Royal Chess

1// Shared "first-person" board view: tilts the canvas toward the player (CSS perspective) and maps
2// pointer input back through the inverse projection, so game code keeps using flat canvas coordinates.
3(() => {
4  const c = document.getElementById("c");
5  if (!c) return;
6  const TILT = 30; // degrees
7  function apply() {
8    const h = c.offsetHeight || 800;
9    c.style.transformOrigin = "50% 100%";
10    c.style.transform = "perspective(" + Math.round(h * 1.3) + "px) rotateX(" + TILT + "deg)";
11    c.style.willChange = "transform";
12  }
13  apply();
14  if (window.ResizeObserver) new ResizeObserver(apply).observe(c);
15  addEventListener("resize", apply);
16  function toLocal(sx, sy) {
17    const w = c.offsetWidth, h = c.offsetHeight;
18    const m = new DOMMatrix(getComputedStyle(c).transform);
19    const dx = sx - (c.offsetLeft + w / 2), dy = sy - (c.offsetTop + h);
20    const a = m.m11 - dx * m.m14, b = m.m21 - dx * m.m24, e = dx * m.m44 - m.m41;
21    const d = m.m12 - dy * m.m14, f = m.m22 - dy * m.m24, g = dy * m.m44 - m.m42;
22    const det = a * f - b * d;
23    if (!det) return null;
24    return { x: (e * f - b * g) / det + w / 2, y: (a * g - e * d) / det + h, w, h };
25  }
26  let busy = false;
27  ["pointerdown", "pointermove", "pointerup"].forEach((type) => {
28    addEventListener(type, (e) => {
29      if (busy || e.target !== c || !e.isTrusted) return;
30      const p = toLocal(e.clientX, e.clientY);
31      if (!p) return;
32      e.stopImmediatePropagation();
33      const r = c.getBoundingClientRect();
34      const ev = new PointerEvent(type, {
35        bubbles: true, cancelable: true, pointerId: e.pointerId, pointerType: e.pointerType, isPrimary: e.isPrimary,
36        button: e.button, buttons: e.buttons, clientX: r.left + (p.x / p.w) * r.width, clientY: r.top + (p.y / p.h) * r.height,
37      });
38      busy = true;
39      try { c.dispatchEvent(ev); } finally { busy = false; }
40    }, true);
41  });
42})();
43