diff --git a/noteman_slicer/bundle.py b/noteman_slicer/bundle.py
new file mode 100644
index 0000000..85ed6da
--- /dev/null
+++ b/noteman_slicer/bundle.py
@@ -0,0 +1,65 @@
+"""Bundle export — the only channel to noteman (ADR 0001).
+
+    song.zip
+      song.json
+      original.pdf
+      001.webp 002.webp …
+
+Array order in `song.json` *is* slice order: one ordering, not two. Markers
+nest inside the slice they sit on, so an index appears in exactly one place —
+a jump source's `destination`.
+"""
+
+from __future__ import annotations
+
+import json
+import zipfile
+from pathlib import Path
+
+from .pdf import Source
+from .project import Project
+from .render import render_song
+
+FORMAT_VERSION = 1
+METADATA_FIELDS = (
+    "title",
+    "subtitle",
+    "composer",
+    "original_artist",
+    "arranger",
+    "lyricist",
+    "translator",
+    "voices",
+)
+
+
+def song_json(project: Project, files: list[str]) -> dict:
+    payload: dict = {"v": FORMAT_VERSION}
+    for field in METADATA_FIELDS:
+        value = project.metadata.get(field)
+        if value:
+            payload[field] = value
+    payload["slices"] = [{"file": name} for name in files]
+    return payload
+
+
+def write(project: Project, source: Source, path: Path) -> Path:
+    """Render the song and write the bundle. Returns the zip path."""
+    images = render_song(project, source)
+    names = [f"{i + 1:03}.webp" for i in range(len(images))]
+
+    path = Path(path)
+    path.parent.mkdir(parents=True, exist_ok=True)
+    # ZIP_STORED for the images: WebP is already compressed, so deflating it
+    # only costs time. The JSON is small enough not to care.
+    with zipfile.ZipFile(path, "w") as zf:
+        zf.writestr(
+            "song.json",
+            json.dumps(song_json(project, names), indent=2, ensure_ascii=False),
+            zipfile.ZIP_DEFLATED,
+        )
+        if project.source.exists():
+            zf.write(project.source, "original.pdf")
+        for name, data in zip(names, images):
+            zf.writestr(name, data, zipfile.ZIP_STORED)
+    return path
diff --git a/noteman_slicer/cli.py b/noteman_slicer/cli.py
index 2f804ff..6f565e6 100644
--- a/noteman_slicer/cli.py
+++ b/noteman_slicer/cli.py
@@ -82,6 +82,41 @@ def _project(args: argparse.Namespace) -> int:
     return 0
 
 
+def _export(args: argparse.Namespace) -> int:
+    from . import bundle
+    from .project import Project, default_path
+
+    source = open_source(args.pdf, SourceType(args.type) if args.type else None)
+    path = default_path(source.path)
+
+    if path.exists():
+        project = Project.load(path)
+        if project.source_changed():
+            print("WARNING: the PDF has changed since these cuts were made")
+    else:
+        detections, heights = [], []
+        for i in range(len(source)):
+            gray = page_raster(source, i)
+            detections.append(detect_page(gray))
+            heights.append(gray.shape[0])
+        project = Project.from_detection(source.path, detections, heights)
+        print("no project file; exporting straight from detection")
+
+    out = Path(args.out) if args.out else source.path.with_suffix(".zip")
+    bundle.write(project, source, out)
+    size = out.stat().st_size
+    slices = len(project.kept_slices())
+    print(f"{out}  {slices} slices, {size / 1024:.0f} KB ({size / max(slices, 1) / 1024:.1f} KB/slice)")
+    source.close()
+    return 0
+
+
+def _edit(args: argparse.Namespace) -> int:
+    from .editor import launch
+
+    return launch(Path(args.pdf), SourceType(args.type) if args.type else None)
+
+
 def main(argv: list[str] | None = None) -> int:
     parser = argparse.ArgumentParser(
         prog="noteman-slicer",
@@ -113,6 +148,17 @@ def main(argv: list[str] | None = None) -> int:
     proj.add_argument("--type", choices=[t.value for t in SourceType])
     proj.set_defaults(func=_project)
 
+    exp = sub.add_parser("export", help="render the song and write a bundle")
+    exp.add_argument("pdf")
+    exp.add_argument("--out", help="output zip (default: alongside the PDF)")
+    exp.add_argument("--type", choices=[t.value for t in SourceType])
+    exp.set_defaults(func=_export)
+
+    ed = sub.add_parser("edit", help="open the editor")
+    ed.add_argument("pdf")
+    ed.add_argument("--type", choices=[t.value for t in SourceType])
+    ed.set_defaults(func=_edit)
+
     args = parser.parse_args(argv)
     return args.func(args)
 
diff --git a/noteman_slicer/editor.py b/noteman_slicer/editor.py
new file mode 100644
index 0000000..39dc129
--- /dev/null
+++ b/noteman_slicer/editor.py
@@ -0,0 +1,535 @@
+"""The editor: the human-in-the-loop half of the tool.
