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Folio

Ruler and zoom window

Two page tools added to the notebook editor. Both are screen-space helpers: neither adds model, storage or format state.

Reference

The ruler design follows XNotes for Android by Shardul Vikram Singh (https://github.com/shardulvs/xnotes-android, MIT licence, copyright 2026). The XNotes sources studied were app/src/main/java/com/xnotes/canvas/Ruler.kt (geometry, hit-testing, edge projection, axis snapping) and the ruler branches of InteractionController.kt (grab order, two-finger transform, pen magnet). No code was copied; Ruler.kt here is a fresh implementation of the same ideas on android.graphics, so the MIT notice does not need to be carried, but the design credit stays.

XNotes does not have a zoom window. That feature follows the zoom-window panel seen in another note-taking app (a box on the page, a magnified writing panel below with locate, left, right, undo, redo and close controls). Nothing was taken from that app’s code.

Ruler

Files: apps/mobile/androidApp/.../Ruler.kt (geometry and drawing), InkCanvasView.kt (touch, snapping), NotebookEditor.kt (rail button).

Behaviour

  • Rail button Ruler (selected = visible, unselected = hidden). Visibility is not persisted.
  • An infinite band fixed in screen space. It does not scroll or scale with the page, so only the perpendicular offset from the centre line matters for hit-testing and snapping.
  • Graduations are millimetres, half-centimetres and centimetres, measured from the band centre. Spacing is xdpi / 2.54 * zoom pixels per centimetre, so it measures page content at the current zoom. Minor ticks drop out below 3.2 px; label steps widen (1, 2, 5, 10 cm) when zoomed out. Labels skip the buttons, handles and angle read-outs.
  • One finger on the band moves it. Two fingers on the band move and twist it. A rotation handle (two, at ±26% of the short screen side) rotates about the centre. Rotation pulls onto 0/90/180/270 within 4°.
  • Two buttons at the centre: lock position and lock angle. A locked band swallows finger touches on it.
  • Angle read-outs at both ends, 0–359°.
  • Pen magnet: a pen stroke or shape that starts within 24 dp of either long edge is projected onto that edge for the whole stroke (rulerSnapSide, applied in gesturePosition). The band is drawn above ink and is never grabbed by the pen except on its handles and buttons, so writing along the edge is safe.
  • The eraser, lasso and other tools are not snapped.

Known gaps

  • Snapping has only been exercised with synthetic touches on the emulator, not a real stylus.
  • Numbers are drawn along the band and appear upside down past 90° of rotation.
  • No length read-out and no protractor mode yet (XNotes has neither beyond the angle box).

Zoom window

Files: InkCanvasView.kt (CanvasMode.ZoomWindow, zoomWindow, showRegion, documentAspect), NotebookEditor.kt (panel and menu entry).

Entry: 100% rail button → Zoom → Zoom window (next to Free zoom-in).

Behaviour

  • A blue box marks the area on the page. Drag it to move it, drag the bottom-right dots to resize, or tap empty page to put it there. While the mode is active the main page does not write.
  • The bottom 40% of the screen is a second InkCanvasView showing exactly the box region, filling the panel width. Writing there (pen or finger) edits the same page, so ink appears in the box live.
  • Panel buttons: locate (move the box to the current view), left and right (step by 90% of the box width, wrapping to the next or previous line on paged notebooks), undo, redo, close (removes the box and returns to the pen).
  • The box height follows the panel’s shape: height = width * ratio, where ratio = (panelH * mainW) / (panelW * mainH). This is why documentAspect exists. A paged page normally stretches to its view (documentHeight = view height), which would stretch ink in a differently sized second view; with documentAspect = mainH / mainW both views map ink uniformly.
  • Works on infinite canvases (box is not clamped there).

Known gaps

  • The box and panel are not persisted; closing the editor drops them.
  • Pinch and finger panning are off in the panel; use the arrow buttons or move the box.
  • Only one zoom window at a time. The ruler can be shown on the main page while the window is open but is not mirrored into the panel.