fix(exposure): scale the luma window's reach by each channel's own range

docs/PIPELINE.md

@@ -66,7 +66,7 @@ These steps run in order. Each stage passes its buffer to the next. A helper nam
     *   **Negative (Color Negative / B&W Negative)**: raw low signal (dense highlights) maps to Floor ($0.0$). Raw high signal (film base, shadows) maps to Ceiling ($1.0$). Range: 0.01% to 99.99%.
     *   **Transparency** takes none of this metering. See *Transparency transfer* in §3.
 
-    Bounds are sampled on **two independent axes** (`_sample_log_bounds`). A **luma** pass fixes the floor and ceiling mean (center plus span), and a **color** pass fixes each channel's deviation from it, so color balance is tunable without compressing the luminance range. Mono gives zero deviation at any clip.
+    Bounds are sampled on **two independent axes** (`_sample_log_bounds`). A **color** pass reads each channel's floor and ceiling reference, and a **luma** pass sets how far the window reaches past them, so color balance is tunable without compressing the luminance range. The reach is a fraction of each channel's own span $s_c$ between its references, averaged over the channels: $f_c = r_c^f + (\bar h_f + k)\,s_c$, with $\bar h_f$ the mean of $(l_c^f - r_c^f)/s_c$ and the ceiling alike. A film whose channels differ in density range, a faded negative for example, keeps a gray scale neutral. $k$ is the base's mean-minus-median channel offset over the mean span. Mono gives zero deviation at any clip.
     *   **Luma Range Clip** (`luma_range_clip`): the *luminance* percentile window. **Positive** values tighten it symmetrically. **Zero** uses outlier-resistant extremes: a block-median prefilter rejects dust and speculars, and a small base clip excludes tiny outlier populations. **Negative** values push the bounds *outward*, leaving lifted blacks and unclipped highlights as headroom.
     *   **Color Clip** (`color_range_clip`): the sampling depth for per-channel color deviation. A larger value samples deeper for a more outlier-resistant balance; a smaller value samples nearer the extremes. The neutral default is `base_color_clip` ($1.0$); the slider spans log-interpolated values either side.
 

negpy/features/exposure/normalization.py

@@ -919,8 +919,8 @@ def analyze_log_exposure_bounds(
         color deviation (white balance / orange-mask cast). A tighter (larger) clip
         gives a more robust channel balance; a gentler (smaller) clip samples nearer
         the extremes. Default neutral is base_color_clip.
-    The luminance centre+span comes from the luma sampling, the per-channel color
-    offsets from the color sampling, so the cast clip is tunable without compressing
+    The per-channel refs come from the color sampling, and the luma sampling sets how
+    far past them the window reaches, so the cast clip is tunable without compressing
     highlights. Identical channels (mono) give zero deviation at any clip.
     """
     img_log = to_log_density(image)
@@ -959,7 +959,7 @@ def analyze_log_exposure_bounds_from_log(
 
     floors, ceils = _sample_log_bounds(img_log, percentile_clip, base_luma, sorted_grid)
 