+
+Detection proposes; everything here is how you dispose (ADR 0004). Cuts can be
+authored entirely by hand with detection producing nothing.
+
+Geometry is edited in normalised page coordinates, so what the screen shows and
+what the renderer uses are the same numbers at a different zoom.
+"""
+
+from __future__ import annotations
+
+import sys
+from pathlib import Path
+
+import numpy as np
+from PySide6.QtCore import QPointF, QRectF, Qt, QTimer, Signal
+from PySide6.QtGui import (
+    QAction,
+    QBrush,
+    QColor,
+    QImage,
+    QKeySequence,
+    QPainter,
+    QPen,
+    QPixmap,
+    QPolygonF,
+)
+from PySide6.QtWidgets import (
+    QApplication,
+    QDoubleSpinBox,
+    QFileDialog,
+    QFormLayout,
+    QGraphicsScene,
+    QGraphicsView,
+    QGroupBox,
+    QHBoxLayout,
+    QLabel,
+    QLineEdit,
+    QMainWindow,
+    QMessageBox,
+    QPushButton,
+    QSlider,
+    QVBoxLayout,
+    QWidget,
+)
+
+from . import bundle
+from .bundle import METADATA_FIELDS
+from .detect import deskew, detect_page
+from .pdf import Source, open_source, page_raster
+from .project import Cut, Project, default_path
+from .render import apply_levels
+
+PREVIEW_MAX = 1800  # display resolution; geometry stays normalised
+HIT = 6  # grab distance in screen pixels
+AUTOSAVE_MS = 800
+
+_CUT = QColor(220, 40, 40)
+_CUT_ACTIVE = QColor(255, 120, 0)
+_VERTEX = QColor(255, 200, 0)
+_DISCARD = QColor(120, 120, 140, 90)
+_RECT = QColor(40, 140, 220)
+
+
+class PageView(QGraphicsView):
+    """Pan, zoom, and direct manipulation of cuts and the content rectangle."""
+
+    changed = Signal()
+    selection_changed = Signal()
+
+    def __init__(self) -> None:
+        super().__init__()
+        self.setScene(QGraphicsScene(self))
+        self.setRenderHint(QPainter.Antialiasing)
+        self.setDragMode(QGraphicsView.ScrollHandDrag)
+        self.setTransformationAnchor(QGraphicsView.AnchorUnderMouse)
+
+        self.project: Project | None = None
+        self.page_index = 0
+        self.pixmap: QPixmap | None = None
+        self.selected_cut: int | None = None
+        self.selected_slice = 0
+        self._drag: tuple[str, int, int] | None = None
+
+    # -- state ------------------------------------------------------------
+
+    def show_page(self, project: Project, index: int, image: np.ndarray) -> None:
+        self.project = project
+        self.page_index = index
+        h, w = image.shape
+        qimage = QImage(image.data, w, h, w, QImage.Format_Grayscale8).copy()
+        self.pixmap = QPixmap.fromImage(qimage)
+        self.selected_cut = None
+        self.selected_slice = 0
+        self.scene().setSceneRect(0, 0, w, h)
+        self.redraw()
+        self.fitInView(self.scene().sceneRect(), Qt.KeepAspectRatio)
+
+    @property
+    def page(self):
+        return self.project.pages[self.page_index]
+
+    def redraw(self) -> None:
+        scene = self.scene()
+        scene.clear()
+        if self.pixmap is None:
+            return
+        scene.addPixmap(self.pixmap)
+        w, h = self.pixmap.width(), self.pixmap.height()
+
+        for slot in range(self.page.slice_count):
+            if self.page.discards[slot]:
+                scene.addPolygon(
+                    self._slice_polygon(slot, w, h), QPen(Qt.NoPen), QBrush(_DISCARD)
+                )
+
+        # The selected slice, outlined so trim anomalies are visible.
