Abstract:
A process and a system is described for generating an MPEG output bitstream starting from an MPEG input bitstream. The output bitstream has a resolution modified with respect to the resolution of the input bitstream. -In the input bitstream, first portions that substantially do not affect and second portions that do affect resolution variation are distinguished. The second portions are then subjected to a function of modification of the resolution obtained by filtering the second portions in a domain of the discrete cosine transform, and then are transferred to the output bitstream. A corresponding computer program product is also provided.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to the processing of bitstreams encoded according to the MPEG standard.  
           [0003]    The MPEG (Moving Pictures Experts Group) standard proposes a set of algorithms dedicated to the compression of sequences of digital (audio/video) signals. The subject of the specification does not regard so much the use of these tools in the encoding phase as rather the way of interpreting the syntax of the encoded bitstream and the use of said tools during decoding (i.e., when carrying out decompression). The techniques used are based on the reduction in spatial and temporal redundancy of the sequence.  
           [0004]    2. Description of the Related Art  
           [0005]    In general, according to the MPEG standard, reduction in spatial redundancy is obtained by independently compressing the individual images, using a discrete cosine transform (DCT), quantization and Huffman coding.  
           [0006]    Reduction in temporal redundancy is obtained by exploiting the correlation that exists between successive and/or temporally close images in the sequence. Approximately it is assumed that each portion of an image could be expressed locally as the translation of a portion of a previous and/or subsequent image in the sequence.  
           [0007]    For this purpose, the MPEG standard reviews three types of images indicated by I (Intra-Coded Frame), P (Predicted Frame), and B (Bidirectionally Predicted Frame).  
           [0008]    The images I are encoded in an altogether independent way; the images P are encoded with respect to a previous image I or P in the sequence; finally, the images B are encoded with respect to two images of an I type or P type, one preceding and the other following in the sequence.  
           [0009]    A typical succession of images may be as follows: IBBPBBPBBIB . . .  
           [0010]    This is the order in which the images are displayed, but since each image P is encoded with respect to the preceding image I or P, and each image B is encoded with respect to the preceding and following image I or P, it is necessary for the decoder to receive the images P before the image B, and the images I before the image P. Consequently, the order of transmission of the images will be IPBBPBBIBB . . .  
           [0011]    The images are processed by the encoder in a sequential way in the order indicated, and subsequently sent to a decoder which decodes them and re-orders them, so enabling their subsequent display. To encode an image B it is necessary for the encoder to maintain the images I and P—encoded and then decoded previously—to which the image B refers, in a special memory referred to as “frame memory”, and this operation requires an appropriate amount of memory.  
           [0012]    The above methodology finds a valid example of implementation in the MPEG 2 and MPEG 4 standards.  
           [0013]    In this connection, the diagram of FIG. 1 illustrates, in the form of a block diagram, the typical structure of a video MPEG encoder.  
           [0014]    The system, designated as a whole by  10 , comprises, in the first place, a module  11  designed to carry out filtering of the chrominance (chroma) component of the video signal passing from the format 4:2:2 to the format 4:2:0. Basically, the module  11  contains a lowpass filter which operates on the chrominance component, replacing each pixel with a weighted sum of the surrounding pixels that are set on the same column multiplied by appropriate coefficients. This enables the subsequent sub-sampling by two to obtain a halved vertical definition of the chrominance.  
           [0015]    The reference number  12  designates a frame-ordering module made up of one or more frame memories. The module  12  is designed to supply at output the frames in the encoding order required by the syntax of the MPEG standard.  
           [0016]    For example, if the input sequence is IBBPBBP, etc., the order at output will be IPBBPBB . . .  
           [0017]    As has already been explained, I (Intra-Coded Picture) is a frame and/or a half-frame containing temporal redundancy; P (Predicted Picture) is a frame and/or a half-frame the temporal redundancy of which with respect to a preceding image I or P (which has been previously encoded/decoded) has been removed; by B (Bidirectionally Predicted Picture) a frame and/or half-frame is indicated the temporal redundancy of which with respect to the preceding image I and the subsequent image P (or else, the preceding image P and the subsequent image P, or again, the preceding image P and the subsequent image I) has been removed. In both cases, the images I and P are to be considered already encoded/decoded.  
