Esa Kataja
c00a00bb2a
Both scan problems reported from testing came from the same place: the content rectangle defaulted to the whole page, so the mechanism meant to handle margin junk never engaged. Ketun joululaulu has a vertical scan streak down the right margin and Engel a shadow, and because trim is tight and per slice, either one sets that slice's width, which sets the song's widest slice, which scales the whole song down. Detection can propose it. Staff lines are long horizontal runs; scan shadows, spine darkening and glass streaks are vertical, so a wide flat opening keeps one and erases the other. Three corrections were needed against the corpus: - Search only rows inside detected systems. A horizontal artefact above or below the music is itself a long horizontal run reaching the paper edge, which put Engel's left bound at 0. - Take a percentile of the staff-line extents, not the maximum. Where an artefact touches a staff line the two merge into one component: on Ketun p2 the merged line ends at 1575px against 1544px on the clean page. - Take the left bound from the brackets too. A bracket sits left of every staff line, so a staff-line bound crops it off — visible immediately when comparing exported slices. Anchors are now a dataclass carrying their left edge rather than a (top, bottom) tuple. Engel now drops 12-14% of page width and its music fills 1920px instead of leaving the shadow's dead space; Ketun drops 12%. Existing projects keep their saved rectangle; the editor's new Auto-fit and Auto-fit all buttons re-propose it without disturbing cuts.
338 lines
13 KiB
Python
338 lines
13 KiB
Python
"""Detection: skew, systems, cuts, staff height.
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Everything here is a *suggestion* the user confirms or edits (ADR 0004).
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Nothing downstream may assume a result is right.
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Systems are anchored on the vertical bracket that spans their staves, not on
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gaps in the row-darkness profile: a row profile cannot tell an inter-staff gap
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from an inter-system gap, and gets the count wrong on every page of a
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multi-voice choral score (ADR 0006). The row profile is still needed, to expand
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each anchor to its true ink extent — a bracket stops at the last staff line,
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but the slice must include the lyrics printed below it.
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"""
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from __future__ import annotations
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from dataclasses import dataclass, field
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import cv2
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import numpy as np
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SKEW_LIMIT_DEG = 5.0
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SKEW_COARSE_STEP = 1.0
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SKEW_FINE_STEP = 0.1
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_SKEW_WORK_SCALE = 0.25
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_INK = 128 # below this is ink, above is paper
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_ANCHOR_KERNEL = 0.03 # vertical open kernel, as a fraction of page height
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_ANCHOR_MIN = 0.04 # a bracket is at least this tall, as a fraction of page
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_PROFILE_FLOOR = 0.02 # ink-run threshold, as a fraction of the profile peak
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_EXPAND_REACH = 1.5 # how far past the bracket a system's ink reaches, in staff heights
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_STAFF_KERNEL = 0.05 # horizontal open kernel, as a fraction of page width
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_STAFF_MIN_WIDTH = 0.2 # a staff line spans at least this share of the page
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_CONTENT_MARGIN = 0.01 # slack past the staff ends, for ledger lines and lyrics
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_EDGE_PERCENTILE = 15 # tolerate this share of staff lines merged into scan artefacts
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@dataclass
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class Anchor:
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"""A system's vertical bracket: where it is, and how far left it reaches."""
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top: int
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bottom: int
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left: int
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@dataclass
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class System:
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"""One line of music: the ink extent that becomes a slice."""
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top: int
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bottom: int
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staff_height: float | None = None
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@property
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def height(self) -> int:
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return self.bottom - self.top
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@dataclass
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class PageDetection:
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skew: float
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systems: list[System] = field(default_factory=list)
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cuts: list[int] = field(default_factory=list)
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content: tuple[float, float, float, float] = (0.0, 0.0, 1.0, 1.0)
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@property
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def bracketless(self) -> bool:
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"""True when no bracket was found and the row profile was used alone."""
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return not self.systems or all(s.staff_height is None for s in self.systems)
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def row_darkness(gray: np.ndarray) -> np.ndarray:
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return (255 - gray.astype(np.float32)).sum(axis=1)
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def deskew_angle(gray: np.ndarray) -> float:
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"""Angle maximising row-darkness variance — staff lines are the signal.
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Coarse then fine, on a downscaled copy: 31 warps instead of 101.
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"""
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work = cv2.resize(gray, None, fx=_SKEW_WORK_SCALE, fy=_SKEW_WORK_SCALE,
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interpolation=cv2.INTER_AREA)
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def score(angle: float) -> float:
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return float(row_darkness(_rotate(work, angle, cv2.INTER_LINEAR)).var())
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coarse = np.arange(-SKEW_LIMIT_DEG, SKEW_LIMIT_DEG + 1e-9, SKEW_COARSE_STEP)
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best = max(coarse, key=score)
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fine = np.arange(best - SKEW_COARSE_STEP, best + SKEW_COARSE_STEP + 1e-9, SKEW_FINE_STEP)
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fine = fine[np.abs(fine) <= SKEW_LIMIT_DEG]
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return round(float(max(fine, key=score)), 2)
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def _rotate(gray: np.ndarray, angle: float, flags: int = cv2.INTER_CUBIC) -> np.ndarray:
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if angle == 0.0:
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return gray
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h, w = gray.shape
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m = cv2.getRotationMatrix2D((w / 2, h / 2), angle, 1.0)
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return cv2.warpAffine(gray, m, (w, h), flags=flags, borderValue=255)
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def deskew(gray: np.ndarray, angle: float) -> np.ndarray:
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return _rotate(gray, angle)
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def system_anchors(gray: np.ndarray) -> list[Anchor]:
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"""The vertical brackets, one per system."""
