A method comprising: obtaining a set of residual data, the set of residual data being useable to reconstruct a representation of a signal at a first level of quality using data based on a representation of the signal at a second level of quality, the first level of quality being higher than the second level of quality, the representation of the signal at the first level of quality being arranged as a two-dimensional array of signal elements the two-dimensional array of signal elements comprising at least first and second rows of signal elements, wherein a residual in the set of residual data is associated with a respective signal element in the representation of the signal at the first level of quality; deriving a set of transformed residual data based on the set of residual data, at least one transformed residual in the set of transformed residual data being derived from a plurality of residuals in the set of residual data; and outputting data including: data at least derived from the representation of the signal at the second level of quality; and data at least derived from the set of transformed residual data, wherein at least one transformed residual in the set of transformed residual data is derived from: a first number of residuals associated with signal elements from the first row of signal elements in the representation of the signal at the first level of quality, wherein the first number is at least two; and a second number of residuals associated with signal elements from the second row of signal elements in the representation of the signal at the first level of quality, wherein the second number is different from the first number. obtaining a set of residual data, the set of residual data being useable to reconstruct a representation of a signal at a first level of quality using data based on a representation of the signal at a second level of quality, the first level of quality being higher than the second level of quality, the representation of the signal at the first level of quality being arranged as a two-dimensional array of signal elements the two-dimensional array of signal elements comprising at least first and second rows of signal elements, wherein a residual in the set of residual data is associated with a respective signal element in the representation of the signal at the first level of quality; deriving a set of transformed residual data based on the set of residual data, at least one transformed residual in the set of transformed residual data being derived from a plurality of residuals in the set of residual data; and outputting data including: data at least derived from the representation of the signal at the second level of quality; and data at least derived from the set of transformed residual data, data at least derived from the representation of the signal at the second level of quality; and data at least derived from the set of transformed residual data, wherein at least one transformed residual in the set of transformed residual data is derived from: a first number of residuals associated with signal elements from the first row of signal elements in the representation of the signal at the first level of quality, wherein the first number is at least two; and a second number of residuals associated with signal elements from the second row of signal elements in the representation of the signal at the first level of quality, wherein the second number is different from the first number. a first number of residuals associated with signal elements from the first row of signal elements in the representation of the signal at the first level of quality, wherein the first number is at least two; and a second number of residuals associated with signal elements from the second row of signal elements in the representation of the signal at the first level of quality, wherein the second number is different from the first number.