Depending on the source and destination face sizes, you may need to implement a resize_crop() function that centralizes the faces in the images and extracts the bounding rectangle of the control points to be used as the input images; otherwise, while iterative blending, the area of the morphed face may keep on increasing/decreasing. 

You just implemented the mesh warping algorithm for face morphing. Alternatively, you can implement the Beier-Neely field morphing algorithm too for face morphing. Implement this algorithm from the references/links provided in the following section, See also.