buildMesh method

@override ShapeMesh buildMesh([ ShapeUv uv = ShapeUv.natural ])

Its triangles, textured by uv.

Implementation

@override
ShapeMesh buildMesh([ShapeUv uv = ShapeUv.natural]) {
  final (cx, cy) = _centre;
  final hz = Primitive3D.size(depth) / 2;
  final n = _steps;
  final b = MeshBuilder(uv);
  (double, double, double) at(int i, double r, double z) {
    final a = _at(i);
    return (r * math.cos(a) - cx, -r * math.sin(a) - cy, z);
  }

  // The curved faces: outside facing out, inside facing the middle.
  for (final (r, out) in [(_outer, 1.0), (_inner, -1.0)]) {
    if (r <= 0) continue;
    final first = b.vertexCount;
    final length = uv.anyway(r * _angle);
    for (var i = 0; i <= n; i++) {
      final a = _at(i);
      final nx = math.cos(a) * out, ny = -math.sin(a) * out;
      for (final z in [-hz, hz]) {
        final (x, y, _) = at(i, r, z);
        final u = uv.metres ? length * i / n : i / n;
        final v = uv.metres ? z * uv.sz + hz * uv.sz : (z + hz) / (2 * hz);
        b.vertex(x, y, z, nx, ny, 0, u, v);
      }
    }
    for (var i = 0; i < n; i++) {
      final a = first + i * 2;
      b.quad(a, a + 1, a + 3, a + 2);
    }
  }
  // Front and back.
  for (var i = 0; i < n; i++) {
    for (final z in [-hz, hz]) {
      b.face(
        [
          at(i, _inner, z),
          at(i, _outer, z),
          at(i + 1, _outer, z),
          at(i + 1, _inner, z),
        ],
        0,
        0,
        z < 0 ? -1 : 1,
      );
    }
  }
  // Its two ends.
  if (!_closed) {
    for (final (i, sign) in [(0, 1.0), (n, -1.0)]) {
      final a = _at(i);
      b.face(
        [
          at(i, _inner, -hz),
          at(i, _outer, -hz),
          at(i, _outer, hz),
          at(i, _inner, hz),
        ],
        math.sin(a) * sign,
        math.cos(a) * sign,
        0,
      );
    }
  }
  final mesh = b.build();
  return smooth ? mesh : mesh.faceted();
}