Gate seg-reuse to the birefnet backend

reuse_as_seg swaps one BiRefNet-family model for another (the validated
equivalence); it must not silently override an explicitly chosen anime-seg
backend. With --seg-backend anime-seg the segmenter now keeps its own
forward and the cross-check veto runs independently on top -- both
combinable, no flag juggling. Default birefnet path is untouched
(verified byte-identical); anime-seg + cross-check verified to run both
models.

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

README.md

@@ -43,7 +43,10 @@ segmentation mask by default**, skipping the primary seg model entirely (one
 less model to load, ~20 s faster per image on CPU). Pilot-validated
 (TestImage3 / FixImage1): trimap 99.8% identical, no structural change to
 fingers, hair wisps or thin lines. Disable with `--no-cross-check-as-seg` to
-run the dedicated seg model instead.
+run the dedicated seg model instead. The reuse is a same-family swap, so it
+applies to the `birefnet` backend only: with `--seg-backend anime-seg` the
+anime segmenter keeps its own forward and the cross-check veto still runs
+independently on top.
 
 The segmentation trimap defaults to `directional` mode (chroma + seg + a hue-direction
 split: it keeps a background-coloured garment such as a white shirt while dropping a

bgfilter/pipeline.py

@@ -111,7 +111,16 @@ def _run_image(
     )
     second_alpha = None
     if settings.segmentation.enabled:
-        if settings.cross_check.enabled and settings.cross_check.reuse_as_seg:
+        reuse = (
+            settings.cross_check.enabled
+            and settings.cross_check.reuse_as_seg
+            # Reuse swaps one BiRefNet-family model for another (validated
+            # equivalent topology). A different backend chosen explicitly
+            # (anime-seg) keeps its own forward; cross-check still runs
+            # independently on top of it.
+            and settings.segmentation.backend == "birefnet"
+        )
+        if reuse:
             # One HR-matting forward serves both the trimap topology and the
             # cross-check second opinion; the primary seg model is never loaded.
             second_alpha = pipeline._second_opinion(rgb)

bgfilter/settings.py

@@ -78,7 +78,9 @@ class CrossCheckSettings:
     # inference serves both trimap topology and the second opinion, skipping the
     # primary seg model entirely (saves its load + forward). Pilot-validated on
     # TestImage3/FixImage1: trimap 99.8% identical, no structural change. Only
-    # takes effect when both cross_check and segmentation are enabled.
+    # applies to the birefnet segmentation backend (a same-family swap); with
+    # anime-seg the chosen segmenter keeps its own forward and the cross-check
+    # runs independently. Needs cross_check and segmentation both enabled.
     reuse_as_seg: bool = True
     # Compute precision of the cross-check forward: "fp32" or "bf16" (autocast;
     # needs bf16-capable hardware, else falls back to fp32 with a warning).

configs/default.yaml

@@ -72,6 +72,8 @@ cross_check:
   # drives both trimap topology and the veto, skipping the primary seg model
   # (saves its ~9s forward + ~0.9 GB load; CPU pipeline ~66s -> ~45s). Pilot:
   # trimap 99.8% identical, no structural change (fingers/wisps/bowstring OK).
+  # Only applies to the birefnet backend (same-family swap); anime-seg keeps
+  # its own forward, with the cross-check running independently on top.
   # Disable with --no-cross-check-as-seg. No effect when cross_check is off.
   reuse_as_seg: true
   # Cross-check forward precision, same semantics as segmentation.precision