public override void computeAABB( out AABB aabb, ref Transform transform, int childIndex )
{
Debug.Assert( childIndex < vertices.Count );
int i1 = childIndex;
int i2 = childIndex + 1;
if( i2 == vertices.Count )
i2 = 0;
var v1 = MathUtils.mul( ref transform, vertices[i1] );
var v2 = MathUtils.mul( ref transform, vertices[i2] );
aabb.lowerBound = Vector2.Min( v1, v2 );
aabb.upperBound = Vector2.Max( v1, v2 );
}