is obtained from the corresponding photogram at time t and at maximum level k through the relationship following the construction of photograms at different levels the motion field computation is performed at different levels in the present invention the motion field is obtained when coding through an iterative procedure such procedure first computes a motion field at the lowest resolution by minimising a function of the optical flow type in order then to compute motion fields for greater and greater resolutions rising along the pyramid of resolutions till the greatest one is reached for which the final motion field will be computed at all other resolutions the motion field is computed by minimising a cost function in which a term represents the similarity between solution to be computed and already obtained solution for the level below minimising can be obtained by solving a system of equations let us assume a generic resolution k u2212 m and a generic time instant t the two photograms being taken into account related to such resolution at time t and at time t 1 are g t f t k u2212 m and g t 1 f t 1 k u2212 m according to the optical flow technique a matrix is built which contains the first space derivatives with respect to cartesian coordinates x and y on two diagonals and which is equal to zero in the other elements a k u2212 m diag g t 1 x diag g t 1 y 2 the optical flow is initialised with horizontal u 0 x y and vertical v 0 x y motion fields assuming in such a way that the pixel in x y is moved by a vector the first term of the function.