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h = gray.shape[0]
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binary = (gray < _INK).astype(np.uint8)
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kernel = cv2.getStructuringElement(cv2.MORPH_RECT, (1, max(3, int(h * _ANCHOR_KERNEL))))
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strokes = cv2.morphologyEx(binary, cv2.MORPH_OPEN, kernel)
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count, _, stats, _ = cv2.connectedComponentsWithStats(strokes, 8)
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tall = [
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Anchor(
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int(stats[i, cv2.CC_STAT_TOP]),
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int(stats[i, cv2.CC_STAT_TOP] + stats[i, cv2.CC_STAT_HEIGHT]),
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int(stats[i, cv2.CC_STAT_LEFT]),
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)
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for i in range(1, count)
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if stats[i, cv2.CC_STAT_HEIGHT] > h * _ANCHOR_MIN
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]
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# Tallest first, keeping only strokes that don't overlap one already kept:
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# a system's barlines all overlap its bracket, so each system yields one.
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# The kept stroke is the tallest, which is the bracket rather than a barline.
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anchors: list[Anchor] = []
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for candidate in sorted(tall, key=lambda a: a.bottom - a.top, reverse=True):
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if any(not (candidate.bottom < a.top or candidate.top > a.bottom) for a in anchors):
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continue
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anchors.append(candidate)
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return sorted(anchors, key=lambda a: a.top)
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def content_columns(
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gray: np.ndarray, anchors: list[Anchor] | None = None
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) -> tuple[float, float]:
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"""Where the music is horizontally, as normalised x bounds.
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Staff lines are long *horizontal* runs; a scan-edge shadow, a spine
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darkening and the vertical line a dirty scanner glass leaves are all
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*vertical*. Opening with a wide flat kernel keeps the first and erases the
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others, so the staff lines' own bounding box is the music area.
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`anchors` does two jobs. It restricts the search to rows known to hold
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systems — without that, a horizontal scan artefact above or below the music
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is itself a long horizontal run reaching the paper edge, which is exactly
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the measurement being avoided. And its brackets give the true left bound:
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a bracket sits *left of every staff line*, so a bound taken from staff
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lines alone crops it off, and a bracket is notation, not artefact.
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This matters more than it looks: trim is tight and per slice, so one dark
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band down the margin sets that slice's width, which sets the song's widest
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slice, which scales the whole song down.
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"""
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height, width = gray.shape
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binary = (gray < _INK).astype(np.uint8)
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if anchors:
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keep = np.zeros(height, bool)
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for anchor in anchors:
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keep[max(0, anchor.top) : min(height, anchor.bottom)] = True
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binary[~keep] = 0
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kernel = cv2.getStructuringElement(cv2.MORPH_RECT, (max(3, int(width * _STAFF_KERNEL)), 1))
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lines = cv2.morphologyEx(binary, cv2.MORPH_OPEN, kernel)
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count, _, stats, _ = cv2.connectedComponentsWithStats(lines, 8)
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runs = [
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(stats[i, cv2.CC_STAT_LEFT], stats[i, cv2.CC_STAT_LEFT] + stats[i, cv2.CC_STAT_WIDTH])
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for i in range(1, count)
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if stats[i, cv2.CC_STAT_WIDTH] > width * _STAFF_MIN_WIDTH
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]
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if not runs:
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return 0.0, 1.0
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# Percentiles, not the extremes. Where a scan-edge band happens to touch
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# the end of a staff line the two merge into one component, and that
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# component then reaches into the artefact — on Ketun joululaulu p2 the
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# merged line ends at 1575px against 1544px on the clean page. A page has
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# dozens of staff lines and only a few are contaminated, so a percentile
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# lands on the true edge while the extreme lands on the worst artefact.
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lefts = np.array([r[0] for r in runs], float)
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rights = np.array([r[1] for r in runs], float)
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margin = width * _CONTENT_MARGIN
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left = float(np.percentile(lefts, _EDGE_PERCENTILE))
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if anchors:
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left = min(left, min(a.left for a in anchors))
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right = float(np.percentile(rights, 100 - _EDGE_PERCENTILE))
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return max(0.0, left - margin) / width, min(float(width), right + margin) / width
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def ink_runs(gray: np.ndarray) -> list[tuple[int, int]]:
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"""Rows containing ink, despeckled — specks are the known failure mode."""
