Image data encoding apparatus, comprising: an entropy encoder for selectively encoding data items representing image data to be encoded by a first, context adaptive binary arithmetic coding (CABAC), encoding system or by a second, bypass, encoding system, so as to generate encoded binarized symbols; the image data representing one or more pictures of a picture sequence, each picture comprising two or more output data units representing respective sub-sections of the picture, each sub-section being decodable and reconstructable independently of other sub-sections in that picture and picture sequence; the entropy encoder being configured to generate an output data stream subject to a constraint defining an upper limit to the number of binarized symbols that may be expressed by any individual output data unit relative to the size in bytes of that output data unit, in which the entropy encoder is configured to apply the constraint to each output data unit, and to provide padding data, for each output data unit which does not meet the constraint, so as to increase the size in bytes of that output data unit in order to meet the constraint; characterized in that the constraint is defined by: N < = K 1 * B + K 2 * CU in which: N = number of binarized symbols in the output data unit; K1 is a constant; B = number of encoded bytes for the output data unit; K2 is a variable dependent upon properties of minimum size coding units employed by the image data encoding apparatus; and CU = size of the sub-section represented by the output data unit expressed as a number of coding units of minimum size. an entropy encoder for selectively encoding data items representing image data to be encoded by a first, context adaptive binary arithmetic coding (CABAC), encoding system or by a second, bypass, encoding system, so as to generate encoded binarized symbols; the image data representing one or more pictures of a picture sequence, each picture comprising two or more output data units representing respective sub-sections of the picture, each sub-section being decodable and reconstructable independently of other sub-sections in that picture and picture sequence; the entropy encoder being configured to generate an output data stream subject to a constraint defining an upper limit to the number of binarized symbols that may be expressed by any individual output data unit relative to the size in bytes of that output data unit, in which the entropy encoder is configured to apply the constraint to each output data unit, and to provide padding data, for each output data unit which does not meet the constraint, so as to increase the size in bytes of that output data unit in order to meet the constraint; characterized in that the constraint is defined by: N < = K 1 * B + K 2 * CU in which: N = number of binarized symbols in the output data unit; K1 is a constant; B = number of encoded bytes for the output data unit; K2 is a variable dependent upon properties of minimum size coding units employed by the image data encoding apparatus; and CU = size of the sub-section represented by the output data unit expressed as a number of coding units of minimum size. N = number of binarized symbols in the output data unit; K1 is a constant; B = number of encoded bytes for the output data unit; K2 is a variable dependent upon properties of minimum size coding units employed by the image data encoding apparatus; and CU = size of the sub-section represented by the output data unit expressed as a number of coding units of minimum size.