diff --git a/AGENTS.md b/AGENTS.md
index 92213e3..be6c5db 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -89,11 +89,13 @@ original names (zero behavior change). Three strategies:
 shift), `stitch_batch_livestitch_into` (nearest-integer + touched bbox),
 and `stitch_batch_nearest` (plain nearest-integer, edge-clamped).
 
-`canvas` and `counts` accumulate **in place**; callers normalize as
-`canvas / np.maximum(counts, 1)` — these functions never normalize. The
-Fourier-shift and livestitch paths flip each patch up-down before
-placement; `stitch_batch_nearest` does not. All edge handling clamps to
-the canvas (no wrap-around). Pure numpy + `scipy.fft`; depends only on
+The accumulators (`stitch_batch_*`) update `canvas`/`counts` **in place**
+and never normalize; `normalize_mosaic(canvas, counts, min_overlap=0.5)` is
+the companion that averages them into a display mosaic (`canvas / counts`,
+under-covered pixels → `NaN`, plus a median `fill_value` for `NaN`-blind
+renderers). The Fourier-shift and livestitch paths flip each patch up-down
+before placement; `stitch_batch_nearest` does not. All edge handling clamps
+to the canvas (no wrap-around). Pure numpy + `scipy.fft`; depends only on
 `fourier_shift` from `preprocess.py`.
 
 **Known gotcha — the three strategies are not pixel-interchangeable**
diff --git a/README.md b/README.md
index 06047ae..243ec47 100644
--- a/README.md
+++ b/README.md
@@ -158,11 +158,11 @@ Change the spatial extent of frames. Three variants by use case.
 ## Stitching (`ptychoml.stitch`)
 
 Patch-placement helpers that accumulate a batch of reconstructed ViT
-patches into a running `(canvas, counts)` mosaic. Both arrays accumulate
-in place; the displayed/written mosaic is `canvas / np.maximum(counts, 1)`
-(no normalization happens inside these functions — the caller picks the
-min-overlap threshold). `positions_px` is `(N, 2)` in canvas pixel
-coordinates `(y, x)` pointing at patch centers.
+patches into a running `(canvas, counts)` mosaic. The accumulators don't
+normalize — `normalize_mosaic(canvas, counts)` turns the pair into the
+displayed/written mosaic (`canvas / counts`, under-covered pixels masked to
+`NaN`). `positions_px` is `(N, 2)` in canvas pixel coordinates `(y, x)`
+pointing at patch centers.
 
 | Function | Purpose |
 |---|---|
@@ -170,8 +170,9 @@ coordinates `(y, x)` pointing at patch centers.
 | `stitch_batch_into(canvas, counts, patches, positions_px, *, pad=1)` | Accumulate one batch into `(canvas, counts)` using the Fourier-shift path. Scatter-add is associative, so per-batch accumulation matches one-shot stitching (up to FFT noise). |
 | `stitch_batch_livestitch_into(canvas, counts, patches, positions_px)` | Nearest-integer accumulation that also returns the `(y0, y1, x0, x1)` bounding box touched this batch — lets a live writer repaint only the changed sub-rectangle. Returns `(0, 0, 0, 0)` when nothing overlapped. |
 | `stitch_batch_nearest(canvas, counts, patches, positions_px)` | Plain nearest-integer scatter-add; clamps at canvas edges (no wrap). Simplest variant, handy as a JIT/cache warm-up kernel. |
+| `normalize_mosaic(canvas, counts, min_overlap=0.5)` | Average a `(canvas, counts)` pair into a display mosaic: covered pixels (`counts >= min_overlap`) become `canvas / counts`, under-covered pixels become `NaN`. Returns `(fill_value, mosaic)` where `fill_value` is the median of covered pixels (a neutral background for renderers that treat `NaN` as zero). The normalization companion to the `stitch_batch_*` accumulators. |
 