           [0018]    The reference number  13  designates the module for estimating motion, i.e., the block that is able to remove the temporal redundancy of the images P and B.  
           [0019]    It is to be recalled that the above block works only on the most energetic component (and hence one that is rich in information) of the images that make up the sequence to be encoded, such as the luminance sequence.  
           [0020]    One of the important concepts for carrying out encoding is the estimation of the motion, and the MPEG standard is based upon the considerations specified below.  
           [0021]    A set of pixels of an image frame may be set in a position of the subsequent image obtained by translation of the image in the previous frame.  
           [0022]    Suppose, for example, that this set of pixels is a square of 16×16 pixels. This set of data, together with the color information associated to it, is usually referred to as “macroblock”.  
           [0023]    Of course, the changes in position of the objects may expose to the filming camera parts that were previously not seen, as well as modifications in the shapes of the objects themselves (for example, as a result of a zooming function, etc.).  
           [0024]    The family of algorithms that are able to identify and associate the said portions of images is referred to as “estimation of motion”. This association makes it possible to calculate the portion of difference image, thus removing the redundant temporal information and rendering the subsequent process of compression by means of a DCT, quantization and entropic encoding more effective.  
           [0025]    The reference number  14  designates a module or block that implements, on the signal coming from an adder node  23  (which will be explained in greater detail later), the DCT according to the MPEG standard. The image I and the images P and B, considered as error images, are divided into 8×8 blocks Y, U, V, on which DCT transformation is applied.  
           [0026]    The reference number  15  designates a quantizer module. Here the 8×8 block resulting from DCT transformation is divided by a matrix, referred to as “quantization matrix”, such as to reduce, more or less drastically, the dimension in number of bits of the DCT coefficients. In this case, the tendency is to remove the information associated to the higher frequencies which are less visible to the human eye. The result is re-ordered and sent to the subsequent block, designated by  16 , which implements the run-length coding (RLC) and the variable-length coding (VLC).  
           [0027]    In particular, RLC aims at taking into account the fact that the code words at output from the quantizer module  15  tend to contain zero coefficients in a more or less high number, followed by non-zero values. The zero values which precede the first non-zero value are counted, and this count constitutes the first portion of a word, the second portion of which is the non-zero coefficient. This method of packeting data is defined as “run-length coding”.  
           [0028]    The result thus obtained undergoes variable-length coding (VLC), also known as Huffman coding.  
           [0029]    This type of coding takes into account the fact that some pairs of values tend to assume more likely values than others. The more likely values are coded with very short words (2/3/4 bits), whereas the less likely values are coded with longer words. Statistically, the number of bits produced at output is smaller than the number of bits at input, or rather the number of bits that there would be if the said coding were not carried out.  
           [0030]    In order to be able to construct the final syntax envisaged by the MPEG standard, the data generated by the variable-length encoder (output from the module 16), the quantization matrices, the vectors of motion (output from the module 13), and other syntactic elements are sent to an assembler module, designated as a whole by 17 and comprising a multiplexer  17   a  and a buffer  17   b.    
           [0031]    The limit size of the buffer is specified by the standard itself and cannot be exceeded.  
           [0032]    The quantization block  15  presides over respect of the said limit, rendering more or less drastic the process of division of the DCT coefficients according to whether the latter are more or less close to filling the buffer and according to the energy of the 8×8 source block taken upstream of the process of estimation of motion and DCT transformation.  
           [0033]    The reference numbers  18  and  19  designate two modules that basically implement a feedback loop to the estimation-of-motion function represented by the module  13 .  
           [0034]    In particular, the module designated by  18  performs on the data undergoing quantization in the module  15  an inverse-quantization function.  