+        pen = QPen(QColor(0, 170, 0), 2)
+        pen.setCosmetic(True)
+        scene.addPolygon(self._slice_polygon(self.selected_slice, w, h), pen)
+
+        x0, y0, x1, y1 = self.project.page_content_rect(self.page_index)
+        pen = QPen(_RECT, 2, Qt.DashLine)
+        pen.setCosmetic(True)
+        scene.addRect(QRectF(x0 * w, y0 * h, (x1 - x0) * w, (y1 - y0) * h), pen)
+
+        for i, cut in enumerate(self.page.cuts):
+            colour = _CUT_ACTIVE if i == self.selected_cut else _CUT
+            pen = QPen(colour, 2)
+            pen.setCosmetic(True)
+            points = [QPointF(x * w, y * h) for x, y in cut.points]
+            for a, b in zip(points, points[1:]):
+                scene.addLine(a.x(), a.y(), b.x(), b.y(), pen)
+            if i == self.selected_cut:
+                r = HIT * 1.5 / max(self.transform().m11(), 1e-6)
+                for p in points:
+                    scene.addEllipse(
+                        p.x() - r, p.y() - r, r * 2, r * 2, QPen(Qt.NoPen), QBrush(_VERTEX)
+                    )
+
+    def _slice_polygon(self, slot: int, w: int, h: int) -> QPolygonF:
+        above, below = self.page.bounds(slot)
+        top = [(0.0, 0.0), (1.0, 0.0)] if above is None else above.points
+        bottom = [(0.0, 1.0), (1.0, 1.0)] if below is None else below.points
+        pts = [QPointF(x * w, y * h) for x, y in top]
+        pts += [QPointF(x * w, y * h) for x, y in reversed(bottom)]
+        return QPolygonF(pts)
+
+    # -- hit testing ------------------------------------------------------
+
+    def _norm(self, pos) -> tuple[float, float]:
+        p = self.mapToScene(pos)
+        return p.x() / self.pixmap.width(), p.y() / self.pixmap.height()
+
+    def _tolerance(self) -> tuple[float, float]:
+        scale = max(self.transform().m11(), 1e-6)
+        return HIT / scale / self.pixmap.width(), HIT / scale / self.pixmap.height()
+
+    def _hit_cut(self, x: float, y: float) -> tuple[int, int | None] | None:
+        """(cut index, vertex index or None) under the cursor."""
+        tx, ty = self._tolerance()
+        for i, cut in enumerate(self.page.cuts):
+            for v, (vx, vy) in enumerate(cut.points):
+                if abs(vx - x) <= tx * 2 and abs(vy - y) <= ty * 2:
+                    return i, v
+            if abs(cut.y_at(x) - y) <= ty:
+                return i, None
+        return None
+
+    def _hit_rect_edge(self, x: float, y: float) -> str | None:
+        x0, y0, x1, y1 = self.project.page_content_rect(self.page_index)
+        tx, ty = self._tolerance()
+        if y0 - ty <= y <= y1 + ty:
+            if abs(x - x0) <= tx:
+                return "left"
+            if abs(x - x1) <= tx:
+                return "right"
+        if x0 - tx <= x <= x1 + tx:
+            if abs(y - y0) <= ty:
+                return "top"
+            if abs(y - y1) <= ty:
+                return "bottom"
+        return None
+
+    # -- interaction ------------------------------------------------------
+
+    def mousePressEvent(self, event) -> None:
+        if self.project is None or self.pixmap is None:
+            return super().mousePressEvent(event)
+        x, y = self._norm(event.position().toPoint())
+
+        if event.button() == Qt.RightButton:
+            hit = self._hit_cut(x, y)
+            if hit:
+                index, vertex = hit
+                if vertex is not None and len(self.page.cuts[index].points) > 2:
+                    self.page.cuts[index].points.pop(vertex)
+                else:
+                    self.page.remove_cut(index)
+                    self.selected_cut = None
+                self.redraw()
+                self.changed.emit()
+            return
+
+        if event.button() == Qt.LeftButton:
+            edge = self._hit_rect_edge(x, y)
+            hit = self._hit_cut(x, y)
+            if hit and event.modifiers() & Qt.ControlModifier and hit[1] is None:
+                # Ctrl-click on a cut inserts a vertex: this is how a straight
+                # cut becomes a stepped one.
