Kokkoja: first public assembly

Assembled from the development repository by tools/make_release.sh.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

client/js/stroke_geometry.js

@@ -0,0 +1,63 @@
+// @ts-check
+/**
+ * The shape of a stroke, shared by everything that draws one: the screen
+ * (renderer.js) and the page file (page_file.js).
+ *
+ * A stroke is a chain of *parts*, one per point.  The part for a point runs
+ * from the midpoint before it to the midpoint after it, curving through the
+ * point, with a width taken from the pen pressure at that point.  At the
+ * ends of the stroke the part runs to the end point itself.
+ */
+
+import { POINT_SIZE, pointCount } from './page_model.js';
+
+/** @typedef {import('./page_model.js').Stroke} Stroke */
+
+/**
+ * @typedef {object} Part
+ * @property {number} fromX
+ * @property {number} fromY
+ * @property {number} x      The point itself, the control point of the curve.
+ * @property {number} y
+ * @property {number} toX
+ * @property {number} toY
+ * @property {number} width
+ * @property {boolean} dot   The stroke is a single point: a filled circle
+ *                           of diameter `width` at (x, y).
+ */
+
+/**
+ * The drawn width of a stroke at one of its points.  The stroke's nominal
+ * width is drawn at pressure 0.4, about the pressure of ordinary writing
+ * (Andy's averaged 0.34 to 0.45 on a Cintiq 16 and rarely passed 0.8).  A
+ * firm press, 0.75, is about 2.6 times as wide.
+ * @param {Stroke} stroke
+ * @param {number} index  Point index.
+ */
+export function widthAt(stroke, index) {
+  const pressure = stroke.points[index * POINT_SIZE + 2];
+  return stroke.width * (0.36 + 4 * pressure * pressure);
+}
+
+/**
+ * The part of a stroke that belongs to point `k`.
+ * @param {Stroke} stroke
+ * @param {number} k
+ * @returns {Part}
+ */
+export function partAt(stroke, k) {
+  const p = stroke.points;
+  const last = pointCount(stroke) - 1;
+  const x = p[k * POINT_SIZE];
+  const y = p[k * POINT_SIZE + 1];
+  return {
+    fromX: k === 0 ? x : (p[(k - 1) * POINT_SIZE] + x) / 2,
+    fromY: k === 0 ? y : (p[(k - 1) * POINT_SIZE + 1] + y) / 2,
+    x,
+    y,
+    toX: k === last ? x : (x + p[(k + 1) * POINT_SIZE]) / 2,
+    toY: k === last ? y : (y + p[(k + 1) * POINT_SIZE + 1]) / 2,
+    width: widthAt(stroke, k),
+    dot: last === 0,
+  };
+}