           [0035]    The signals thus obtained undergo inverse DCT (IDCT) in the module  19 . In practice, the DCT function is inverted and applied to the 8×8 block at output from the process of inverse quantization. The function performed in the module  19  enables passage from the domain of spatial frequencies to the pixel domain, obtaining at output:  
           [0036]    the decoded frame (half-frame) I that is to be stored in an appropriate frame memory for subsequent removal of temporal redundancy, with respect thereto, from the subsequent images P and B; and  
           [0037]    the decoded prediction error frame (half-frame) P and B which is added to the information previously removed during the step of estimation of motion; in the P case, this resulting sum, stored in an appropriate frame memory, is used during the process of estimation of motion for the subsequent images P and B.  
           [0038]    The above is performed in the module designated, as a whole, by  20 , where the frame memories are usually distinct from the re-ordering memories.  
           [0039]    The reference number  21  designates the rate-control module which interacts for this purpose with the output of the module  14  and the output of the buffer  17   b , supplying a corresponding control signal mQuant to the module  15 .  
           [0040]    Finally, the reference numbers  22  and  23  designate two adder nodes in which the following are respectively added:  
           [0041]    the output of the IDCT module  19  and the output, designated by  24 , on which the data relating to the motion vectors are transferred from the module  20  to the estimation-of-motion module  13 ; and  
           [0042]    the output of the re-ordering module  12  and the output of the module  20 , and this in view of supply to the module  14  which implements the DCT function.  
           [0043]    The foregoing obviously corresponds to altogether current know-how for persons skilled in the sector, a know-how which is here recalled merely for purposes of reference.  
           [0044]    The same also applies to the structure of an MPEG decoder as represented in FIG. 2.  
           [0045]    In the above-mentioned figure it is possible to note that the said demodulator, designated as a whole by  30 , in the first place carries out, in a module designated by  31 , detection of the so-called “headers” in the framework of the MPEG-encoded bitstream and the subsequent accumulation of the data received within a buffer  32  designed to absorb any discontinuities in the said stream.  
           [0046]    The module  33  is responsible for performing the functions of demultiplexing, inverse VLC decoding, and inverse decoding of the run-level pairs in view of forwarding of the data thus obtained to a module  34 . Here, under the control of the signal mQuant supplied by the module  33  itself on a line  35 , the inverse-quantization function is performed.  
           [0047]    The signal thus obtained is then passed onto to a module  36  which performs the inverse DCT function, the aim being to proceed, in an adder node  37  to reconstruction of the output signal according to the signal generated by the motocompensation node  38  which receives, from the module  33 , the data regarding the motion vectors on a line  39 . In the node  37  also the prediction error is calculated for decoding the subsequent images P and B (line  40 ).  
           [0048]    It may therefore be stated that the processes illustrated in FIGS. 1 and 2 are two concurrent processes cascaded together.  
           [0049]    In the actual use of the MPEG standard it is therefore possible to transmit (or record) films, or, in general, video sequences on a variety of channels and media, each of which has its own characteristics of capacity, speed and cost.  
           [0050]    For example, the distribution of a film starting from the master recording may take place on a DVD medium, via satellite, via radio antenna, or via cable.  
           [0051]    The band available for transmission may therefore be different from the one envisaged in the step of decoding of the video sequence according to the MPEG standard; or else the bitrate obtained during the step of encoding of the video sequences may be in excess as compared to the bitrate allowed by the transmission channel/recording medium.  
           [0052]    Consider, for example, encoding a 6-Mbit/s sequence according to the MPEG 2 standard.  
           [0053]    If the attempt were made to use a 384-kbit/s UMTS (third-generation cellphone transmission standard) channel, it would be possible to consider decoding the sequence, returning to the pixel domain, and then re-encode the 384-kbit/s sequence afterwards.  
           [0054]    In the case where re-encoding was applied to images having the same spatial resolution as the decoded ones, the quality of the bitstream obtained would be very poor in terms of signal-to-noise ratio.  
           [0055]    In addition, the decoder located in the receiving terminal should be able to decode bitstreams in compliance with an MPEG specification containing a different spatial resolution from the one with which the bitstream generated by the first video encoder would comply.  
