abstract class Face {
 List<int> indices;
 Vector3 normal;
 List vertexNormals, vertexColors, vertexTangents;
 Color color;
 int materialIndex;
 Vector3 centroid;

 /// normalOrVertexNormals and colorOrVertexColors can be either a [Vector3] or a [List<Vector3>]
 Face( this.indices, normalOrVertexNormals, colorOrVertexColors, this.materialIndex ) {

   normal = normalOrVertexNormals is Vector3 ? normalOrVertexNormals :  new Vector3.zero();
   vertexNormals = normalOrVertexNormals is List ? normalOrVertexNormals : [];

   color = colorOrVertexColors is Color ? colorOrVertexColors : new Color();
   vertexColors = colorOrVertexColors is List ? colorOrVertexColors : [];

   vertexTangents = [];

   centroid = new Vector3.zero();
 }

 int get size => indices.length;

 Face setFrom(Face other) {
   normal.setFrom(other.normal);
   color.copy( other.color );
   centroid.setFrom(other.centroid);

   materialIndex = other.materialIndex;

   vertexNormals = other.vertexNormals.map((Vector3 v) => v.clone()).toList();
   vertexColors = other.vertexColors.map((Vector3 v) => v.clone()).toList();
   vertexTangents = other.vertexTangents.map((Vector3 v) => v.clone()).toList();

   return this;
 }

 Face clone();
}