1. A method of calibrating an image display system comprising a plurality of projectors oriented in the direction of an image projection screen, the method comprising: recovering non-parametric calibration data for the display system; generating a non-parametric model of the display system from the non-parametric calibration data; generating local parametric models relating to the display surface of the projection screen using canonical surface data representing, at least in part, the image projection screen; comparing the local parametric models with data points defined by the non-parametric calibration data; using the comparison of the local parametric models with the data points defined by the non-parametric calibration data to identify one or more local errors in the non-parametric calibration data; converting the local errors in the non-parametric calibration data to data points defined at least in part by the local parametric models; and operating the projectors to project an image on the image projection screen by utilizing a hybrid calibration model comprising data points taken from the non-parametric model and data points taken from one or more local parametric models to convert the local errors in the non-parametric calibration data.