fourdgsPoseAt function

FourdgsPose? fourdgsPoseAt(FourdgsPoseSampled track, double t)

The pose at scene time t, or null when the record has no samples.

Outside the sample range the pose is clamped, never extrapolated: before the first sample it is the first sample, at or after the last it is the last. Extrapolating produces a platform that accelerates away from the scene at the ends of the clip, which is never what the capture did.

Implementation

FourdgsPose? fourdgsPoseAt(FourdgsPoseSampled track, double t) {
  checkSceneTime(t);
  final n = track.sampleCount;
  if (n == 0) return null;
  if (t <= track.timeAt(0)) return _sample(track, 0);
  if (t >= track.timeAt(n - 1)) return _sample(track, n - 1);

  // Times are strictly increasing (enforced at parse), so a bisection is exact.
  var lo = 0;
  var hi = n - 1;
  while (hi - lo > 1) {
    final mid = (lo + hi) >> 1;
    if (track.timeAt(mid) <= t) {
      lo = mid;
    } else {
      hi = mid;
    }
  }

  if (track.interpolation == trajectoryStep) {
    return _sample(track, lo);
  }
  if (track.interpolation != trajectoryLinear) {
    // Unknown-but-legal, not malformed — but there is no defensible way to
    // invent the rule, and picking linear would silently answer a question the
    // file asked differently. Naming it is the whole obligation.
    throw FourdgsMalformedFile(
      "trajectory '${track.name}' uses interpolation ${track.interpolation}, "
      'which this build does not implement',
    );
  }

  final u = interpolationFraction(t, track.timeAt(lo), track.timeAt(lo + 1));
  final a = _sample(track, lo);
  final b = _sample(track, lo + 1);
  return FourdgsPose(
    rotation: fourdgsSlerp(a.rotation, b.rotation, u),
    translation: <double>[
      finiteLerp(a.translation[0], b.translation[0], u),
      finiteLerp(a.translation[1], b.translation[1], u),
      finiteLerp(a.translation[2], b.translation[2], u),
    ],
  );
}