A data encoding method comprises generating an ordered array of data values representing an image region and having an array order; deriving a quantization parameter indicating a quantization degree so that the ordered array of data values, when encoded using that quantization degree, meets one or more predetermined criteria; quantizing the data values using the derived quantization parameter to generate respective quantized values; detecting a number of non-zero quantized values; if no more than a threshold number of first values in the array order are non-zero, selecting a different quantization parameter and quantizing the data values using a quantization degree indicated by the selected different quantization parameter; and entropy encoding at least the non-zero quantized values.