1. A method of preprocessing input data corresponding to picture elements (pixels) that represent arbitrarily shaped objects, said input data comprising for each object a texture part, corresponding to the values of the pixels of said object, and an object mask, subdividing said input data into a first and a second subset of data respectively corresponding to fully or partially opaque pixels and to transparent pixels in said texture part, said preprocessing method being provided for determining DCT (discrete cosine transform) coefficients corresponding to said opaque pixels and comprising for each considered object the steps of: (1) partitioning the object plane into bidimensional blocks; (2) introducing in the picture area defined by said block a set of basis vectors chosen in order to express an estimate of the original pixel values as a linear combination of said basis vectors; (3) defining a cost function V to measure the distortion between the original expression of the pixel values and said estimate of this original expression; (4) finding the coefficients allowing to minimize the cost function &psgr;; characterized in that: (a) said cost function T is given by a relation of the type: 6 ψ = ( f opaque, &Sum; i = 1 64 &it; &it; c i &it; b opaque &af; ( i ) ) where f is the column-vector of the pixels of the concerned block, ((b i ), i&nlt;(1 to 64)) are the basis vectors of a 8×8 DCT, f opaque is the restriction of f to the opaque pixels of said block, ((b opaque ), i&nlt;(1 to 64)) are the restriction of said basis vectors to the location of the opaque pixels of the block, and 7 &Sum; i = 1 64 &it; &it; c i &it; b opaque &af; ( i ) is called the reconstruction of f opaque ; (b) said finding step itself comprises the following operations: initialization of the following parameters, including: iteration parameter k&equals;0; initial estimation of f opaque E &equals;0; initial reconstruction coefficients c i 0 &equals;0; extraction of the basis vectors restricted to the opaque pixels and calculation of the projection coefficients: p i 0 &equals;&lcub;( f opaque −f opaque E ), b opaque(i)&rcub; with &lcub; &rcub; denoting the cross-correlation function, i varying from 1 to 64, and (b opaque ) being said restricted basis vectors; iteration(s), each of said iteration being provided for performing the following sub-steps: &lsqb;a&rsqb; finding the index i* of the basis vector which best contributes to minimize the cost function; &lsqb;b&rsqb; updating the reconstruction of f opaque E according to the relation f opaque E ( k&plus; 1)&equals; f opaque E ( k ) &plus;p i k ·b opaque(i) &lsqb;c&rsqb; and updating the reconstruction coefficients c i k&plus;1 &equals;C i k for i≠i* and c j* k&plus;1 &equals;c i* k &plus;p i* k and the projection coefficients p i* l&plus;1 ; interruption of said iterations if said cost function &psgr; is below a given threshold or if a predetermined number of iterations is reached.