1. An encoding method for the compression of a video sequence including successive frames organized in groups of frames, each frame being decomposed by means of a three-dimensional (3D) wavelet transform leading to a given number of successive resolution levels, said encoding method being based on the hierarchical subband encoding process called “set partitioning in hierarchical trees” (SPIHT) and leading from the original set of picture elements (pixels) of each group of frames to wavelet transform coefficients encoded with a binary format and constituting a hierarchical pyramid, said coefficients (i,j) being organized into a spatio-temporal orientation tree rooted in the lowest frequency (or approximation subband) resulting from the 3D wavelet transform and completed by an offspring in the higher frequency subbands, the coefficients of said tree being ordered into partitioning sets involving the pixels and corresponding to respective levels of significance, said sets being defined by means of magnitude tests leading to a classification of the significance information in three ordered lists called list of insignificant sets (LIS), list of insignificant pixels (LIP) and list of significant pixels (LSP), said tests being carried out in order to divide said original set of pixels into said partitioning sets according to a division process that continues until each significant coefficient is encoded within said binary representation, and said spatio-temporal orientation tree defining the spatio-temporal relationship inside said hierarchical pyramid, said method being further characterized in that, according to the algorithm indicated in the appendix A: (a) starting by the pixels in the highest subbands and finishing by those belonging to the lowest ones, a full exploration of the subbands is performed during the initialization step of the process, and the set significance level SSL of each subtree of said tree in the root pixels is calculated and stored, according to the following procedure: if the coefficient (i,j) has no child, SSL(i,j)&equals;−1; else SSL(i,j)&equals;max &lcub;PSL(k,l), SSL(k,l)&rcub;; (with k and l &Egr; O(i,j), the offspring corresponding to the coefficient (i,j), and PSL is the pixel significance level given by: PSL ( i,j )&equals; floor ( log 2 x ( i,j )), (b) in the sorting step of the process, a comparison between SSL(i,j) and the current significance level n replaces the call to the function that computes the significance of a tree relatively to n: if SSL(i,j)<n, the subtree whose the root is (i,j) is insignificant relatively to n, else, the subtree is significant.