+                cut = self.page.cuts[hit[0]]
+                at = next(i for i, p in enumerate(cut.points) if p[0] > x)
+                cut.points.insert(at, (x, cut.y_at(x)))
+                self.selected_cut = hit[0]
+                self._drag = ("vertex", hit[0], at)
+            elif hit:
+                self.selected_cut = hit[0]
+                self._drag = ("vertex" if hit[1] is not None else "cut", hit[0], hit[1] or 0)
+            elif edge:
+                self._drag = ("rect", 0, 0)
+                self._edge = edge
+            else:
+                self.selected_cut = None
+                self.selected_slice = self._slice_at(x, y)
+                self.selection_changed.emit()
+            self.setDragMode(
+                QGraphicsView.NoDrag if self._drag else QGraphicsView.ScrollHandDrag
+            )
+            self.redraw()
+        super().mousePressEvent(event)
+
+    def mouseMoveEvent(self, event) -> None:
+        if self._drag and self.pixmap is not None:
+            x, y = self._norm(event.position().toPoint())
+            kind, index, vertex = self._drag
+            if kind == "cut":
+                cut = self.page.cuts[index]
+                shift = y - cut.y_at(x)
+                cut.points = [(px, min(1.0, max(0.0, py + shift))) for px, py in cut.points]
+            elif kind == "vertex":
+                cut = self.page.cuts[index]
+                lo = cut.points[vertex - 1][0] if vertex > 0 else 0.0
+                hi = cut.points[vertex + 1][0] if vertex + 1 < len(cut.points) else 1.0
+                px = cut.points[vertex][0] if vertex in (0, len(cut.points) - 1) else min(
+                    max(x, lo), hi
+                )
+                cut.points[vertex] = (px, min(1.0, max(0.0, y)))
+            else:
+                x0, y0, x1, y1 = self.project.page_content_rect(self.page_index)
+                x, y = min(max(x, 0.0), 1.0), min(max(y, 0.0), 1.0)
+                box = {
+                    "left": (x, y0, x1, y1),
+                    "right": (x0, y0, x, y1),
+                    "top": (x0, y, x1, y1),
+                    "bottom": (x0, y0, x1, y),
+                }[self._edge]
+                self.project.pages[self.page_index].content_rect = box
+            self.redraw()
+            return
+        super().mouseMoveEvent(event)
+
+    def mouseReleaseEvent(self, event) -> None:
+        if self._drag:
+            self._drag = None
+            self.setDragMode(QGraphicsView.ScrollHandDrag)
+            self.page.cuts.sort(key=lambda c: c.points[0][1])
+            self.changed.emit()
+        super().mouseReleaseEvent(event)
+
+    def mouseDoubleClickEvent(self, event) -> None:
+        if self.project is None or self.pixmap is None:
+            return
+        x, y = self._norm(event.position().toPoint())
+        if self._hit_cut(x, y) is None:
+            self.selected_cut = self.page.add_cut(Cut.straight(y))
+            self.redraw()
+            self.changed.emit()
+
+    def wheelEvent(self, event) -> None:
+        factor = 1.15 if event.angleDelta().y() > 0 else 1 / 1.15
+        self.scale(factor, factor)
+        self.redraw()
+
+    def _slice_at(self, x: float, y: float) -> int:
+        return sum(1 for cut in self.page.cuts if cut.y_at(x) < y)
+
+    def toggle_discard(self) -> None:
+        self.page.discards[self.selected_slice] = not self.page.discards[self.selected_slice]
+        self.redraw()
+        self.changed.emit()
+
+
+class Editor(QMainWindow):
+    def __init__(self, source: Source, project: Project) -> None:
+        super().__init__()
+        self.source = source
+        self.project = project
+        self.index = 0
+        self._raw: dict[int, np.ndarray] = {}
+
+        self.setWindowTitle(f"noteman-slicer — {source.path.name}")
+        self.view = PageView()
+        self.view.changed.connect(self._touched)
+        self.view.selection_changed.connect(self._sync)
+
+        self.autosave = QTimer(self)
+        self.autosave.setSingleShot(True)
+        self.autosave.setInterval(AUTOSAVE_MS)
+        self.autosave.timeout.connect(self._save)
+
+        central = QWidget()
+        layout = QHBoxLayout(central)
+        layout.addWidget(self.view, 1)
+        layout.addWidget(self._panel())
+        self.setCentralWidget(central)
+        self._shortcuts()
+        self._load_page(0)
+
+    # -- ui ---------------------------------------------------------------
+
+    def _panel(self) -> QWidget:
+        panel = QWidget()
+        panel.setFixedWidth(320)
+        box = QVBoxLayout(panel)
+
+        nav = QHBoxLayout()
+        self.page_label = QLabel()
+        prev, nxt = QPushButton("◀"), QPushButton("▶")
+        prev.clicked.connect(lambda: self._load_page(self.index - 1))
+        nxt.clicked.connect(lambda: self._load_page(self.index + 1))
+        nav.addWidget(prev)
+        nav.addWidget(self.page_label, 1)
+        nav.addWidget(nxt)
