pdfgutter: two facing pages photographed as one, found and cut

parity/oracle/pdfgutter.py

@@ -0,0 +1,81 @@
+"""Decide 2-up scans the way the book pipeline did, for comparison.
+
+The four functions below are the originals, copied out of the preprocessing
+module verbatim — importing it would pull in a PDF renderer and a page tree,
+and none of that decides anything. What decides is the arithmetic, which is
+what this compares.
+
+    python3 oracle/pdfgutter.py < cases.ndjson
+
+One case per line, tagged with what it is asking about. Writes one JSON
+answer per line.
+"""
+
+from __future__ import annotations
+
+import json
+import sys
+
+import numpy as np
+
+_MIN_RATIO = 1.15
+_MIN_GUTTER_FRAC = 0.6
+_TEXT_COL_INK = 0.02
+
+
+def _ratio(width: float, height: float) -> float:
+    return width / height if height else 0.0
+
+
+def _has_gutter(ink: np.ndarray) -> bool:
+    w = len(ink)
+    center = ink[int(0.46 * w):int(0.54 * w)]
+    left = ink[int(0.10 * w):int(0.44 * w)]
+    right = ink[int(0.56 * w):int(0.90 * w)]
+    if center.size == 0 or left.size == 0 or right.size == 0:
+        return False
+    return bool(
+        center.min() < _TEXT_COL_INK
+        and np.median(left) > _TEXT_COL_INK
+        and np.median(right) > _TEXT_COL_INK
+    )
+
+
+def _sample_indices(n: int, k: int = 8) -> list[int]:
+    if n <= 6:
+        return list(range(n))
+    lo, hi = 2, n - 3
+    step = max(1, (hi - lo) // k)
+    return list(range(lo, hi + 1, step))[:k]
+
+
+def _verdict(ratios: list[float], gutters: list[bool]) -> dict:
+    median_ratio = float(np.median(ratios)) if ratios else 0.0
+    gutter_frac = (sum(gutters) / len(gutters)) if gutters else 0.0
+    return {
+        "median_ratio": median_ratio,
+        "gutter_fraction": gutter_frac,
+        "two_up": bool(median_ratio > _MIN_RATIO and gutter_frac >= _MIN_GUTTER_FRAC),
+    }
+
+
+def answer(case: dict) -> dict:
+    kind = case["kind"]
+    if kind == "ratio":
+        return {"ratio": _ratio(case["width"], case["height"])}
+    if kind == "gutter":
+        return {"gutter": _has_gutter(np.asarray(case["ink"], dtype=float))}
+    if kind == "sample":
+        return {"indices": _sample_indices(case["pages"])}
+    if kind == "verdict":
+        return _verdict(case["ratios"], case["gutters"])
+    raise SystemExit(f"no such case: {kind}")
+
+
+for line in sys.stdin:
+    if not line.strip():
+        continue
+    case = json.loads(line)
+    out = {"name": case["name"], "kind": case["kind"]}
+    out.update(answer(case))
+    sys.stdout.write(json.dumps(out, sort_keys=True) + "\n")