Default to BiRefNet + CPU; add --seg-backend to pick the segmenter

- Segmentation backend now defaults to birefnet (ZhengPeng7/BiRefNet); anime-seg is
  selected at runtime with --seg-backend anime-seg, which also swaps in the matching
  weights (unknown backends raise). Config files stay for detailed tuning, not
  backend selection.
- Default device is now cpu everywhere (dataclass defaults, default.yaml,
  birefnet.yaml); pass --device cuda for GPU. Avoids a hard CUDA-required failure on
  machines without a CUDA-enabled PyTorch build.
- Docs: README + workflow status note updated (backend via --seg-backend, CPU
  default, examples no longer force --device cuda).

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

bgfilter/cli.py

@@ -30,6 +30,7 @@ def main(
     fg_safe_radius_ratio: float | None = typer.Option(None, "--fg-safe-radius-ratio", min=0.0),
     despill: bool | None = typer.Option(None, "--despill/--no-despill"),
     trimap_mode: str | None = typer.Option(None, "--trimap-mode", help="Trimap mode (segmentation pipeline): directional | seg | directional-hard-bg"),
+    seg_backend: str | None = typer.Option(None, "--seg-backend", help="Segmentation backend: birefnet (default) | anime-seg"),
 ) -> None:
     try:
         settings = override_settings(
@@ -45,6 +46,7 @@ def main(
             fg_safe_radius_ratio=fg_safe_radius_ratio,
             despill=despill,
             trimap_mode=trimap_mode,
+            seg_backend=seg_backend,
         )
 
         if input_dir is not None:

bgfilter/config.py

@@ -17,6 +17,9 @@ from .settings import (
 
 T = TypeVar("T")
 
+# Canonical weights per segmentation backend (selected at runtime via --seg-backend).
+_SEG_MODELS = {"birefnet": "ZhengPeng7/BiRefNet", "anime-seg": "skytnt/anime-seg"}
+
 
 def _load_yaml(path: str | Path) -> dict[str, Any]:
     try:
@@ -100,6 +103,14 @@ def override_settings(settings: PipelineSettings, **overrides: Any) -> PipelineS
     seg_updates: dict[str, Any] = {}
     if overrides.get("device") is not None:
         seg_updates["device"] = overrides["device"]
+    if overrides.get("seg_backend") is not None:
+        backend = overrides["seg_backend"]
+        if backend not in _SEG_MODELS:
+            raise ValueError(
+                f"Unknown --seg-backend '{backend}'. Use one of: {', '.join(_SEG_MODELS)}."
+            )
+        seg_updates["backend"] = backend
+        seg_updates["model_name"] = _SEG_MODELS[backend]
 
     screen_color = (
         overrides["screen_color"]

bgfilter/settings.py

@@ -93,7 +93,7 @@ class DespillSettings:
 @dataclass(frozen=True)
 class ModelSettings:
     model_name: str = "hustvl/vitmatte-small-composition-1k"
-    device: str = "cuda"
+    device: str = "cpu"
     matting_method: str = "vitmatte"
     fallback_to_chroma_alpha: bool = False
 
@@ -101,9 +101,9 @@ class ModelSettings:
 @dataclass(frozen=True)
 class SegmentationSettings:
     enabled: bool = True       # False = chroma-only pipeline (no seg model)
-    backend: str = "anime-seg"  # "anime-seg" or "birefnet"
-    model_name: str = "skytnt/anime-seg"
-    device: str = "cuda"
+    backend: str = "birefnet"  # "birefnet" (general) or "anime-seg" (anime characters)
+    model_name: str = "ZhengPeng7/BiRefNet"
+    device: str = "cpu"
     input_size: int = 1024