A method of generating a measure of quality for a decoded form of a video signal, the video signal having: an original form; an encoded form in which the video signal has been encoded using a compression algorithm utilising a variable quantiser step size such that the encoded signal has a quantiser step size parameter associated therewith and utilising differential coding such that the encoded signal contains representations of the prediction residual of the signal; and said decoded form in which the encoded video signal has been at least in part reconverted to the original form, the method comprising: generating a quality measure dependent on said quantiser step size parameter according to a predetermined relationship, generating a masking measure for each of a plurality of types of regions present in pictures formed by decoding the video signal, and applying a different weighting to each masking measure, wherein each masking measure is dependent on a measure of spatial complexity of the regions of the corresponding type, and combining the quality measure and the plurality of weighted quality masking measures to generate a combined quality measure for the decoded form of the video signal and wherein one type of region comprises an active region within a given focus of attention region of a picture in said video sequence for which a prediction residual measure exceeds a prediction residual threshold or a measure of spatial complexity exceeds a spatial complexity threshold as a first condition, and characterised in that: at least one other type of region comprises a background region of the focus of attention region of the picture in said video sequence for which another condition is met, wherein the other condition comprises: the prediction residual measure not exceeding the prediction residual threshold; and a measure of spatial complexity not exceeding a spatial complexity threshold. generating a quality measure dependent on said quantiser step size parameter according to a predetermined relationship, generating a masking measure for each of a plurality of types of regions present in pictures formed by decoding the video signal, and applying a different weighting to each masking measure, wherein each masking measure is dependent on a measure of spatial complexity of the regions of the corresponding type, and combining the quality measure and the plurality of weighted quality masking measures to generate a combined quality measure for the decoded form of the video signal and wherein one type of region comprises an active region within a given focus of attention region of a picture in said video sequence for which a prediction residual measure exceeds a prediction residual threshold or a measure of spatial complexity exceeds a spatial complexity threshold as a first condition, and characterised in that: at least one other type of region comprises a background region of the focus of attention region of the picture in said video sequence for which another condition is met, wherein the other condition comprises: the prediction residual measure not exceeding the prediction residual threshold; and a measure of spatial complexity not exceeding a spatial complexity threshold. at least one other type of region comprises a background region of the focus of attention region of the picture in said video sequence for which another condition is met, wherein the other condition comprises: the prediction residual measure not exceeding the prediction residual threshold; and a measure of spatial complexity not exceeding a spatial complexity threshold. the prediction residual measure not exceeding the prediction residual threshold; and a measure of spatial complexity not exceeding a spatial complexity threshold.