Add the render pipeline and bundle export

Project state plus PDF in, finished slice images out. Slices are cut as
polygons rather than row ranges, so a stepped cut yields a slice with a
transparent notch instead of one that covers its neighbour.

Masking paints white, which the ink-to-alpha step turns into full
transparency — the same outcome the spec asks for, one step earlier.

Scale normalises every slice to the median staff height before fitting
the song to 1920px, so a rescanned page sits at the same note size as
its neighbours. The cap only ever shrinks: a song narrower than 1920
stays narrower.

Alpha quantisation rounds to 16 values spanning 0-255 inclusive.
Flooring, as first written, capped full ink at 240 and left every note
6% transparent — caught by decoding an exported slice rather than by
reading the code.

Ketun joululaulu exports 24 slices at a uniform 1489px, under the cap
and correctly not upscaled from its 200 DPI source; Feliz Navidad 20;
Elaman nalka 18.

Closes #21
Closes #22
Closes #23
Closes #24
Closes #25
Closes #27

noteman_slicer/render.py

@@ -0,0 +1,234 @@
+"""Render project state into finished slice images.
+
+    load raster → deskew → levels → content rect → cut → discard
+                → trim → scale → pad → ink→alpha → encode
+
+The order is not arbitrary. Levels runs before anything geometric so the trim
+bounding box is computed on the image that actually ships; the content
+rectangle runs before cutting so margin junk never enters a slice; and trim
+runs before scale because the scale factor derives from the widest *trimmed*
+slice.
+
+Output is final — nothing downstream reprocesses it (ADR 0001).
+"""
+
+from __future__ import annotations
+
+from dataclasses import dataclass
+
+import cv2
+import numpy as np
+
+from .detect import deskew, staff_height
+from .pdf import Source, page_raster
+from .project import Cut, Project
+
+MAX_WIDTH = 1920
+ALPHA_LEVELS = 16  # quantising alpha costs nothing visible and ~32% of the bytes
+_SPECK_AREA = 300  # ink blobs smaller than this don't anchor a trim
+
+
+@dataclass
+class SliceImage:
+    """One rendered slice, before scaling."""
+
+    page: int
+    index: int
+    gray: np.ndarray
+    staff: float | None
+
+    @property
+    def width(self) -> int:
+        return self.gray.shape[1]
+
+
+def apply_levels(gray: np.ndarray, black: int, white: int) -> np.ndarray:
+    """Map [black, white] onto the full range with a lookup table.
+
+    A global LUT, not an adaptive method: CLAHE and adaptive thresholding are
+    tuned for text and eat the thin stuff on notation — hairpin tips, slur ends,
+    ledger lines, tapered beams.
+    """
+    if (black, white) == (0, 255):
+        return gray
+    lo, hi = min(black, white), max(black, white)
+    if hi <= lo:
+        return gray
+    ramp = np.clip((np.arange(256) - lo) * 255.0 / (hi - lo), 0, 255)
+    return cv2.LUT(gray, ramp.astype(np.uint8))
+
+
+def page_pixels(project: Project, source: Source, index: int) -> np.ndarray:
+    """A page straightened and levelled, ready to be cut."""
+    page = project.pages[index]
+    gray = deskew(page_raster(source, index), page.skew)
+    black, white = project.page_levels(index)
+    return apply_levels(gray, black, white)
+
+
+def _boundary(cut: Cut | None, width: int, height: int, *, bottom: bool) -> list[tuple[int, int]]:
+    """A cut as pixel points spanning the page, or the page edge when absent."""
+    if cut is None:
+        y = height if bottom else 0
+        return [(0, y), (width, y)]
+    return [(int(round(x * width)), int(round(y * height))) for x, y in cut.points]
+
+
+def slice_mask(project: Project, index: int, slot: int, shape: tuple[int, int]) -> np.ndarray:
+    """Which pixels of a page belong to one slice.
+
+    A slice bounded by a stepped cut is not rectangular, so this is a polygon
+    rather than a row range: the top boundary left to right, then the bottom
+    boundary right to left.
+    """
+    height, width = shape
+    page = project.pages[index]
+    above, below = page.bounds(slot)
+
+    polygon = _boundary(above, width, height, bottom=False)
+    polygon += _boundary(below, width, height, bottom=True)[::-1]
+
+    mask = np.zeros(shape, np.uint8)
+    cv2.fillPoly(mask, [np.array(polygon, np.int32)], 255)
+
+    # The content rectangle is applied here rather than as a separate crop, so
+    # margin junk can never enter a slice in the first place.
+    x0, y0, x1, y1 = project.page_content_rect(index)
+    box = np.zeros(shape, np.uint8)
+    box[int(y0 * height) : int(y1 * height), int(x0 * width) : int(x1 * width)] = 255
+    return cv2.bitwise_and(mask, box)
+
+
+def _ink_bbox(gray: np.ndarray) -> tuple[int, int, int, int] | None:
+    """Tight bounds of the ink, ignoring specks.
+
+    One scan fleck at the far left would otherwise anchor the trim and shift
+    that slice relative to every other one.
