A method of coding a video signal, comprising: (a) analysing pictures to obtain, in respect of each region thereof analysed, a measure of predictive power, each measure being dependent on the similarity of the region to a further region for which it is to be used as a predictor, wherein the said further region is a region of another frame, wherein the analysing comprises: i) performing a trial coding of the signal in a reverse direction by a video coder that makes a decision for each region as to whether that region is coded by inter-frame differential coding or not, and include in its output a flag so indicating; ii) examining the results of the trial coding to determine for each region whether it meets a criterion of similarity to a further region for which it is to be used as a predictor, wherein the criterion of similarity is that the flag in respect of the region under consideration is indicative of a decision to use inter-frame differential coding; iii) each measure of predictive power is calculated cumulatively in that, if the criterion of similarity is met, setting the measure of predictive power for that region to be a fixed increment of the measure of predictive power of the region that is used as its predictor, and if the criterion of similarity is not met, setting the measure of predictive power for that region to be a fixed amount; and (b) coding the signal in a forward direction, using differential coding including inter-frame differential coding, with a quantisation parameter that varies as a function of the measures of predictive power. (a) analysing pictures to obtain, in respect of each region thereof analysed, a measure of predictive power, each measure being dependent on the similarity of the region to a further region for which it is to be used as a predictor, wherein the said further region is a region of another frame, wherein the analysing comprises: i) performing a trial coding of the signal in a reverse direction by a video coder that makes a decision for each region as to whether that region is coded by inter-frame differential coding or not, and include in its output a flag so indicating; ii) examining the results of the trial coding to determine for each region whether it meets a criterion of similarity to a further region for which it is to be used as a predictor, wherein the criterion of similarity is that the flag in respect of the region under consideration is indicative of a decision to use inter-frame differential coding; iii) each measure of predictive power is calculated cumulatively in that, if the criterion of similarity is met, setting the measure of predictive power for that region to be a fixed increment of the measure of predictive power of the region that is used as its predictor, and if the criterion of similarity is not met, setting the measure of predictive power for that region to be a fixed amount; and i) performing a trial coding of the signal in a reverse direction by a video coder that makes a decision for each region as to whether that region is coded by inter-frame differential coding or not, and include in its output a flag so indicating; ii) examining the results of the trial coding to determine for each region whether it meets a criterion of similarity to a further region for which it is to be used as a predictor, wherein the criterion of similarity is that the flag in respect of the region under consideration is indicative of a decision to use inter-frame differential coding; iii) each measure of predictive power is calculated cumulatively in that, if the criterion of similarity is met, setting the measure of predictive power for that region to be a fixed increment of the measure of predictive power of the region that is used as its predictor, and if the criterion of similarity is not met, setting the measure of predictive power for that region to be a fixed amount; and (b) coding the signal in a forward direction, using differential coding including inter-frame differential coding, with a quantisation parameter that varies as a function of the measures of predictive power.