Despill across full foreground to cut edge green spill

Green spill bleeds into pixels ViTMatte marks as solid foreground (alpha~1),
but despill was gated to the soft edge band (0.005<alpha<0.995) and weakened
exactly there: the alpha tent peaked at 0.5 and bg_confidence (low on the
character edge) further suppressed the correction.

- (1) Apply despill to the whole foreground (alpha > edge_low) instead of the
  soft band only; neutral pixels stay untouched via the green-excess term.
- (2) Weight by alpha at full strength across the opaque range (alpha>=0.5),
  easing off only where mostly transparent, instead of a tent peaking at 0.5.
- (3) Drop bg_confidence as a despill gate; localise purely by green excess.

Controlled comparison (same alpha, only despill changed), edge_green_excess:
  ViTMatte TestImage  p95 0.239->0.176, mean 0.131->0.061
  ViTMatte TestImage2 p95 0.231->0.165, mean 0.115->0.052

Removes now-unused DespillSettings fields edge_high and bg_confidence_weight.

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

bgfilter/despill.py

@@ -9,7 +9,6 @@ from .settings import DespillSettings
 def despill_green(
     rgb: np.ndarray,
     alpha: np.ndarray,
-    bg_confidence: np.ndarray,
     model: BackgroundModel,
     settings: DespillSettings,
 ) -> tuple[np.ndarray, np.ndarray]:
@@ -17,7 +16,11 @@ def despill_green(
     if not settings.enabled:
         return rgb.copy(), np.zeros(alpha.shape, dtype=np.float32)
 
-    edge = (alpha > settings.edge_low) & (alpha < settings.edge_high)
+    # (1) Despill across the whole foreground, not only the soft edge band.
+    # Green frequently bleeds into pixels ViTMatte marks as solid foreground
+    # (alpha ~ 1); the old upper bound left those uncorrected. Neutral pixels
+    # stay untouched because the green-excess term below is zero for them.
+    foreground = alpha > settings.edge_low
     if settings.edge_expand_radius > 0:
         from .deps import require_cv2
 
@@ -26,23 +29,26 @@ def despill_green(
         kernel = cv2.getStructuringElement(
             cv2.MORPH_ELLIPSE, (radius * 2 + 1, radius * 2 + 1)
         )
-        edge = cv2.dilate(edge.astype(np.uint8), kernel).astype(bool)
+        foreground = cv2.dilate(foreground.astype(np.uint8), kernel).astype(bool)
 
     r = rgb_f[..., 0]
     g = rgb_f[..., 1]
     b = rgb_f[..., 2]
     neutral_green = np.maximum(r, b) + settings.green_excess_margin
     excess = np.maximum(g - neutral_green, 0.0)
+
+    # (2) Keep full strength across the whole opaque range (alpha >= 0.5) and
+    # ease off only where the pixel is mostly transparent. The previous tent
+    # peaked at alpha 0.5 and starved exactly the solid hair that needs despill.
     alpha_weight = np.maximum(
-        np.clip((1.0 - np.abs(alpha - 0.5) * 2.0), 0.0, 1.0),
-        settings.alpha_weight_floor,
-    )
-    bg_weight = (
-        settings.bg_confidence_weight * np.clip(bg_confidence, 0.0, 1.0)
-        + (1.0 - settings.bg_confidence_weight)
+        np.clip(alpha * 2.0, 0.0, 1.0), settings.alpha_weight_floor
     )
+
+    # (3) Localise purely by green excess. bg_confidence is intentionally not a
+    # gate: it is low on the character edge where the spill lives, so using it
+    # suppressed the very correction we want.
     green_weight = np.clip(excess / max(settings.green_excess_margin * 8.0, 1e-6), 0.0, 1.0)
-    mask = edge.astype(np.float32) * alpha_weight * bg_weight * green_weight
+    mask = foreground.astype(np.float32) * alpha_weight * green_weight
 
     out = rgb_f.copy()
     target_green = np.minimum(g, neutral_green)

bgfilter/pipeline.py

@@ -76,7 +76,7 @@ def _run_image(
         rgb, alpha, bg_confidence, model, settings.foreground
     )
     corrected_rgb, despill_mask = despill_green(
-        foreground.rgb, alpha, bg_confidence, model, settings.despill
+        foreground.rgb, alpha, model, settings.despill
     )
     save_rgba(output_path, corrected_rgb, alpha)
 

bgfilter/settings.py

@@ -54,11 +54,9 @@ class ForegroundSettings:
 class DespillSettings:
     enabled: bool = True
     edge_low: float = 0.005
-    edge_high: float = 0.995
     strength: float = 0.92
     green_excess_margin: float = 0.015
     edge_expand_radius: int = 2
-    bg_confidence_weight: float = 0.65
     alpha_weight_floor: float = 0.35
 
 

configs/default.yaml

@@ -26,9 +26,7 @@ foreground:
 despill:
   enabled: true
   edge_low: 0.005
-  edge_high: 0.995
   strength: 0.92
   green_excess_margin: 0.015
   edge_expand_radius: 2
-  bg_confidence_weight: 0.65
   alpha_weight_floor: 0.35