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Add non-flat background mode (--no-chroma / chroma.enabled: false)

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No colour key at all: segmentation alone drives the trimap (mode forced
to 'seg', whose chroma terms degrade to no-ops on a zero background-
confidence map), ViTMatte still refines the unknown band at full
resolution, and pymatting still estimates edge foreground colour. Every
colour-keyed stage is bypassed: background auto-detect, the directional
hue split, chroma alpha suppression, despill, the cross-check veto (its
suspect zone is background-hued by definition; with reuse_as_seg the
second opinion still serves as the segmenter), and the unmix fallback.

Guards: requires segmentation.enabled and matting_method 'vitmatte'
(clear errors otherwise); despill/foreground handle model=None.

Validation: flat-background default path is bit-identical pre/post
(alpha and RGB |D|max = 0 on TestImage3). On a known-alpha subject
composited over a gradient+blotch background, recovered alpha scores
MAE 0.0018 / IoU@0.5 0.993 (0.995 with --cross-check-as-seg); the same
input correctly fails auto-detection in default mode.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
This commit is contained in:
孔令弘
2026-07-14 11:13:08 +08:00
parent f22af16517
commit e61fbe84f0
8 changed files with 78 additions and 18 deletions
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bgfilter/pipeline.py
+34 -13
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@@ -132,10 +132,26 @@ def run_image(
def _process_rgb(rgb: np.ndarray, pipeline: MattingPipeline) -> MattingResult:
settings = pipeline.settings
screen_color = parse_hex_color(settings.screen_color) if settings.screen_color else None
bg_confidence, model = compute_bg_confidence(
rgb, settings=settings.chroma, screen_color=screen_color
)
if settings.chroma.enabled:
screen_color = parse_hex_color(settings.screen_color) if settings.screen_color else None
bg_confidence, model = compute_bg_confidence(
rgb, settings=settings.chroma, screen_color=screen_color
)
else:
if not settings.segmentation.enabled:
raise RuntimeError(
"chroma.enabled: false (non-flat background mode) needs the segmentation "
"pipeline; enable segmentation or re-enable chroma."
)
if settings.model.matting_method == "chroma":
raise RuntimeError(
"matting_method 'chroma' needs the chroma key; use 'vitmatte' or re-enable chroma."
)
# Non-flat background: no colour key exists. A zero background-confidence
# map makes every chroma-fused formula degrade to its seg-only form; the
# colour-keyed stages (hue split, suppression, veto, despill) are skipped.
bg_confidence = np.zeros(rgb.shape[:2], dtype=np.float32)
model = None
second_alpha = None
if settings.segmentation.enabled:
reuse = (
@@ -154,7 +170,9 @@ def _process_rgb(rgb: np.ndarray, pipeline: MattingPipeline) -> MattingResult:
seg_mask = second_alpha
else:
seg_mask = pipeline._segment(rgb)
mode = settings.trimap.mode
# The directional modes are colour tests against the key colour; without
# one, the seg-topology trimap is the only meaningful choice.
mode = settings.trimap.mode if settings.chroma.enabled else "seg"
if mode == "seg":
trimap, trimap_stats = fuse_trimap(seg_mask, bg_confidence, settings.trimap)
elif mode in ("directional", "directional-hard-bg"):
@@ -173,14 +191,17 @@ def _process_rgb(rgb: np.ndarray, pipeline: MattingPipeline) -> MattingResult:
raw_alpha, alpha_source = pipeline._predict_alpha(rgb, trimap, bg_confidence)
alpha = enforce_trimap(raw_alpha, trimap)
alpha = suppress_alpha_by_chroma(
alpha, bg_confidence, trimap, settings.alpha_post,
# The matte-confidence gate only makes sense for a real matting prediction;
# a chroma-seeded alpha is itself colour evidence, so no gate there.
raw_alpha=raw_alpha if alpha_source == "vitmatte" else None,
)
if settings.chroma.enabled:
alpha = suppress_alpha_by_chroma(
alpha, bg_confidence, trimap, settings.alpha_post,
# The matte-confidence gate only makes sense for a real matting prediction;
# a chroma-seeded alpha is itself colour evidence, so no gate there.
raw_alpha=raw_alpha if alpha_source == "vitmatte" else None,
)
alpha = clean_alpha(alpha, trimap, settings.alpha_post)
if settings.cross_check.enabled:
# The veto's suspect zone is defined by background hue, so it needs the key
# colour; without chroma the second opinion still serves as seg (reuse above).
if settings.cross_check.enabled and settings.chroma.enabled:
lab = convert_color_spaces(rgb)[1]
if second_alpha is None:
second_alpha = pipeline._second_opinion(rgb)
@@ -199,7 +220,7 @@ def _process_rgb(rgb: np.ndarray, pipeline: MattingPipeline) -> MattingResult:
metadata = {
"alpha_source": alpha_source,
"background_model": model.to_dict(),
"background_model": model.to_dict() if model is not None else None,
"trimap": trimap_stats,
"settings": asdict(settings),
}