-    # Color pass: per-channel deviations recombined onto the luma mean centre and span. The
+    # Color pass: per-channel refs, widened to the luma window. The
     # ceils (thin end, base-anchored) come from per-channel percentiles at color_clip. The
     # floors (dense end, scene content) prefer the same-pixel chroma-gated band refs and fall
     # back to the percentile pass when the band holds no trustworthy neutrals, and always for
@@ -969,10 +969,22 @@ def analyze_log_exposure_bounds_from_log(
         sp = _same_pixel_color_floor_refs(img_log, floors, ceils, (c_ceils[0], c_ceils[1], c_ceils[2]), color_clip)
         if sp is not None:
             c_floors = [sp[0], sp[1], sp[2]]
-    mean_lf, mean_lc = sum(floors) / 3.0, sum(ceils) / 3.0
-    mean_cf, mean_cc = sorted(c_floors)[1], sorted(c_ceils)[1]
-    floors = [mean_lf + (c_floors[ch] - mean_cf) for ch in range(3)]
-    ceils = [mean_lc + (c_ceils[ch] - mean_cc) for ch in range(3)]
+    spans = [c_floors[ch] - c_ceils[ch] for ch in range(3)]
+    if min(abs(s) for s in spans) > 1e-6:
+        # The luma window's reach past the color refs, in units of each channel's own span. A
+        # reach in log units would tint a film whose channels differ in density range.
+        reach_f = sum((floors[ch] - c_floors[ch]) / spans[ch] for ch in range(3)) / 3.0
+        reach_c = sum((ceils[ch] - c_ceils[ch]) / spans[ch] for ch in range(3)) / 3.0
+        # The window sits off the luma mean by the base's mean-minus-median channel offset, the
+        # placement the print constants are tuned to.
+        shift = (sum(c_ceils) / 3.0 - sorted(c_ceils)[1]) / (sum(spans) / 3.0)
+        floors = [c_floors[ch] + (reach_f + shift) * spans[ch] for ch in range(3)]
+        ceils = [c_ceils[ch] + (reach_c + shift) * spans[ch] for ch in range(3)]
+    else:
+        mean_lf, mean_lc = sum(floors) / 3.0, sum(ceils) / 3.0
+        mean_cf, mean_cc = sorted(c_floors)[1], sorted(c_ceils)[1]
+        floors = [mean_lf + (c_floors[ch] - mean_cf) for ch in range(3)]
+        ceils = [mean_lc + (c_ceils[ch] - mean_cc) for ch in range(3)]
 
     return LogNegativeBounds(
         (floors[0], floors[1], floors[2]),

negpy/services/assets/thumbnail_fingerprint.py

@@ -12,7 +12,7 @@
 from negpy.domain.models import WorkspaceConfig
 
 # Bump in any change that alters rendered pixels for unchanged settings, a changed default included.
-THUMBNAIL_RENDER_VERSION = 3
+THUMBNAIL_RENDER_VERSION = 4
 
 # Marks a thumbnail made from the source preview, which runs none of the frame's settings.
 QUICK = "quick"

tests/test_same_pixel_bounds.py

@@ -30,17 +30,19 @@ def _ramp() -> np.ndarray:
     return log
 
 
-def _old_color_recombined(img_log: np.ndarray, color_clip: float = 1.0):
-    """The pre-change recombination (independent per-channel percentiles)."""
+def _percentile_recombined(img_log: np.ndarray, color_clip: float = 1.0):
+    """The recombination on independent per-channel percentiles at both ends."""
     from negpy.features.exposure.models import EXPOSURE_CONSTANTS
 
     floors, ceils = _sample_log_bounds(img_log, 0.0, float(EXPOSURE_CONSTANTS["base_luma_clip"]))
     c_floors, c_ceils = _sample_log_bounds(img_log, color_clip, 0.0)
-    mean_lf, mean_lc = sum(floors) / 3.0, sum(ceils) / 3.0
-    mean_cf, mean_cc = sorted(c_floors)[1], sorted(c_ceils)[1]
+    spans = [c_floors[ch] - c_ceils[ch] for ch in range(3)]
+    reach_f = sum((floors[ch] - c_floors[ch]) / spans[ch] for ch in range(3)) / 3.0
+    reach_c = sum((ceils[ch] - c_ceils[ch]) / spans[ch] for ch in range(3)) / 3.0
+    shift = (sum(c_ceils) / 3.0 - sorted(c_ceils)[1]) / (sum(spans) / 3.0)
     return (
-        [mean_lf + (c_floors[ch] - mean_cf) for ch in range(3)],
-        [mean_lc + (c_ceils[ch] - mean_cc) for ch in range(3)],
+        [c_floors[ch] + (reach_f + shift) * spans[ch] for ch in range(3)],
+        [c_ceils[ch] + (reach_c + shift) * spans[ch] for ch in range(3)],
     )
 
 
@@ -63,17 +65,17 @@ def test_colored_dense_content_no_longer_reads_as_cast():
     new_err = np.abs(_dev(bounds.floors) - truth).max()
     assert new_err < 0.02, f"same-pixel floors polluted (err={new_err:.3f})"
 