-**The three strategies are not pixel-interchangeable.** The Fourier-shift
+**The three placement strategies are not pixel-interchangeable.** The Fourier-shift
 and livestitch paths flip each patch up-down before placement (matching
 the ptycho-vit convention) while `stitch_batch_nearest` does not, and the
 three use slightly different center-rounding conventions (so a patch
@@ -192,7 +193,7 @@ Allocate the mosaic once, then call the same function each batch
 
 ```python
 import numpy as np
-from ptychoml.stitch import stitch_batch_livestitch_into
+from ptychoml.stitch import stitch_batch_livestitch_into, normalize_mosaic
 
 H, W = 2048, 2048
 canvas = np.zeros((H, W), dtype=np.float32)  # running sum of patch values
@@ -203,12 +204,12 @@ for patches, positions_px in stream:         # patches (B, ph, pw); positions (B
     canvas, counts, (y0, y1, x0, x1) = stitch_batch_livestitch_into(
         canvas, counts, patches, positions_px,
     )
-    mosaic = canvas / np.maximum(counts, 1)   # normalize for display/write
-    repaint(mosaic[y0:y1, x0:x1])             # bbox = only the region that changed this batch
+    fill, mosaic = normalize_mosaic(canvas, counts)   # under-covered pixels -> NaN
+    repaint(np.where(np.isnan(mosaic), fill, mosaic)[y0:y1, x0:x1])  # bbox = changed region
 
 # --- Offline / batch: a single call with every patch ---
 canvas, counts, _ = stitch_batch_livestitch_into(canvas, counts, all_patches, all_positions)
-mosaic = canvas / np.maximum(counts, 1)
+fill, mosaic = normalize_mosaic(canvas, counts)
 ```
 
 For sub-pixel accuracy use `stitch_batch_into` (drop the bbox return);
diff --git a/ptychoml/__init__.py b/ptychoml/__init__.py
index 38dda66..778cd0c 100644
--- a/ptychoml/__init__.py
+++ b/ptychoml/__init__.py
@@ -28,6 +28,7 @@
     zero_pad_to_target,
 )
 from .stitch import (
+    normalize_mosaic,
     place_patches_fourier_shift,
     stitch_batch_into,
     stitch_batch_livestitch_into,
@@ -62,6 +63,7 @@
     "mask_hot_pixels",
     "mask_hot_pixels_by_count",
     "normalize_intensity",
+    "normalize_mosaic",
     "place_patches_fourier_shift",
     "preprocess_diffraction",
     "remap_positions",
diff --git a/ptychoml/stitch.py b/ptychoml/stitch.py
index cb2b4d0..85cf5b3 100644
--- a/ptychoml/stitch.py
+++ b/ptychoml/stitch.py
@@ -349,3 +349,36 @@ def stitch_batch_nearest(
             canvas[sy0:sy1, sx0:sx1] += patches[i, py0:py1, px0:px1]
             counts[sy0:sy1, sx0:sx1] += 1.0
     return canvas, counts
+
+
+def normalize_mosaic(canvas, counts, min_overlap=0.5):
+    """Average a stitched ``(canvas, counts)`` pair into a display mosaic.
+
+    ``canvas`` is the running sum of placed patch values and ``counts`` the
+    running occupancy, both produced by the ``stitch_batch_*`` functions (which
+    deliberately leave normalization to the caller). A pixel is "covered" when
+    ``counts >= min_overlap``; covered pixels become ``canvas / counts`` and
+    under-covered pixels become ``NaN``.
+
+    Returns ``(fill_value, mosaic)`` where:
+
+    * ``mosaic`` is ``float32`` with ``NaN`` in the under-covered regions.
+    * ``fill_value`` is the median of the covered pixels — a neutral background
+      for renderers that treat ``NaN`` as zero (paint ``NaN`` with
+      ``fill_value`` before display). ``0.0`` when nothing is covered.
+
+    With the default ``min_overlap=0.5`` and integer-valued ``counts``, a pixel
+    counts as covered once it has been written at least once (count ``>= 1``).
+    Raise ``min_overlap`` to require multiple overlapping patches.
+
+    Source: holoptycho/vit_inference.py ``MosaicWriterOp._normalise_full``.
+    """
+    canvas = np.asarray(canvas)
+    counts = np.asarray(counts)
+    valid = counts >= min_overlap
+    if valid.any():
+        avg = canvas / np.where(valid, counts, 1.0)
+        fill = float(np.median(avg[valid]))
+        mosaic = np.where(valid, avg, np.nan).astype(np.float32)
+        return fill, mosaic
+    return 0.0, np.full(canvas.shape, np.nan, dtype=np.float32)
diff --git a/tests/test_stitch.py b/tests/test_stitch.py
index 020783a..186b9a0 100644
--- a/tests/test_stitch.py
+++ b/tests/test_stitch.py
@@ -3,6 +3,7 @@
 import pytest
 