+    """
+    ink = (gray < 200).astype(np.uint8)
+    count, _, stats, _ = cv2.connectedComponentsWithStats(ink, 8)
+    boxes = [
+        (
+            stats[i, cv2.CC_STAT_LEFT],
+            stats[i, cv2.CC_STAT_TOP],
+            stats[i, cv2.CC_STAT_LEFT] + stats[i, cv2.CC_STAT_WIDTH],
+            stats[i, cv2.CC_STAT_TOP] + stats[i, cv2.CC_STAT_HEIGHT],
+        )
+        for i in range(1, count)
+        if stats[i, cv2.CC_STAT_AREA] >= _SPECK_AREA
+    ]
+    if not boxes:
+        return None
+    return (
+        min(b[0] for b in boxes),
+        min(b[1] for b in boxes),
+        max(b[2] for b in boxes),
+        max(b[3] for b in boxes),
+    )
+
+
+def cut_slice(page: np.ndarray, mask: np.ndarray) -> np.ndarray | None:
+    """Extract one slice: everything outside its region becomes paper.
+
+    Paper here means white, which the ink→alpha step turns into full
+    transparency — so a stepped slice's notch composites invisibly on the
+    viewer's sheet rather than covering the neighbouring system.
+    """
+    isolated = np.where(mask > 0, page, np.uint8(255))
+    box = _ink_bbox(isolated)
+    if box is None:
+        return None
+    x0, y0, x1, y1 = box
+    return isolated[y0:y1, x0:x1]
+
+
+def render_slices(project: Project, source: Source) -> list[SliceImage]:
+    """Every kept slice, trimmed but not yet scaled."""
+    out: list[SliceImage] = []
+    for index in range(len(project.pages)):
+        page = page_pixels(project, source, index)
+        for slot in range(project.pages[index].slice_count):
+            if project.pages[index].discards[slot]:
+                continue
+            gray = cut_slice(page, slice_mask(project, index, slot, page.shape))
+            if gray is None:
+                continue  # a kept slice that turned out to hold no ink
+            out.append(SliceImage(index, slot, gray, staff_height(gray, 0, gray.shape[0])))
+    return out
+
+
+def scale_song(slices: list[SliceImage], cap: int = MAX_WIDTH) -> list[np.ndarray]:
+    """Normalise every slice to one staff height, then fit the song to the cap.
+
+    Two steps, both per song. Staff-height normalisation is what makes a
+    rescanned page — or a re-engraved system — sit at the same note size as its
+    neighbours; width-based scaling cannot, because width depends on how much
+    music is in a system rather than on how big it is drawn.
+
+    The cap is a ceiling, never a target: a song that comes out narrower stays
+    narrower, since enlarging a scan past its own resolution buys softness and
+    bytes and no detail.
+    """
+    if not slices:
+        return []
+
+    measured = [s.staff for s in slices if s.staff]
+    target = float(np.median(measured)) if measured else 0.0
+
+    factors = [target / s.staff if (target and s.staff) else 1.0 for s in slices]
+    widest = max(s.width * f for s, f in zip(slices, factors))
+    song = min(1.0, cap / widest) if widest else 1.0
+
+    out = []
+    for s, f in zip(slices, factors):
+        k = f * song
+        if abs(k - 1.0) < 1e-3:
+            out.append(s.gray)
+            continue
+        interp = cv2.INTER_AREA if k < 1 else cv2.INTER_CUBIC
+        out.append(cv2.resize(s.gray, None, fx=k, fy=k, interpolation=interp))
+    return out
+
+
+def pad_right(images: list[np.ndarray]) -> list[np.ndarray]:
+    """Bring every slice to the song's width, flush left.
+
+    A short system simply ends earlier; the padding is paper, so it disappears
+    when ink becomes alpha.
+    """
+    if not images:
+        return []
+    width = max(i.shape[1] for i in images)
+    return [
+        i
+        if i.shape[1] == width
+        else cv2.copyMakeBorder(i, 0, 0, 0, width - i.shape[1], cv2.BORDER_CONSTANT, value=255)
+        for i in images
+    ]
+
+
+def encode(gray: np.ndarray) -> bytes:
+    """Ink black, paper transparent, lossless WebP.
+
+    Lossless rather than lossy not because lossy looks bad — measured, it
+    doesn't — but because it is 58% *larger* on line art (ADR 0003).
+    """
+    alpha = 255 - gray
+    if ALPHA_LEVELS < 256:
+        # Round to the nearest of ALPHA_LEVELS values spanning 0–255 inclusive.
+        # Flooring instead would cap full ink at 240 and leave every note
+        # slightly transparent.
+        step = 255 / (ALPHA_LEVELS - 1)
+        alpha = (np.round(alpha / step) * step).astype(np.uint8)
+    rgba = np.zeros((*gray.shape, 4), np.uint8)
+    rgba[:, :, 3] = alpha
+    ok, buf = cv2.imencode(".webp", rgba, [cv2.IMWRITE_WEBP_QUALITY, 101])
+    if not ok:
+        raise RuntimeError("WebP encoding failed")
+    return buf.tobytes()
+
+
+def render_song(project: Project, source: Source) -> list[bytes]:
+    """The whole raster pipeline: project + PDF in, finished slice images out."""
+    slices = render_slices(project, source)
+    return [encode(image) for image in pad_right(scale_song(slices))]