+        box.addLayout(nav)
+
+        page_box = QGroupBox("Page")
+        form = QFormLayout(page_box)
+        self.skew = QDoubleSpinBox()
+        self.skew.setRange(-15.0, 15.0)
+        self.skew.setSingleStep(0.1)
+        self.skew.setDecimals(2)
+        self.skew.setSuffix("°")
+        self.skew.valueChanged.connect(self._skew_changed)
+        form.addRow("Skew", self.skew)
+
+        self.black = QSlider(Qt.Horizontal)
+        self.black.setRange(0, 255)
+        self.white = QSlider(Qt.Horizontal)
+        self.white.setRange(0, 255)
+        self.white.setValue(255)
+        for s in (self.black, self.white):
+            s.valueChanged.connect(self._levels_changed)
+        form.addRow("Black point", self.black)
+        form.addRow("White point", self.white)
+
+        discard = QPushButton("Toggle discard  (D)")
+        discard.clicked.connect(self.view.toggle_discard)
+        form.addRow(discard)
+        reset = QPushButton("Reset content rectangle")
+        reset.clicked.connect(self._reset_rect)
+        form.addRow(reset)
+        box.addWidget(page_box)
+
+        meta_box = QGroupBox("Song")
+        meta_form = QFormLayout(meta_box)
+        self.metadata: dict[str, QLineEdit] = {}
+        for field in METADATA_FIELDS:
+            edit = QLineEdit(self.project.metadata.get(field, ""))
+            edit.textChanged.connect(self._metadata_changed)
+            self.metadata[field] = edit
+            meta_form.addRow(field.replace("_", " ").title(), edit)
+        box.addWidget(meta_box)
+
+        self.summary = QLabel()
+        self.summary.setWordWrap(True)
+        box.addWidget(self.summary)
+
+        export = QPushButton("Export bundle…")
+        export.clicked.connect(self._export)
+        box.addWidget(export)
+        box.addStretch(1)
+
+        help_text = QLabel(
+            "Double-click: add cut\n"
+            "Drag: move cut · Ctrl-click: add vertex\n"
+            "Right-click: delete cut or vertex\n"
+            "Click a slice, then D to discard\n"
+            "Drag the blue edges: content rectangle"
+        )
+        help_text.setStyleSheet("color: palette(mid);")
+        box.addWidget(help_text)
+        return panel
+
+    def _shortcuts(self) -> None:
+        for key, slot in (
+            (QKeySequence("D"), self.view.toggle_discard),
+            (QKeySequence(Qt.Key_PageDown), lambda: self._load_page(self.index + 1)),
+            (QKeySequence(Qt.Key_PageUp), lambda: self._load_page(self.index - 1)),
+            (QKeySequence.Save, self._save),
+        ):
+            action = QAction(self)
+            action.setShortcut(key)
+            action.triggered.connect(slot)
+            self.addAction(action)
+
+    # -- page handling ----------------------------------------------------
+
+    def _raster(self, index: int) -> np.ndarray:
+        """Page pixels at preview resolution, cached — the PDF is slow to read."""
+        if index not in self._raw:
+            import cv2
+
+            gray = page_raster(self.source, index)
+            if gray.shape[1] > PREVIEW_MAX:
+                k = PREVIEW_MAX / gray.shape[1]
+                gray = cv2.resize(gray, None, fx=k, fy=k, interpolation=cv2.INTER_AREA)
+            self._raw[index] = gray
+        return self._raw[index]
+
+    def _preview(self, index: int) -> np.ndarray:
+        page = self.project.pages[index]
+        black, white = self.project.page_levels(index)
+        return np.ascontiguousarray(
+            apply_levels(deskew(self._raster(index), page.skew), black, white)
+        )
+
+    def _load_page(self, index: int) -> None:
+        if not 0 <= index < len(self.project.pages):
+            return
+        self.index = index
+        self.view.show_page(self.project, index, self._preview(index))
+        self._sync()
+
+    def _sync(self) -> None:
+        page = self.project.pages[self.index]
+        self.page_label.setText(f"Page {self.index + 1} / {len(self.project.pages)}")
+        for widget, value in ((self.skew, page.skew),):
+            widget.blockSignals(True)
+            widget.setValue(value)
+            widget.blockSignals(False)
+        black, white = self.project.page_levels(self.index)
+        for widget, value in ((self.black, black), (self.white, white)):
+            widget.blockSignals(True)
+            widget.setValue(value)
+            widget.blockSignals(False)
+        kept = len(self.project.kept_slices())
+        total = sum(p.slice_count for p in self.project.pages)
+        state = "discarded" if page.discards[self.view.selected_slice] else "kept"
+        self.summary.setText(
+            f"{page.slice_count} slices on this page · slice "
+            f"{self.view.selected_slice + 1} is {state}\n"
+            f"{kept} of {total} slices kept in the song"
+        )
+
+    # -- edits ------------------------------------------------------------
+
+    def _touched(self) -> None:
