theseus/bundled-addons/pdf-editor/lib/signature.js

// The signature pad.
//
// What comes out is a list of strokes, each a list of points normalised to
// 0..1 inside the pad's box, y measured downwards. Not a picture. That matters
// for three reasons: the same signature can be dropped at any size on any page
// and stay sharp, it survives into the saved PDF as vector line art rather
// than a fuzzy PNG, and it is a few hundred bytes in storage instead of tens
// of kilobytes of base64.
//
// The pad also trims the drawing to its own bounding box before normalising,
// so a signature scrawled in the top-left corner of the pad still fills the
// box it gets placed in.

export class SignaturePad {
  constructor(canvas) {
    this.canvas = canvas;
    this.ctx = canvas.getContext("2d");
    this.strokes = [];      // in canvas pixel space while drawing
    this.cur = null;
    this._onChange = null;
    canvas.addEventListener("pointerdown", (e) => this._down(e));
    canvas.addEventListener("pointermove", (e) => this._move(e));
    window.addEventListener("pointerup", () => this._up());
    this.clear();
  }

  onChange(fn) { this._onChange = fn; }

  clear() {
    this.strokes = [];
    this.cur = null;
    this._paint();
    this._onChange?.(false);
  }

  get isEmpty() { return !this.strokes.some((s) => s.length > 1); }

  _pt(e) {
    const r = this.canvas.getBoundingClientRect();
    // The canvas is CSS-scaled to the modal's width, so a client point has to
    // come back through that ratio to land in the backing store's pixels.
    return [
      (e.clientX - r.left) * (this.canvas.width / r.width),
      (e.clientY - r.top) * (this.canvas.height / r.height),
    ];
  }
  _down(e) {
    if (e.button !== 0) return;
    this.cur = [this._pt(e)];
    this.strokes.push(this.cur);
    this.canvas.setPointerCapture?.(e.pointerId);
    e.preventDefault();
  }
  _move(e) {
    if (!this.cur) return;
    this.cur.push(this._pt(e));
    this._paint();
  }
  _up() {
    if (!this.cur) return;
    if (this.cur.length < 2) this.strokes.pop();
    this.cur = null;
    this._paint();
    this._onChange?.(!this.isEmpty);
  }

  _paint() {
    const { ctx, canvas } = this;
    ctx.save();
    ctx.setTransform(1, 0, 0, 1, 0, 0);
    ctx.fillStyle = "#ffffff";
    ctx.fillRect(0, 0, canvas.width, canvas.height);
    // A baseline, so people sign at a sensible height rather than in a corner.
    ctx.strokeStyle = "rgba(20,30,50,.18)";
    ctx.lineWidth = 1;
    ctx.beginPath();
    ctx.moveTo(24, canvas.height * 0.72);
    ctx.lineTo(canvas.width - 24, canvas.height * 0.72);
    ctx.stroke();
    ctx.strokeStyle = "#101418";
    ctx.lineWidth = 2.6;
    ctx.lineCap = "round";
    ctx.lineJoin = "round";
    for (const s of this.strokes) {
      if (s.length < 2) continue;
      ctx.beginPath();
      ctx.moveTo(s[0][0], s[0][1]);
      for (let i = 1; i < s.length; i++) ctx.lineTo(s[i][0], s[i][1]);
      ctx.stroke();
    }
    ctx.restore();
  }

  /**
   * @returns {{strokes:[number,number][][], aspect:number}|null}
   *   strokes normalised into 0..1 over the drawing's own bounds; `aspect` is
   *   height/width of those bounds, so the caller can place it undistorted.
   */
  result() {
    const live = this.strokes.filter((s) => s.length > 1);
    if (!live.length) return null;
    const xs = live.flat().map((p) => p[0]);
    const ys = live.flat().map((p) => p[1]);
    const x0 = Math.min(...xs), x1 = Math.max(...xs);
    const y0 = Math.min(...ys), y1 = Math.max(...ys);
    // A single horizontal or vertical line has zero extent on one axis; floor
    // it so the division below cannot produce Infinity.
    const w = Math.max(1, x1 - x0), h = Math.max(1, y1 - y0);
    return {
      strokes: live.map((s) => s.map(([x, y]) => [(x - x0) / w, (y - y0) / h])),
      aspect: Math.min(1.2, Math.max(0.12, h / w)),
    };
  }
}