 @@ def test_a_box_result_on_the_sdk_grid_matches_the_live_painter():
         background="pure_color",
         canvas_bgr=(0x16, 0x13, 0x12),
         colormap=cmap,
+        policy=policy,
     )
     frame = backend.ImageFrame(
         base,