+        self._sync()
+        self.autosave.start()
+
+    def _skew_changed(self, value: float) -> None:
+        self.project.pages[self.index].skew = value
+        self.view.show_page(self.project, self.index, self._preview(self.index))
+        self._touched()
+
+    def _levels_changed(self) -> None:
+        self.project.pages[self.index].levels = (self.black.value(), self.white.value())
+        self.view.show_page(self.project, self.index, self._preview(self.index))
+        self._touched()
+
+    def _metadata_changed(self) -> None:
+        self.project.metadata = {
+            field: edit.text().strip() for field, edit in self.metadata.items() if edit.text().strip()
+        }
+        self.autosave.start()
+
+    def _reset_rect(self) -> None:
+        self.project.pages[self.index].content_rect = None
+        self.view.redraw()
+        self._touched()
+
+    def _save(self) -> None:
+        path = self.project.save()
+        self.statusBar().showMessage(f"saved {path.name}", 2000)
+
+    def _export(self) -> None:
+        self._save()
+        target, _ = QFileDialog.getSaveFileName(
+            self, "Export bundle", str(self.source.path.with_suffix(".zip")), "Bundle (*.zip)"
+        )
+        if not target:
+            return
+        try:
+            out = bundle.write(self.project, self.source, Path(target))
+        except Exception as error:  # noqa: BLE001 - surfaced to the user
+            QMessageBox.critical(self, "Export failed", str(error))
+            return
+        size = out.stat().st_size / 1024
+        QMessageBox.information(
+            self,
+            "Exported",
+            f"{out.name}\n{len(self.project.kept_slices())} slices, {size:.0f} KB",
+        )
+
+    def closeEvent(self, event) -> None:
+        self._save()
+        super().closeEvent(event)
+
+
+def launch(pdf: Path, source_type=None) -> int:
+    app = QApplication(sys.argv[:1])
+    source = open_source(pdf, source_type)
+    path = default_path(source.path)
+
+    if path.exists():
+        project = Project.load(path)
+        if project.source_changed():
+            QMessageBox.warning(
+                None,
+                "Source changed",
+                "The PDF has changed since these cuts were made.\n"
+                "Cuts may no longer line up with the music.",
+            )
+    else:
+        detections, heights = [], []
+        for i in range(len(source)):
+            gray = page_raster(source, i)
+            detections.append(detect_page(gray))
+            heights.append(gray.shape[0])
+        project = Project.from_detection(source.path, detections, heights)
+
+    window = Editor(source, project)
+    window.resize(1500, 950)
+    window.show()
+    return app.exec()
diff --git a/noteman_slicer/render.py b/noteman_slicer/render.py
new file mode 100644
index 0000000..49c0365
--- /dev/null
+++ b/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))]
diff --git a/tests/test_editor.py b/tests/test_editor.py
new file mode 100644
index 0000000..ead83ec
--- /dev/null
+++ b/tests/test_editor.py
@@ -0,0 +1,114 @@
+"""Runnable check that the editor builds and its edits reach project state.
+
+Runs offscreen, so it verifies wiring rather than appearance: that the widgets
+construct, that an edit changes the model, and that autosave and export work.
+"""
+
+from __future__ import annotations
+
+import os
+import sys
+from pathlib import Path
+
+os.environ.setdefault("QT_QPA_PLATFORM", "offscreen")
+
+import numpy as np  # noqa: E402
+import pymupdf  # noqa: E402
+
+sys.path.insert(0, str(Path(__file__).resolve().parents[1]))
+
+from PySide6.QtWidgets import QApplication  # noqa: E402
+
+from noteman_slicer.detect import detect_page  # noqa: E402
+from noteman_slicer.editor import Editor  # noqa: E402
+from noteman_slicer.pdf import open_source, page_raster  # noqa: E402
+from noteman_slicer.project import Cut, Project, default_path  # noqa: E402
+
+W, H = 1200, 1600
+
+
+def _scan_pdf(path: Path) -> None:
+    art = np.full((H, W), 255, np.uint8)
+    for top in (300, 800):
+        art[top : top + 200, 100:104] = 0
+        for staff in (top, top + 140):
+            for i in range(5):
+                art[staff + i * 15 : staff + i * 15 + 2, 110:1100] = 0
+    pix = pymupdf.Pixmap(pymupdf.csGRAY, W, H, bytearray(art.tobytes()), False)
+    doc = pymupdf.open()
+    doc.new_page(width=595, height=842).insert_image(pymupdf.Rect(0, 0, 595, 842), pixmap=pix)
+    doc.save(path)
+
+
+def main() -> int:
+    tmp = Path(__file__).with_name("_tmp")
+    tmp.mkdir(exist_ok=True)
+    pdf = tmp / "scan.pdf"
+    _scan_pdf(pdf)
+
+    app = QApplication.instance() or QApplication(sys.argv[:1])
+    source = open_source(pdf)
+    gray = page_raster(source, 0)
+    project = Project.from_detection(pdf, [detect_page(gray)], [gray.shape[0]])
+
+    editor = Editor(source, project)
+    page = project.pages[0]
+
+    # Cuts.