           [0056]    With regard to the MPEG 2 standard and/or the MPEG 4 standard, there thus emerges the problem of dynamic re-adaptation of the resolution of the compressed images into a bitstream encoded in compliance with an MPEG standard.  
           [0057]    In order to achieve the above target, it is possible to envisage a procedure according to which the MPEG bitstream is simply decoded, and then the horizontal resolution and vertical resolution are changed by operating in the pixel domain. This could be done, for example, passing from a horizontal and vertical resolution Hor×Vert to a final resolution (Hor/N)×(Vert/M), where N and M are positive fractional integers, then to proceed to the subsequent signal re-encoding which undergoes change of resolution by means of an MPEG encoder.  
           [0058]    For example, proceeding as described above, it is possible to have the following cases of change of resolution:  
                                   INITIAL SPATIAL RESOLUTION   FINAL SPATIAL RESOLUTION                   HDTV (1920 × 1088   SDTV_1 (960 × 544 or       or 1920 × 1152)   960 × 576) →           N = M = 2       HDTV (1920 × 1088   SDTV_2 (720 × 480       or 1920 × 1152)   or 720 × 576) →           N = 2.67 M = 2.267 or M = 2       CIF (352 × 240 or 352 × 288)   QCIF → N = M = 2       SDTV_2   CIF → N = M = 2       SDTV   HALF SDTV → N = 2 M = 1                  
 
           [0059]    The above solution is in actual fact very complex from a computational point of view, as may be easily understood if reference is made to the diagram of FIG. 3, which is a schematic illustration of a solution based, precisely, on the criterion of obtaining the change of resolution by operating in the pixel domain.  
           [0060]    If we assume that we are operating on an input bitstream IS encoded according to the MPEG 2 or MPEG 4 standard, the reference number  50  designates a decoder that performs a transformation of the MPEG bitstream (whether it be the specification 2 or the specification 4 is in itself irrelevant) into decoded images ID with a number of pixels per image equal to Hor×Vert.  
           [0061]    The reference number  60  designates a filtering module with downsampling which performs change of resolution bringing said resolution to the value (Hor/N)×(Vert/M).  
           [0062]    The said module may be, for example, a module able to perform change of resolution on the basis of a classic technique that uses finite-impulse-response (FIR) filters.  
           [0063]    The FIR filter in question performs a transformation based upon the availability of a certain number N of pixels for each component of luminance and chrominance of the image. These pixels are multiplied by appropriate weights, and the results are accumulated and divided by the sum of said weights. Finally, some of the pixels are not transmitted in the resulting image, depending upon the mutation factor of the chosen resolution.  
           [0064]    Starting from an encoded bitstream with arbitrary bitrate B 1 , it is always possible to obtain an encoded bitstream with bitrate B 2  by simply connecting the output of the decoder  50  to the input of the change-of-resolution block  60 . The output from the latter is then connected to the input of the encoder  70  programmed to encode at an Mbit/s bitrate B 2 .  
           [0065]    The signal that has undergone change of resolution in the module  60  is then fed to an MPEG encoder  70  which is able to generate a syntax in conformance with the MPEG 2 standard or MPEG 4 standard in view of the transmission schematically represented in T.  
           [0066]    Finally, downstream of transmission (it is to be recalled that, for the purposes of the present invention, here the term “transmission” also includes recording on a physical medium, such as a DVD) the MPEG (re)encoded signal is fed to a decoder  90  which is able to read and decode the bitstream received according to a syntax in conformance with the MPEG standard (either MPEG 2 or MPEG 4) and with a resolution (Hor/N)×(Vert/M), in view of the generation of an output video sequence OS.  
           [0067]    If the block diagrams of FIGS. 1 and 2 are borne in mind, it will be immediately realized that the sequence of processes illustrated in FIG. 3 presents a decidedly high computational complexity.  
           [0068]    If the block diagrams of FIGS. 1 and 2 are borne in mind, it will be immediately realized that the sequence of processes illustrated in FIG. 3 presents a decidedly high computational complexity.  
