/* Express co as term of movement along 2 edges of a triangle. */
static float2 packed_edge_coordinate(const float2 v1,
                                     const float2 v2,
                                     const float2 v3,
                                     const float2 co)
{
  float2 v13 = v3 - v1;
  float2 v12 = v2 - v1;
  float2 point_2b = co - v13;
  float2 isect_point;

  isect_line_line_v2_point(v1, v2, co, point_2b, isect_point);
  float len_v12 = len_v2(v12);
  float len_v12_isect = len_v2(isect_point - v1);
  float d1 = len_v12_isect / len_v12;

  float len_v13 = len_v2(v13);
  float len_v13_isect = len_v2(co - isect_point);
  float d2 = len_v13_isect / len_v13;
  return float2(d1, d2);
}