coverage method
This volume's coverage at cameraPosition, 0..1 (before weight).
Implementation
double coverage(Vector3 cameraPosition) {
final b = bounds;
if (b == null) return 1.0;
final dist = b.distanceTo(cameraPosition);
if (blendDistance <= 0) return dist <= 0 ? 1.0 : 0.0;
final c = 1.0 - dist / blendDistance;
return c.clamp(0.0, 1.0);
}