           [0069]    The transcoding operation represented in the diagram of FIG. 3 entails, in fact, as far as the decoder  50  is concerned, the execution of the following steps:  
           [0070]    inverse Huffman coding;  
           [0071]    inverse Run-Length coding;  
           [0072]    inverse quantization;  
           [0073]    inverse discrete cosine transform;  
           [0074]    motocompensation;  
           [0075]    filtering; and  
           [0076]    change of resolution (where envisaged).  
           [0077]    For the encoder  70 , the following operations become necessary:  
           [0078]    pre-processing;  
           [0079]    estimation of motion;  
           [0080]    calculation of prediction error;  
           [0081]    cosine transform;  
           [0082]    quantization;  
           [0083]    run-length coding;  
           [0084]    Huffman coding;  
           [0085]    inverse quantization;  
           [0086]    inverse discrete cosine transform; and  
           [0087]    motocompensation.  
           [0088]    Finally, for the receiving decoder, the following operations must be carried out:  
           [0089]    inverse Huffman coding;  
           [0090]    inverse run-length coding;  
           [0091]    inverse quantization;  
           [0092]    inverse discrete cosine transform; and  
           [0093]    motocompensation.  
           [0094]    The computational cost lies almost entirely in the estimation of motion, followed by the direct and inverse cosine transforms and motocompensation. Quantization and the (direct and inverse) run-length and Huffman codings constitute, instead, a contribution smaller than the previous ones to the overall cost.  
           [0095]    The quality of the resulting output bitstream OS derives, instead, from the information content of the quantized coefficients. This depends upon the implementation of the encoder (the decoder is uniquely defined by ISO/IEC 13818-2 Directives for the MPEG 2 standard and by ISO/IEC 14496-2 Directives for the MPEG 4 standard), upon the effectiveness of its estimator of motion, and upon the quality and precision of the rate control.  
           [0096]    A solution on the whole similar to the one described above with reference to FIG. 3 is illustrated in U.S. Pat No. 6,005,621. In particular, this document describes a method for explicit transcoding which envisages complete decoding of an input bitstream, scaling of the images, and subsequent encoding of a number of output bitstreams with different resolutions by means of a technique of hierarchical estimation of motion in the spatial domain.  
         BRIEF SUMMARY OF THE INVENTION  
         [0097]    Aspects of the present invention provide a solution that is capable of enabling change of resolution of an MPEG bitstream without having to resort to the extremely burdensome solution illustrated previously.  
           [0098]    Aspects of the invention also regard the corresponding system (which can be implemented, for example, in the form of a dedicated processor, such as a DSP), as well as the corresponding computer program product, namely, the set of program codes which may be loaded in the memory of a digital processor, in particular of the general-purpose type, and which may enable the processor in question to carry out the process according to aspects of the invention.  
           [0099]    Basically, the solution according to aspects of the invention envisages the merging of a decoder with an encoder in an ensemble designed specifically for variation or else conservation of the bitrate of a bitstream.  
           [0100]    The solution according to aspects of the invention enables reduction in computational complexity and an improvement or conservation of the quality of the output signal with respect to the input signal.  
           [0101]    The solution according to aspects of the invention makes it possible to achieve dynamic re-adaptation of the resolution of compressed images in a bitstream encoded according to the MPEG standard with a high-quality result and with the possibility of using system architectures that are optimized in terms of storage capacity thanks to recourse to structures based upon multipliers that implement the inverse-quantization function and the filters. This applies in particular to the possibility of using the multiplying structure designed to implement inverse quantization also for the filtering required by the range of resolution.  
           [0102]    The solution according to aspects of the invention basically performs filtering in the domain of the discrete cosine transform (DCT) instead of in the pixel domain. This is perfectly legitimate in so far as DC transformation and the convolution operation (which lies at the basis of filtering) are linear operations, and hence interchangeable in order of execution. The fact that filtering is performed in the DCT domain implies the availability of the decompressed image in said domain, hence the availability of the motocompensation operation in compliance with the MPEG specification.  