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profile = row_darkness(cv2.medianBlur(gray, 3))
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if profile.max() <= 0:
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return []
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inked = profile > profile.max() * _PROFILE_FLOOR
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runs: list[tuple[int, int]] = []
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start: int | None = None
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for i, on in enumerate(inked):
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if on and start is None:
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start = i
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elif not on and start is not None:
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runs.append((start, i))
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start = None
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if start is not None:
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runs.append((start, len(inked)))
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return runs
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def staff_height(gray: np.ndarray, top: int, bottom: int) -> float | None:
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"""Distance between a staff's outer lines, from staff-line spacing."""
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profile = row_darkness(gray[top:bottom])
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if profile.size == 0 or profile.max() <= 0:
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return None
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peaks = np.where(profile > profile.max() * 0.55)[0]
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if peaks.size < 2:
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return None
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centres = []
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run = [peaks[0]]
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for prev, cur in zip(peaks, peaks[1:]):
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if cur - prev > 3:
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centres.append(float(np.mean(run)))
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run = []
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run.append(cur)
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centres.append(float(np.mean(run)))
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if len(centres) < 2:
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return None
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gaps = np.diff(centres)
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# Keep intra-staff gaps; the big ones are the spaces between staves.
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intra = gaps[gaps < np.median(gaps) * 2]
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if intra.size == 0:
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return None
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return float(np.median(intra) * 4) # 5 lines, 4 spaces
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def _gap(run: tuple[int, int], anchor: Anchor) -> int:
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"""Vertical distance between an ink run and a bracket; 0 if they overlap."""
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start, end = run
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if end > anchor.top and start < anchor.bottom:
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return 0
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return anchor.top - end if end <= anchor.top else start - anchor.bottom
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def _assign(
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runs: list[tuple[int, int]],
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anchors: list[Anchor],
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reaches: list[float],
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) -> list[tuple[int, int]]:
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"""Give every ink run to one system, and return each system's extent.
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A run between two systems is resolved by **precedence, not proximity**: the
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system above wins if the run is within its reach. Text printed under a staff
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belongs to that staff, and engravers space lyrics generously — on *Feliz
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Navidad* a lyric line sits 43px under its own system's bracket but only 10px
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above the next one's, so nearest-bracket gives it to the wrong system.
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Distance is measured from the *bracket*, never from a growing extent — a
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title block's credit lines are stacked closely enough that a chaining
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expansion hops from one to the next and walks the whole way up the page.
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One pass over all systems, rather than each bracket expanding on its own, so
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that a run has exactly one owner and extents cannot overlap.
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Known limit: when a lyric line is printed tight enough under its system that
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no blank row separates it from the *next* system's staves, the two fuse into
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a single ink run and no row profile can split them — the lyric is then given
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to the system below and the cut lands high. Dragging the cut is the fix;
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separating them needs a signal this pass doesn't have.
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"""
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bounds = [[a.top, a.bottom] for a in anchors]
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def claim(index: int, run: tuple[int, int]) -> None:
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bounds[index][0] = min(bounds[index][0], run[0])
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bounds[index][1] = max(bounds[index][1], run[1])
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for run in runs:
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gaps = [_gap(run, a) for a in anchors]
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# Ink overlapping a bracket belongs to it — to the one it overlaps most,
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# whatever else is in reach.
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inside = [
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(min(run[1], anchors[i].bottom) - max(run[0], anchors[i].top), i)
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for i, g in enumerate(gaps)
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if g == 0
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]
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if inside:
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claim(max(inside)[1], run)
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continue
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within = [i for i, g in enumerate(gaps) if g <= reaches[i]]
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if not within:
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continue # a title block or a footer: too far from any system
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# Otherwise the system above wins, and only failing that the one below.
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above = [i for i in within if anchors[i].bottom <= run[0]]
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claim(above[-1] if above else within[0], run)
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return [(lo, hi) for lo, hi in bounds]
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def detect_page(gray: np.ndarray, skew: float | None = None) -> PageDetection:
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"""Full proposal for one page raster. `gray` is the *unrotated* page."""
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angle = deskew_angle(gray) if skew is None else skew
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straight = deskew(gray, angle)
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runs = ink_runs(straight)
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anchors = system_anchors(straight)
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if anchors:
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# Staff height is measured on the bracket span, before expansion, so a
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# swallowed title block can't distort it.
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heights = [staff_height(straight, a.top, a.bottom) for a in anchors]
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reaches = [(h or gray.shape[0] * 0.02) * _EXPAND_REACH for h in heights]
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systems = [
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System(top=lo, bottom=hi, staff_height=h)
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for (lo, hi), h in zip(_assign(runs, anchors, reaches), heights)
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]
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else:
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# No bracket: a single-staff melody or lead sheet, where every ink run
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# genuinely is its own system.
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systems = [System(top=t, bottom=b) for t, b in runs]
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cuts = [
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(systems[i].bottom + systems[i + 1].top) // 2 for i in range(len(systems) - 1)
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]
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# Only the horizontal bounds are proposed. Vertically the cuts and the
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# discard flags already isolate the header and footer, and cropping the top
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# would risk clipping a tempo mark or a section label above the first staff.
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left, right = content_columns(straight, anchors)
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return PageDetection(
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skew=angle, systems=systems, cuts=cuts, content=(left, 0.0, right, 1.0)
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)
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