src/veil_pgd/stealth/gate.py

"""The hard stealth gate: pass/fail against configured thresholds."""

from __future__ import annotations

from typing import Callable, Optional

from PIL import Image

from veil_pgd.config import StealthThresholds, get_settings
from veil_pgd.stealth import metrics as m
from veil_pgd.types import StealthReport

# A LPIPS provider is injected (klaus-3 service). If absent, LPIPS is skipped
# (treated as passing) and noted so the gate degrades gracefully offline.
LpipsFn = Callable[[Image.Image, Image.Image], float]


def evaluate_stealth(
    orig: Image.Image,
    mod: Image.Image,
    box: Optional[tuple[int, int, int, int]] = None,
    thresholds: Optional[StealthThresholds] = None,
    lpips_fn: Optional[LpipsFn] = None,
) -> StealthReport:
    th = thresholds or get_settings().stealth
    psnr_v = m.psnr(orig, mod)
    ssim_v = m.ssim(orig, mod)
    de_v = m.delta_e_p95(orig, mod, box)
    lpips_v = lpips_fn(orig, mod) if lpips_fn is not None else float("nan")

    failures: list[str] = []
    if psnr_v < th.psnr_min:
        failures.append(f"psnr {psnr_v:.1f}<{th.psnr_min}")
    if ssim_v < th.ssim_min:
        failures.append(f"ssim {ssim_v:.3f}<{th.ssim_min}")
    if de_v > th.delta_e_p95_max:
        failures.append(f"dE {de_v:.2f}>{th.delta_e_p95_max}")
    if lpips_fn is not None and lpips_v > th.lpips_max:
        failures.append(f"lpips {lpips_v:.3f}>{th.lpips_max}")

    return StealthReport(
        psnr=psnr_v,
        ssim=ssim_v,
        lpips=lpips_v,
        delta_e_p95=de_v,
        passed=len(failures) == 0,
        failures=failures,
    )