           [0103]    The solution according to aspects of the invention therefore provides a method alternative to “explicit” decoding and re-encoding of the type described previously with reference to FIG. 3.  
           [0104]    The solution according to aspects of the invention operates directly in the DCT-encoded domain, at a considerably lower computational cost, at the same time enabling not only regeneration of a bitstream containing a reduced-resolution image, but also the possible viewing of the images on which change of resolution is taking place, precisely at the moment at which the said change is occurring. This option enables the user to examine the quality obtained instant by instant, in order to intervene, if need be, on said quality.  
           [0105]    The solution according to aspects of the invention thus presents two main strong points:  
           [0106]    it avoids the need, when changing resolution, to (re)estimate the motion; the solution according to aspects of the invention in fact re-uses information of the input bitstream to generate the output bitstream; and  
           [0107]    it avoids the need, when changing resolution, to cascade a DCT function and its inverse function IDCT, since the solution according to aspects of the invention operates entirely in the frequency domain.  
           [0108]    In the solution according to aspects of the invention, the portions of bitstream that do not significantly affect reduction of the bitrate are not processed, but simply translated according to the syntax and resolution of the target standard. The motion vectors are appropriately filtered by means of a transformation based upon the availability of a certain number M of motion vectors associated to the macroblocks which are to be merged into the new macroblock, or else surround those that are to be merged into the new macroblock. The motion vectors are multiplied by appropriate weights, and the results are accumulated and divided by the sum of the weights. In fact, the motion field must be appropriately scaled to be associated to the pixel macroblocks that characterize the target resolution.  
           [0109]    The portions of the bitstream that significantly affect the reduction in bitrate are basically the DCT coefficients. In particular, the DCT coefficients undergo the following processing operations:  
           [0110]    inverse variable-length coding (VLC);  
           [0111]    inverse quantization (Q); and  
           [0112]    motocompensation and storage.  
           [0113]    Other features and advantages of the invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings.  
       
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)  
       [0114]    An embodiment of the invention will now be described, purely by way of non-limiting example, with reference to the attached drawings, in which:  
         [0115]    FIGS.  1  to  3 , which regard the prior art, have already been extensively described previously;  
         [0116]    [0116]FIG. 4 illustrates, in the form of a block diagram, an embodiment of the solution according to the invention; and  
         [0117]    [0117]FIG. 5, illustrates one embodiment of the present invention, having components a), b), and c); and  
         [0118]    [0118]FIG. 6 illustrates one embodiment of a solution according to the invention in greater detail. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0119]    Basically, the purpose pursued by the solution illustrated in FIG. 4 is to start from an input bitstream IS (whether MPEG 2 or MPEG 4) and to generate, starting therefrom, an output bitstream OS (again, either MPEG 2 or MPEG 4, according to the requirements), with the possibility of effecting, in addition to the change of resolution, also a change of syntax and/or bitrate.  
         [0120]    It will, on the other hand, be appreciated that it is by no means imperative to make all three of the above changes simultaneously. Consequently, even though with reference to FIG. 4 a solution will be described that is able to perform all three changes in question, an embodiment of the solution according to the invention is suitable for being applied in contexts in which the only change made is the change of resolution.  
         [0121]    The input bitstream IS is fed from a sorting module  100 , which performs a function of parsing of the headers. This function basically aims at distinguishing the portions of the bitstream that are not useful for the purposes of reducing the resolution from the portions which, instead, are useful for this purpose (basically, the DCT coefficients).  
         [0122]    The former portions of bitstream are sent, through a line  102 , to a module  104  which carries out the function of change of resolution and syntax by accessing the syntax fields which store the aforesaid values and by changing their binary coding into the values corresponding to the target resolution and bitrate.  
         [0123]    The latter portions of the bitstream (the ones useful for the purpose of reducing the resolution) are, instead, sent along a line  106  to a block  108  which basically carries out the inverse VLC transform.  