 from ptychoml.stitch import (
+    normalize_mosaic,
     place_patches_fourier_shift,
     stitch_batch_into,
     stitch_batch_livestitch_into,
@@ -228,3 +229,110 @@ def test_place_patches_fourier_shift_boundary_padding():
     assert out.shape == (20, 20)
     assert out[10:, 10:].sum() == 0.0  # nothing leaked to the far corner
     assert out.sum() > 0.0  # the in-bounds portion was placed
+
+
+# ----- normalize_mosaic -----------------------------------------------------
+
+def test_normalize_mosaic_averages_covered_pixels():
+    canvas = np.array([[6.0, 0.0], [9.0, 4.0]], dtype=np.float32)
+    counts = np.array([[3.0, 0.0], [3.0, 2.0]], dtype=np.float32)
+
+    fill, mosaic = normalize_mosaic(canvas, counts, min_overlap=0.5)
+
+    # covered pixels = canvas / counts
+    assert mosaic[0, 0] == pytest.approx(2.0)
+    assert mosaic[1, 0] == pytest.approx(3.0)
+    assert mosaic[1, 1] == pytest.approx(2.0)
+    # the count==0 pixel is under-covered -> NaN
+    assert np.isnan(mosaic[0, 1])
+
+
+def test_normalize_mosaic_masks_undercovered_to_nan():
+    canvas = np.array([[5.0, 5.0]], dtype=np.float32)
+    counts = np.array([[5.0, 0.0]], dtype=np.float32)
+
+    _, mosaic = normalize_mosaic(canvas, counts)
+
+    assert mosaic[0, 0] == pytest.approx(1.0)
+    assert np.isnan(mosaic[0, 1])
+
+
+def test_normalize_mosaic_fill_is_median_of_covered():
+    # covered averaged values are 1, 2, 3 -> median 2; the NaN pixel is excluded
+    canvas = np.array([[1.0, 2.0, 3.0, 7.0]], dtype=np.float32)
+    counts = np.array([[1.0, 1.0, 1.0, 0.0]], dtype=np.float32)
+
+    fill, _ = normalize_mosaic(canvas, counts)
+
+    assert fill == pytest.approx(2.0)
+
+
+def test_normalize_mosaic_threshold_is_inclusive():
+    # count exactly equal to the threshold counts as covered
+    canvas = np.array([[4.0]], dtype=np.float32)
+    counts = np.array([[2.0]], dtype=np.float32)
+
+    _, mosaic = normalize_mosaic(canvas, counts, min_overlap=2.0)
+
+    assert mosaic[0, 0] == pytest.approx(2.0)
+    assert not np.isnan(mosaic[0, 0])
+
+
+def test_normalize_mosaic_higher_threshold_excludes_thin_coverage():
+    canvas = np.array([[2.0, 6.0]], dtype=np.float32)
+    counts = np.array([[1.0, 3.0]], dtype=np.float32)
+
+    _, mosaic = normalize_mosaic(canvas, counts, min_overlap=2.0)
+
+    assert np.isnan(mosaic[0, 0])          # count 1 < 2 -> dropped
+    assert mosaic[0, 1] == pytest.approx(2.0)
+
+
+def test_normalize_mosaic_empty_returns_zero_and_all_nan():
+    canvas = np.zeros((3, 3), dtype=np.float32)
+    counts = np.zeros((3, 3), dtype=np.float32)
+
+    fill, mosaic = normalize_mosaic(canvas, counts)
+
+    assert fill == 0.0
+    assert mosaic.shape == (3, 3)
+    assert np.isnan(mosaic).all()
+
+
+def test_normalize_mosaic_returns_float32():
+    canvas = np.ones((2, 2), dtype=np.float64)
+    counts = np.ones((2, 2), dtype=np.float64)
+
+    _, mosaic = normalize_mosaic(canvas, counts)
+
+    assert mosaic.dtype == np.float32
+
+
+def test_normalize_mosaic_does_not_mutate_inputs():
+    canvas = np.array([[4.0, 0.0]], dtype=np.float32)
+    counts = np.array([[2.0, 0.0]], dtype=np.float32)
+    canvas_ref = canvas.copy()
+    counts_ref = counts.copy()
+
+    normalize_mosaic(canvas, counts)
+
+    np.testing.assert_array_equal(canvas, canvas_ref)
+    np.testing.assert_array_equal(counts, counts_ref)
+
+
+def test_normalize_mosaic_after_livestitch_recovers_patch_value():
+    # stitch two non-overlapping all-ones patches, then normalize: covered
+    # region should read back the patch value (1.0), rest NaN.
+    canvas = np.zeros((40, 40), dtype=np.float32)
+    counts = np.zeros((40, 40), dtype=np.float32)
+    patches = np.ones((2, 6, 6), dtype=np.float32)
+    positions = np.array([[10.0, 10.0], [10.0, 30.0]])
+
+    canvas, counts, _ = stitch_batch_livestitch_into(canvas, counts, patches, positions)
+    fill, mosaic = normalize_mosaic(canvas, counts)
+
+    covered = counts >= 0.5
+    assert covered.sum() == 2 * 6 * 6
+    np.testing.assert_allclose(mosaic[covered], 1.0, atol=1e-6)
+    assert np.isnan(mosaic[~covered]).all()
+    assert fill == pytest.approx(1.0)