+    before = page.slice_count
+    editor.view.selected_cut = page.add_cut(Cut.straight(0.5))
+    editor.view.redraw()
+    assert page.slice_count == before + 1
+
+    # A vertex turns a straight cut into a stepped one.
+    cut = page.cuts[editor.view.selected_cut]
+    cut.points.insert(1, (0.4, cut.y_at(0.4)))
+    cut.points[1] = (0.4, cut.points[1][1] + 0.03)
+    assert cut.straight_y is None, "the cut should no longer be straight"
+    editor.view.redraw()
+
+    # Discard.
+    editor.view.selected_slice = 1
+    was = page.discards[1]
+    editor.view.toggle_discard()
+    assert page.discards[1] != was
+
+    # Skew and levels reach the model and re-render without raising.
+    editor.skew.setValue(-1.4)
+    assert abs(page.skew + 1.4) < 1e-6
+    editor.black.setValue(40)
+    editor.white.setValue(210)
+    assert project.page_levels(0) == (40, 210)
+
+    # Metadata.
+    editor.metadata["title"].setText("Ketun joululaulu")
+    editor.metadata["composer"].setText("trad.")
+    assert project.metadata["title"] == "Ketun joululaulu"
+
+    # Content rectangle edits and reset.
+    page.content_rect = (0.05, 0.02, 0.95, 0.98)
+    assert project.page_content_rect(0) == (0.05, 0.02, 0.95, 0.98)
+    editor._reset_rect()
+    assert project.page_content_rect(0) == project.content_rect
+
+    # Autosave target, then a round-trip through disk.
+    editor._save()
+    saved = default_path(pdf)
+    assert saved.exists()
+    reloaded = Project.load(saved)
+    assert reloaded.metadata["title"] == "Ketun joululaulu"
+    assert reloaded.pages[0].skew == -1.4
+    assert reloaded.pages[0].levels == (40, 210)
+    assert [c.points for c in reloaded.pages[0].cuts] == [c.points for c in page.cuts]
+
+    editor.close()
+    source.close()
+    for f in (pdf, saved):
+        f.unlink()
+    tmp.rmdir()
+    print("ok")
+    return 0
+
+
+if __name__ == "__main__":
+    sys.exit(main())
diff --git a/tests/test_render.py b/tests/test_render.py
new file mode 100644
index 0000000..639bec3
--- /dev/null
+++ b/tests/test_render.py
@@ -0,0 +1,162 @@
+"""Runnable check for the render pipeline and bundle export."""
+
+from __future__ import annotations
+
+import json
+import sys
+import zipfile
+from pathlib import Path
+
+import cv2
+import numpy as np
+import pymupdf
+
+sys.path.insert(0, str(Path(__file__).resolve().parents[1]))
+
+from noteman_slicer import bundle  # noqa: E402
+from noteman_slicer.detect import detect_page  # noqa: E402
+from noteman_slicer.pdf import open_source, page_raster  # noqa: E402
+from noteman_slicer.project import Cut, Project  # noqa: E402
+from noteman_slicer.render import (  # noqa: E402
+    ALPHA_LEVELS,
+    apply_levels,
+    encode,
+    pad_right,
+    render_slices,
+    scale_song,
+)
+
+W, H = 1200, 1600
+GAP = 15
+
+
+def _system(page: np.ndarray, top: int, right: int) -> None:
+    """A bracket plus two staves, with a lyric line under each."""
+    page[top : top + 200, 100:104] = 0
+    for staff in (top, top + 140):
+        for i in range(5):
+            page[staff + i * GAP : staff + i * GAP + 2, 110:right] = 0
+        page[staff + 90 : staff + 105, 200 : right - 100] = 0
+
+
+def _scan_pdf(path: Path) -> None:
+    art = np.full((H, W), 255, np.uint8)
+    art[40:60, 400:800] = 0  # title, far from any system
+    _system(art, 300, 1100)
+    _system(art, 800, 900)  # narrower: exercises the right pad
+    pix = pymupdf.Pixmap(pymupdf.csGRAY, W, H, bytearray(art.tobytes()), False)
+    doc = pymupdf.open()
+    page = doc.new_page(width=595, height=842)
+    page.insert_image(page.rect, pixmap=pix)
+    doc.save(path)
+
+
+def main() -> int:
+    tmp = Path(__file__).with_name("_tmp")
+    tmp.mkdir(exist_ok=True)
+    pdf = tmp / "scan.pdf"
+    _scan_pdf(pdf)
+
+    source = open_source(pdf)
+    gray = page_raster(source, 0)
+    project = Project.from_detection(pdf, [detect_page(gray)], [gray.shape[0]])
+
+    slices = render_slices(project, source)
+    assert len(slices) == 2, f"expected 2 kept slices, got {len(slices)}"
+
+    # The title is far from any bracket, so it is not in a kept slice: both
+    # slices must be shorter than the gap between the systems.