-    old_floors, _ = _old_color_recombined(log)
-    old_err = np.abs(_dev(old_floors) - truth).max()
-    assert old_err > 0.10, "fixture must actually break the percentile pass"
+    pct_floors, _ = _percentile_recombined(log)
+    pct_err = np.abs(_dev(pct_floors) - truth).max()
+    assert pct_err > 0.10, "fixture must actually break the percentile pass"
 
 
 def test_neutral_frame_matches_percentile_pass():
     # With neutral extremes both estimators must agree (no invented tint).
     log = _ramp()
     bounds = analyze_log_exposure_bounds_from_log(log, color_clip=1.0)
-    old_floors, _ = _old_color_recombined(log)
-    np.testing.assert_allclose(_dev(bounds.floors), _dev(old_floors), atol=5e-3)
+    pct_floors, _ = _percentile_recombined(log)
+    np.testing.assert_allclose(_dev(bounds.floors), _dev(pct_floors), atol=5e-3)
 
 
 def test_no_neutral_dense_end_falls_back_bit_exact():
@@ -87,16 +89,35 @@ def test_no_neutral_dense_end_falls_back_bit_exact():
     log[dense, 1::2, 2] -= 0.65
 
     bounds = analyze_log_exposure_bounds_from_log(log, color_clip=1.0)
-    old_floors, old_ceils = _old_color_recombined(log)
-    np.testing.assert_allclose(bounds.floors, old_floors, atol=1e-9)
-    np.testing.assert_allclose(bounds.ceils, old_ceils, atol=1e-9)
+    pct_floors, pct_ceils = _percentile_recombined(log)
+    np.testing.assert_allclose(bounds.floors, pct_floors, atol=1e-9)
+    np.testing.assert_allclose(bounds.ceils, pct_ceils, atol=1e-9)
 
 
 def test_thin_end_stays_percentile_based():
-    # Ceiling refs must be untouched by the same-pixel change (base is anchored).
+    # The ceiling color refs are the percentile ones (base is anchored); the dense refs reach
+    # the ceiling only through the span that scales its luma reach.
     log = _ramp()
     block = slice(0, int(0.08 * _W))
     log[:, block, 0] = _BASE[0] - 0.95
     bounds = analyze_log_exposure_bounds_from_log(log, color_clip=1.0)
-    _, old_ceils = _old_color_recombined(log)
-    np.testing.assert_allclose(bounds.ceils, old_ceils, atol=1e-9)
+    _, pct_ceils = _percentile_recombined(log)
+    np.testing.assert_allclose(bounds.ceils, pct_ceils, atol=5e-3)
+
+
+def test_unequal_channel_ranges_keep_a_gray_scale_neutral():
+    # One gray scale on a film whose channels differ in density range, with speculars past
+    # it: every step must normalize to the same value in all three channels.
+    E = np.linspace(0.0, 1.0, _H, dtype=np.float32)
+    gammas = (0.3, 0.65, 0.2)
+    log = np.empty((_H, _W, 3), np.float32)
+    for ch in range(3):
+        log[:, :, ch] = (_BASE[ch] - gammas[ch] * E)[:, None]
+        log[-1, :8, ch] = _BASE[ch] - gammas[ch] * 1.6
+
+    bounds = analyze_log_exposure_bounds_from_log(log, color_clip=1.0)
+    norm = np.stack(
+        [(log[: _H - 1, 0, ch] - bounds.floors[ch]) / (bounds.ceils[ch] - bounds.floors[ch]) for ch in range(3)],
+        axis=-1,
+    )
+    assert np.ptp(norm, axis=1).max() < 5e-3

tests/thumbnail_fingerprint_defaults.json

@@ -98,6 +98,8 @@
    "autocrop_offset": "0",
    "autocrop_ratio": "\"Free\"",
    "autocrop_rebate_trim": "1.0",
+   "autocrop_square": "false",
+   "autocrop_squared": "false",
    "converge_h": "0.0",
    "converge_v": "0.0",
    "crop_detect_key": "\"\"",
@@ -203,5 +205,5 @@
    "vanadium_strength": "0.0"
   }
  },
- "render_version": 3
+ "render_version": 4
 }