         [0124]    In particular, the motion vectors that derive from this operation are sent back, on a line  110 , to a block  112 , which monitors the function of reshaping of the motion vectors. This is basically a transformation based upon the availability of a certain number M of motion vectors associated to the macroblocks that are to be merged into the new macroblock or surround those that are to be merged into the new macroblock. The motion field must be appropriately scaled to enable the association of pixels that characterize the target resolution to the macroblocks.  
         [0125]    Downstream of the module  108 , there is also performed, in a module  114 , the inverse-quantization function (IQ).  
         [0126]    At output from the module  114  it is possible to obtain:  
         [0127]    the decoded frame (half-frame) I, in the DCT domain, which is to be stored in a frame memory so that it can then contribute to restoring temporal redundancy to subsequent images P and B according to the motocompensation process described in the MPEG standard; and  
         [0128]    the decoded prediction error frame (half-frame) P and B, in the DCT domain, which is to be added to the information previously removed during the step of estimation of motion; in the P case, this resulting sum, with respect to the preceding I or P, is stored in an appropriate frame memory and is then used during the process of compensation of motion for the subsequent images P and B.  
         [0129]    More precisely, storage takes place in a memory block  116  having the function of a frame buffer connected at input both to the output of the module  112  and to the output of a module  118  which performs (according to criteria which will be better described in what follows) the function of a horizontal and vertical downsampling filter. The output of the buffer  116  constitutes the input of a further module  120  designed to perform a horizontal and vertical upsampling filtering function, which is complementary to the function performed by the filter  118 .  
         [0130]    Both the filter  118  and the filter  120  operate according to filtering matrices applied on the respective inputs designated by FM.  
         [0131]    The output of the module  120  (which in practice constitutes the prediction signal of the motocompensation process) is sent to a corresponding motocompensation module  122 , which also receives the signal at output from the module  114 .  
         [0132]    The output signal of the module  122  constitutes, precisely, the input of the filter  118 .  
         [0133]    More specifically, the inverse oversampling function represented by the filter  120  is supplied by the blocks read by the frame memory  116  and is addressed by the vectors obtained at output from the block  112 . The data thus oversampled are sent to the motocompensation unit represented by the block  122 .  
         [0134]    It will be appreciated that the motocompensated images obtained according to the aforesaid criterion have the same resolution Hor×Vert as the input signal. They are not, however, stored in the module  116  with this resolution, but rather with the resolution obtained downstream of the block  118  that performs horizontal and vertical filtering in the DCT domain.  
         [0135]    The latter resolution is precisely the one resulting from the downsampling function, namely (Hor/N)×(Vert/M), with corresponding dimensions in the storage module  116 . In particular, this storage module may have dimensions [(Hor/N)×(Vert/M)×1.5×1.5×2] for storing two frames with a precision of 12 bits per coefficient in the DCT domain and in format  420 , which are useful for the motocompensation process.  
         [0136]    The signal subjected to reduction in resolution at output from the filter  118  can be sent, along a line  118   a,  to a module  119  which carries out the inverse DCT (IDCT) function, in such a way as to render visible, for example on a monitor M, the images on which change of resolution is taking place. This can be done exactly at the moment when the change is occurring: the user of the system can thus inspect the quality obtained instant by instant so as to be able to intervene, if need be, on the process of change of resolution, basically via the matrices FM.  
         [0137]    The reference number  126  designates, instead, a further line on which the results deriving from the inverse VLC coding operation are sent to a module  128 , which basically superintends a redefinition of the macroblock parameters according to the modalities described in greater detail in what follows.  
         [0138]    The aim of the foregoing is to arrive, in the module designated as a whole by  130 , at an action of reshaping of the macroblocks, which, after a prior new VLC coding, performed in the module designated by  132 , are sent back to an output node  134  in which the portions of bitstream originally switched on the line  102  and on the line  106  are again recombined together so as to generate the output bitstream OS.  
         [0139]    In particular, the block  130  operates on the basis of the signal generated by the module  112  which superintends reshaping of the motion vectors. The foregoing is done according to the signal coming from a module  131 , which performs the function of re-quantization of the modified-resolution signal received on a line  118   a  from the filter  118 .  
