referring now to fig1 a system is illustrated for measuring blocking artefacts in an image and storing a measure for visualisation and analysis by a user . a non - intrusive quality assessment system 1 is connected to receive a signal representing an image 3 . the system 1 comprises a parameter extractor processor 6 arranged to extract parameters which are relevant to quality from the image 3 and a store 4 connected to receive and store quality measures . extracted parameters are used by quality measure processor 2 ( which may or may not be part of the same system as processor 6 ) to generate a quality measure which is then sent to analysis and visualisation module 5 ( which may or may not be part of the same system as processors 2 or 6 ) to analyse the extracted measures of quality and to provide a user with a prediction of the perceived quality of the image . details relating to images which have been analysed are stored for later reference . quality prediction models typically produce a set of intermediate parameters from the input signal ( or signals in the case of a full - reference model ) such that each parameter changes in response to the presence and severity of one or more classes of image impairment . said intermediate parameters are then combined to produce a single quality prediction value that correlates with the mean opinion score ( mos ) that would be obtained for the decoded input signal when assessed by human subjects in a subjective experiment . the parameter combination step can be a simple weighted sum . methods for optimising the relative weights of the parameters , like multi - variable regression , are well known to those skilled in the art and are not directly relevant to the present invention . an example of a video quality prediction model that uses an intermediate set of parameters as described above is provided in annex a of itu - t recommendation j . 144 , “ objective perceptual video quality measurement techniques for digital cable television in the presence of a full reference ”, with the weighted sum of the parameters performed according to equation a . 4 - 2 . itu - r recommendation bt - 500 , “ methodology for the subjective assessment of the quality of television pictures ” describes methods of performing subjective experiments for video signals . a sequence of images comprising frames of a video sequence may be analysed and the quality prediction may be updated so that over a period of time the quality prediction relates to a plurality of analysed frames of data comprising a video sequence . referring now to fig2 , 3 and 4 a method of determining a parameter relating to block artefact will now be described . the measure of blockiness used in the present invention is obtained from computing the variation of pixel luminance within defined blocks of the image . in fig2 , an image data frame ( digital image ) 3 is shown schematically decomposed into nblocks each of size blocksizex × blocksizey separated horizontally by blockshiftx pixels , and separated vertically by blockshifty pixels . fig2 shows blocksizex equal to blockshiftx and blocksizey = blockshifty , but it will be appreciated that this need not be the case . the measure is based on the premise that the severity of the perceived blockiness will be proportional to the percentage of pixels within a block for which the standard deviation from the average value is small . for a video signal , this blockiness measure may be computed for each frame of data . a method of determining an intra block measure for a selected block ( k ) is now described . referring firstly to fig3 , an intra block measure is initially determined at step 21 for each block within the decoded image data frame 3 , also referred to herein as a digital image . then at step 22 a measure of blockiness is calculated for the frame in dependence upon the intra block measure determined for each block at step 21 . step 21 , determination of the intra block measure for each block is now described in more detail with reference to fig4 . the blockiness measure for the selected block ( blocking ( k )) is initially set to zero at step 31 . it will be appreciated that the blockiness measure may be set to zero at any point prior to its first use . at step 32 , a first set of standard deviations of a pixel value ( in the preferred embodiment the value used is luminance ) is determined , the set comprising a standard deviation for each row 10 within a selected block , then at step 33 an average of the first set of standard deviations is determined . at step 34 a second set of standard deviations of a pixel value ( in the preferred embodiment the value used is luminance ) is determined , the set comprising a standard deviation for each column 11 within a selected block , then at step 35 the average of the second set of standard deviations is determined . it will be appreciated that the steps involved determining each of these averages may be carried out either in order ( rows and then columns or columns and then rows ) or in parallel ; the precise sequence of steps is not material to the invention . furthermore , the terms ‘ rows ’ and ‘ columns ’ are effectively interchangeable because the orientation of the digital image to be analysed is arbitrary . in fig2 , the rows 10 are shown on the second block and the columns 11 are shown on the first block for clarity only , in practice , in the preferred embodiment of the invention , the first set of standard deviations and the second set of standard deviations are both calculated for each selected block . an intra block measure is determined by increasing the measure ( blocking ( k )) by one at step 38 or 39 if the average of the first set of standard deviations is lower than a predetermined threshold at step 36 or if the average of the second set of standard deviations is lower than a second predetermined threshold at step 37 . the measure ( blocking ( k )) is increased by a penalty value of one again at step 41 if both the first set of standard deviations is lower than a predetermined threshold and average of the second set of standard deviations is lower than a second predetermined threshold at step 40 . in other embodiments the penalty value may be set to a value , possibly non - integer , other than one . the blocking measure ( std2blockiness in the equation below ) is then determined in dependence upon the intra block measure for each block at step 22 ( fig3 ) according to the equation : in order to compute the blocking measure , the image is first decomposed into blocks of blocksizex * blocksizey pixels every blockshiftx in the horizontal direction and every blockshifty pixels in the vertical direction . if blockshiftx = blocksizex and blockshifty = blocksizey , then the blocks will be adjoining ( as shown in fig2 ). however , this is not necessary to the operation of the invention , and the complexity of the invention can be reduced by making blockshiftx greater than blocksizex and blockshifty greater than blocksizey , effectively sub - sampling the image prior to analysis . in the preferred embodiment of the invention blocksizex = blocksizey = 8 , blockshiftx = blockshifty = 32 and the predetermined threshold = 0 . 1 in both cases . if a blocking measure is to be calculated for the whole or portion of a video signal , then the blocking measure ( std2blockiness ) is first calculated for each frame in the video sequence and then averaged to produce a measure for the video sequence being analysed . it will be understood by those skilled in the art that the processes described above may be implemented on a conventional programmable computer , and that a computer program encoding instructions for controlling the programmable computer to perform the above methods may be provided on a computer readable medium . it will be appreciated that certain features of the invention , which are , for clarity , described in the context of separate embodiments , may also be provided in combination in a single embodiment . conversely , various features of the invention which are , for brevity , described in the context of a single embodiment , may also be provided separately , or in any suitable combination . it is to be recognised that various alterations , modifications , and / or additions may be introduced into the constructions and arrangements of parts described above without departing from the scope of the present invention as defined in the appended claims .