An image encoding apparatus comprising: a selector configured to select, from a set of candidate prediction operations each defining at least a prediction direction, a prediction operation for prediction of samples of a current region of a current image, the current region comprising an array of two or more rows and two or more columns of samples; an intra-image predictor configured to derive predicted samples of the current region with respect to one or more of a group of reference samples of the same image in dependence upon a prediction direction, defined by the selected prediction operation, between a current sample to be predicted and a reference position amongst the reference samples; in which, for at least some of the candidate prediction operations, the group of reference samples comprises two or more adjacent linear arrays of reference samples disposed at different respective separations from the current region; and a detector configured to detect whether samples corresponding to any of the two or more adjacent linear arrays of reference samples are unavailable for use in prediction of samples of the current region and, if any of the two or more adjacent linear arrays of reference samples are unavailable, to inhibit selection, by the selector, of a candidate prediction operation dependent upon the unavailable reference samples; in which the detector is configured to detect, separately for reference sample positions disposed vertically or horizontally with respect to the current image region, whether samples corresponding to any of the two or more adjacent linear arrays of reference samples are unavailable for use in prediction of samples of the current region and, if any of the two or more adjacent linear arrays of reference samples are unavailable for reference sample positions disposed vertically or horizontally with respect to the current image region, to inhibit selection, by the selector, of a candidate prediction operation dependent upon the linear array containing the unavailable reference samples. a selector configured to select, from a set of candidate prediction operations each defining at least a prediction direction, a prediction operation for prediction of samples of a current region of a current image, the current region comprising an array of two or more rows and two or more columns of samples; an intra-image predictor configured to derive predicted samples of the current region with respect to one or more of a group of reference samples of the same image in dependence upon a prediction direction, defined by the selected prediction operation, between a current sample to be predicted and a reference position amongst the reference samples; in which, for at least some of the candidate prediction operations, the group of reference samples comprises two or more adjacent linear arrays of reference samples disposed at different respective separations from the current region; and a detector configured to detect whether samples corresponding to any of the two or more adjacent linear arrays of reference samples are unavailable for use in prediction of samples of the current region and, if any of the two or more adjacent linear arrays of reference samples are unavailable, to inhibit selection, by the selector, of a candidate prediction operation dependent upon the unavailable reference samples; in which the detector is configured to detect, separately for reference sample positions disposed vertically or horizontally with respect to the current image region, whether samples corresponding to any of the two or more adjacent linear arrays of reference samples are unavailable for use in prediction of samples of the current region and, if any of the two or more adjacent linear arrays of reference samples are unavailable for reference sample positions disposed vertically or horizontally with respect to the current image region, to inhibit selection, by the selector, of a candidate prediction operation dependent upon the linear array containing the unavailable reference samples.