         [0140]    It will be noted that the inverse VLC decoding operation performed in the module  108  and the (new) VLC coding operation performed in the module  132  are in fact linked together to take into account the MPEG 2 and MPEG 4 standards involved (respectively at input and at output). The aforesaid modules receive at input also the weighting matrices, which may possibly be defined by the user and introduced into the system on a line  136  and used by a module  138 .  
         [0141]    In order to make the change of resolution, an embodiment of the solution according to the invention then proceeds to a filtering in the DCT domain. This takes place according to the modalities illustrated in greater detail in FIGS. 5 and 6.  
         [0142]    Of course it is to be recalled that, as has already been said previously, the functional-block representation provided by FIG. 5 corresponds to processing operations that may be carried out using both dedicated processors and general-purpose processors that are adequately programmed (in a way of itself known, once the functional specifications that it is intended to adopt are known).  
         [0143]    The part a) of FIG. 5 shows, for example, how from four luminance macroblocks (each consisting of 16×16 pixels) designated by Y 1  to Y 4  just one is extracted EY, designated by Y, in the case of a subsampling factor equal to 2.  
         [0144]    The parts of FIG. 5 designated by b) and c) show, instead, that, for the chrominance component, respectively U and V 4:2:0 it is necessary to have available four 8×8 blocks designated by U 1  to U 4  and V 1  to V 4  to merge them, FU and FV, into a single block, designated by U or V, by means of filtering.  
         [0145]    The filtering operation is then based upon steps illustrated in FIG. 6.  
         [0146]    In particular, if a certain number of 8×8 MPEG blocks arranged on one and the same horizontal line of a local buffer are designated by MB 1  to MB 4 , these are made available in a number of at least three to the horizontal filter  1181  comprised in the module  118  in FIG. 4. This implements the multiplication of the above-mentioned macroblocks by an appropriate number of matrices of size H×V, thus obtaining a new set with halved horizontal definition (HDD).  
         [0147]    The blocks thus generated are stored and arranged on the same vertical line of a second local buffer so as to make at least three of them available to the vertical filter  1182  comprised in the module  118  in FIG. 4. This multiplies the macroblocks by an appropriate number of matrices having size H×V, thus obtaining a new set with halved vertical definition.  
         [0148]    In this way, the equivalent macroblock (shown in the right-hand part of FIG. 5) can be sent to the module  116 , to the module  119  that implements the inverse cosine transform for complete decoding of the images (in this way making the images simultaneously available to the transcoded bitstream), and to the module  131 , which, when it receives the macroblock filtered by the module  118 , re-quantizes it according to the quantization parameter obtained at output from the block  128 . The result is then sent to the block  130 .  
         [0149]    The block  130  receives the data from the block  112 , the module  131 , and the module  128  so as to generate the new macroblock, which is sent to the module  132 . The data thus obtained are multiplexed in  134  with the data coming from the module  104 .  
         [0150]    In  128 , the values of the parameters quantizer_scale_code and quantizer_scale_type are redefined.  
         [0151]    As far as the former parameter is concerned, the following alternatives are possible:  
         [0152]    re-using the one present in the input bitstream IS; and  
         [0153]    calculating the weighted mean value for appropriate coefficients, the minimum value, the median value, or else the maximum value of N quantizer_scale_code values associated to the macroblocks present in the input bitstream, the said macroblocks being the ones that supply the block  118 .  
         [0154]    It will be appreciated that the main advantage of an embodiment of the solution according to the present invention derives, in terms of computational gain, from the elimination of the motocompensation, estimate of motion, and inverse and direct cosine transform blocks.  
         [0155]    Of course, without prejudice to the principle of the invention, the details of implementation and the embodiments may vary widely with respect to what is described and illustrated herein, without thereby departing from the scope of the present invention as defined in the annexed claims.  
         [0156]    From the foregoing it will be appreciated that, although specific embodiments of the invention have been described herein for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the invention. Accordingly, the invention is not limited except as by the appended claims.