in brief , a block - compression method and a corresponding compression apparatus are proposed , which envisage execution of a jpeg compression with a pre - set or desired bitrate . the bitrate is obtained using a procedure of a statistical type and is based upon observation by the applicant of a statistical fact : the value of bits per pixel bpp can be set , in fact , in relation to the number of zeros in the quantized coefficients obtained from the discrete cosine transform via a relation of a linear type . fig3 shows a graph that gives , as a function of a zero percentage zp on the abscissa , the corresponding value of bits per pixel bpp on the ordinate . a dashed line rl represents a linear regression calculated on the values of bits per pixel bpp that can be assumed . according to the compression method proposed , it is envisaged to execute a retrieval step of a statistical type that enables identification of a mean interpolation point pmi given by the mean of the values of zero percentage zp obtained at a constant value of bits per pixel bpp . said mean interpolation point pmi depends upon a desired value of bits per pixel t and upon a size of the image s . for this reason , for each desired image size s and value of bits per pixel t , the zero percentage zp that must contain the quantized coefficients is known . consequently , the method proposed substantially regulates the number of zeros of the quantized coefficients in order to satisfy the aforementioned constraint . the method according to the invention hence envisages execution of the following steps . execution of a first mild quantization operation on dct coefficients with a first gain value g 0 in order to obtain a granular histogram h , hence with a value sufficiently low to ensure the highest possible number of coefficients quantized with different values . construction of said histogram h collecting all the quantized coefficients of the image data ; an example of said histogram h is shown in fig4 , where given on abscissa is an index “ i ” of the coefficients and on the ordinate a number nc of coefficients for each given index “ i ”. fixing of a zero - percentage point xp on the abscissa of the histogram h ; all the coefficients falling under said zero - percentage point xp must be zeros . calculation of a new gain value g t starting from the zero - percentage point xp . quantization of the dct coefficients with the new gain value so as to obtain the desired value of bits per pixel t . fig5 shows a block diagram of a coding apparatus that implements the method according to the invention . in fig5 , the blocks having functions similar to those of the blocks of fig1 are designated by the same reference numbers . in said apparatus , the image data i , already converted into the ycbcr form of representation deriving from the original rgb format , are then sent on to the dct block 10 , which produces at output the transform coefficients i dct . said transform coefficients i dct are stored in a dynamic - memory element 90 , in particular a dram ( dynamic random access memory ) and are then supplied to the quantization block 20 . said quantization block 20 receives and uses the scaled quantization level { tilde over ( q )}, which is calculated as the product of the gain factor g and a quantization level q , produced by the quantization table 60 according to equation 1 given above . said quantization block 20 hence produces at output quantized data qd , which contain the quantized dct coefficients . according to one aspect of the invention , a rate - control block 100 is provided , which supplies to a multiplier 95 the gain value g to be multiplied by the value of quantization level q in order to implement equation 1 and to supply the scaled quantization level { tilde over ( q )} to the quantization block 20 . the rate - control block 100 receives at one input thereof a desired value of bits per pixel t and a desired value of size of the image s . according to the invention , the coding apparatus illustrated in fig5 presents at least two distinct operating steps . in a first operating step , the transformed signal i dct is written into the dynamic memory 90 , and then applied thereon in the quantization module 20 is a mild quantization , with an initial gain value g 0 described previously , to produce mildly quantized data qd 0 . said mildly quantized data qd 0 , i . e ., dct quantized coefficients , are sent to a rate - control block 100 so as to populate the histogram h , already described with reference to fig4 , and evaluate the population of the histogram . as will be described in greater detail hereinafter , with reference to fig6 , a final gain value g t is then calculated , which is used to quantize again , in the quantization block 20 , the transformed signal i dct stored in the dynamic memory 9 and produce at output final quantized data qd 1 , which are then supplied at input to the zigzag - ordering block 30 and to the subsequent huffman - coding block 49 to generate the jpeg compressed image o . it should be noted that , notwithstanding the two distinct operating steps , the method is configured as a one - pass method , i . e ., a method executed just once , in so far as the main blocks involved in the jpeg compression are used just once . only the quantization operation is performed twice . fig6 shows a detail of the rate - control block 100 . in said rate - control block 100 , the mildly quantized data qd 0 are sent on to a module for population of the histogram 110 , which populates the histogram h of fig4 . the desired value of bits per pixel t and the value of size of the image s are instead supplied as input data for a search in a pre - loaded look - up table 140 , which supplies at output a threshold value r 0 , which indicates heuristically the correct zero percentage zp . said threshold value r 0 is supplied , together with values of population d of the histogram , generated by the module for the population of the histogram 110 , to a block for determining the index 120 , which derives the index “ i ” of the coefficient . said index “ i ” is supplied to a gain - calculation block 130 , which supplies at output the final gain value g t . said final gain value g t is sent on to one of the inputs of a multiplexer 15 , which receives at another input thereof the initial gain value g 0 , supplied by the pre - loaded look - up table 140 . in the first operating step , the multiplexer 160 selects as gain g said initial gain value g 0 , which is chosen heuristically so as to enable a mild quantization to be carried out at block 20 that will ensure a histogram h with a fine granularity . preferably , with 1024 coefficients in the quantized data qd , g 0 is set equal to 1 / 1024 , in so far as the hardware implementation is simpler . in the second operating step , the multiplexer 150 selects , instead , as gain g the final gain value g t . the block for determining the index 120 at the end of the first operating step calculates the value of the index “ i ” according to the following relation : as may be inferred from equations 4 , the value 0 . 49 is a factor chosen from among all the ones that satisfy the condition of inequality ( i / g & lt ; 0 . 5 ). where with h ( j ) are the values of the histogram h defined previously . the pre - loaded look - up table 140 contains data obtained heuristically . the parameters depend upon the size of the image s and upon the desired value of bits per pixel t . the pre - loaded look - up table 140 contains the initial gain value g 0 and the threshold value r 0 . the initial gain value g 0 is the first multiplier of the quantizer q that is used in the first step . the threshold value r 0 contains the zero percentage zp used in the block for determining the index 120 . table 1 shows an example , corresponding to vga and cif images with 3 . 2 bpp , 1 . 6 bpp , 0 . 8 bpp of desired value of bits per pixel t , of values assumed by the initial gain value g 0 and by the threshold value r 0 . fig7 and 8 show histograms of error distribution corresponding to tests performed on the coding apparatus described with reference to fig5 and 6 . the desired value of bits per pixel t is 3 . 2 and the resolution is vga for the histogram of fig7 and cif for the histogram of fig8 . for the tests , a database was used with a number of images in equal to 110 , captured with a cmos - vga sensor in an “ stv6500 - e01 ” evaluation kit equipped with a sensor “ 502 vga ”, as described in colour sensor evaluation kit vv6501 , stmicroelectronics , edinburgh , www . edb . st . com , 2003 . the histograms give the percentage image number ni % as a function of the percentage deviation dev %. consequently , without prejudice to the principle of the invention , the details of implementation and the embodiments may vary , even significantly , with respect to what is described and illustrated herein , purely by way of non - limiting example , without thereby departing from the scope of the invention , as defined in the ensuing claims .