1. A computer-implemented method comprising: obtaining a before image of a person taken before a portion of the person's body has changed, and an actual after image of the person taken after the portion of person's body has changed; applying model parameters to the before image of the person to generate a forecasted after image that predicts how the portion of the person's body will appear after the portion of the person's body has changed; comparing the actual after image to the forecasted after image to determine an error between the actual after image and the forecasted after image; and adjusting the model parameters based on the error.