1. A method of estimating motion in a picture sequence in which the luminosity of each element is expressed in a digital form and each of these elements is defined by two coordinates (m,n), respectively, indicating the row of the picture line in which the element is present and the row of the element on this m.sup.th line, comprising: (A) preserving the preceding picture with respect to the current picture for constituting a reference picture; (B) defining three groups as fixed elements, moving elements and as exposed elements, in which groups the elements are classified with a delay of L elements according to scanning of the picture by examining a given number of possible group sequences for the L elements succeeding the current element X considered, and in accordance with a given classification criterion, said criterion for the fixed and moving elements being the difference in luminosity between two successive pictures, taking the displacement of the elements of one picture with respect to the other into account for the moving elements, and the difference in luminosity between two adjacent lines for the exposed elements; (C) constructing a classification tree whose 3.sup.L branches at each successive level are equal in number to those of the possible groups and which correspond from the first to the L.sup.th level to the sequences of possible groups for the L considered successive elements of the picture sequence; (D) associating two parameters with each branch of each level: (a) a first parameter referred to as displacement vector representative of the possible displacement of the element with respect to the picture or to the preceding line and determined by means of a recursive displacement estimation method; and (b) a second parameter referred to as cumulated distortion representative of the cumulative sum of the classification errors of the preceding elements, the groups of these elements being those of the tree branches traversed to arrive at the current branch, each classification error being rendered minimum by taking into account the influence of a displacement estimated for an element on the displacement estimated for the L following elements, said influence being only examined, among the 3.sup.L possible branches of the tree, for M branches of the weakest cumulated distortion referred to as surviving branches, where M is a limited number chosen to be substantially smaller than the maximum number of branches 3L; (E) and in accordance with this procedure relating to L elements, taking a decision of the majority type concerning the effective group and concerning the displacement vector of the element situated L elements earlier with respect to the current element, said decision being taken by examining the surviving branches and by classifying each element situated L elements earlier in the group to which the largest number of these surviving branches corresponds, the corresponding displacement vector being thus considered as the displacement of this element situated L elements earlier, said decision procedure being identically repeated but for the L last elements of the picture sequence in which one avoids taking a decision.