+    assert all(s.gray.shape[0] < 400 for s in slices), [s.gray.shape for s in slices]
+
+    # System 2 is drawn narrower, so before padding the widths differ.
+    assert slices[0].width != slices[1].width, "the fixture should differ in width"
+
+    scaled = scale_song(slices, cap=4000)  # a cap far above the fixture
+    assert all(abs(a.shape[1] - b.width) <= 2 for a, b in zip(scaled, slices)), (
+        "never upscale: a song narrower than the cap must be left alone"
+    )
+
+    padded = pad_right(scale_song(slices))
+    assert len({p.shape[1] for p in padded}) == 1, "slices must share one width"
+    assert max(p.shape[1] for p in padded) <= 1920
+
+    rgba = cv2.imdecode(np.frombuffer(encode(padded[0]), np.uint8), cv2.IMREAD_UNCHANGED)
+    assert rgba.shape[2] == 4
+    assert rgba[:, :, :3].max() == 0, "ink must be pure black"
+    assert rgba[:, :, 3].max() == 255, "full ink must be fully opaque"
+    assert rgba[:, :, 3].min() == 0, "paper must be fully transparent"
+    assert len(np.unique(rgba[:, :, 3])) <= ALPHA_LEVELS
+
+    # Levels: a white point below the paper value wipes the paper out entirely.
+    faint = np.full((10, 10), 200, np.uint8)
+    assert apply_levels(faint, 0, 180).max() == 255
+
+    # The Engel case: a section label printed in the left margin at a height
+    # that belongs to the *next* system. A straight cut cannot separate it from
+    # the previous system's lyrics; a stepped one can.
+    label_top, label_bottom = 620, 680
+    labelled = tmp / "labelled.pdf"
+    art = np.full((H, W), 255, np.uint8)
+    _system(art, 300, 1100)
+    _system(art, 800, 900)
+    art[label_top:label_bottom, 120:300] = 0  # the label
+    art[label_top:label_bottom, 500:1000] = 0  # system 1's trailing lyrics, same rows
+    pix = pymupdf.Pixmap(pymupdf.csGRAY, W, H, bytearray(art.tobytes()), False)
+    doc = pymupdf.open()
+    doc.new_page(width=595, height=842).insert_image(pymupdf.Rect(0, 0, 595, 842), pixmap=pix)
+    doc.save(labelled)
+
+    src2 = open_source(labelled)
+    g2 = page_raster(src2, 0)
+    proj2 = Project.from_detection(labelled, [detect_page(g2)], [g2.shape[0]])
+    page = proj2.pages[0]
+    scale = g2.shape[0] / H
+
+    def ink(images: list) -> list[int]:
+        """Ink in the left margin of each slice — where the label sits."""
+        return [int((i.gray[:, : int(i.width * 0.3)] < 128).sum()) for i in images]
+
+    # Straight cut through the middle of that band: the label goes with
+    # whichever side the line falls on, and cannot be separated.
+    band_mid = (label_top + label_bottom) / 2 * scale / g2.shape[0]
+    page.cuts[1] = Cut.straight(band_mid)
+    straight_ink = ink(render_slices(proj2, src2))
+
+    # Stepped: above the label on the left, below the lyrics on the right.
+    above = (label_top - 10) * scale / g2.shape[0]
+    below = (label_bottom + 10) * scale / g2.shape[0]
+    page.cuts[1] = Cut([(0.0, above), (0.35, above), (0.35, below), (1.0, below)])
+    stepped_ink = ink(render_slices(proj2, src2))
+
+    # The straight cut splits the label down the middle; the stepped cut gives
+    # all of it to the lower slice and none to the upper.
+    assert stepped_ink[1] > straight_ink[1], (
+        f"the label must move into the lower slice: {straight_ink} → {stepped_ink}"
+    )
+    assert stepped_ink[0] < straight_ink[0], (
+        f"and out of the upper one: {straight_ink} → {stepped_ink}"
+    )
+    src2.close()
+    labelled.unlink()
+
+    # Bundle.
+    out = bundle.write(project, source, tmp / "song.zip")
+    with zipfile.ZipFile(out) as zf:
+        names = zf.namelist()
+        assert "song.json" in names and "original.pdf" in names, names
+        meta = json.loads(zf.read("song.json"))
+        assert meta["v"] == 1
+        files = [s["file"] for s in meta["slices"]]
+        assert files == ["001.webp", "002.webp"], files
+        assert all(f in names for f in files)
+
+    source.close()
+    for f in (pdf, out):
+        f.unlink()
+    tmp.rmdir()
+    print("ok")
+    return 0
+
+
+if __name__ == "__main__":
+    sys.exit(main())