the following description will illustrate the invention using an exemplary video encoding system . it should be understood , however , that the invention is not limited to use with any particular type of video signal format , video encoding standard or encoding system configuration . the invention is instead more generally applicable to any video encoding system in which it is desirable to improve the efficiency of an encoding process for generating encoded bit streams with respect to computational complexity and compression ratio without adversely affecting perceived picture quality . the term “ video sequence ” should be understood to include any frame or field sequence which is in a form suitable for encoding in accordance with standards such as , for example , h . 261 , h . 263 , motion - jpeg , mpeg - 1 and mpeg - 2 . the terms “ image ” or “ picture ” as used herein refer to a frame or field of a video sequence . the term “ block ” as used herein is intended to include not only macroblocks as defined in the above - noted compression standards , but more generally any grouping of pixel elements in a video frame or field . prior to a detailed description of preferred embodiments of the invention , some background on perceptual coding will be given . the history of perceptual coding can be traced back to the weber - fechner law of experimental psychology . it was found that the human capacity to distinguish a change in magnitude of a stimulus depends on the current magnitude of that stimulus . for example , it is generally easier to distinguish a 2 - pound weight from a 1 - pound weight , than it is to distinguish a 21 - pound weight from a 20 - pound weight . in the 1800s , weber developed a quantitative description of the relationship between stimulus intensity and discrimination now known as weber &# 39 ; s law , namely : where δs is the perceived intensity difference relative to a background stimulus s and k is a constant . fechner applied weber &# 39 ; s law to sensory experience in the 1860s and found that the intensity of a sensation is proportional to the logarithm of the strength of the stimulus : where i is the subjective experienced intensity , t is the threshold , or minimally perceptible stimulus level , and s (& gt ; t ) is the strength of the supra - threshold stimulus . this relationship is referred to as the weber - fechner ( wf ) law . further discussion of the wf law may be found in e . g . boring , “ sensation and perception in the history of experimental psychology ,” new york : appleton - century 1942 . in video , let i denote the actual intensity at a target pixel that is to be encoded , and let i ′ denote the displayed intensity at a spatio - temporal immediate neighbor of the target pixel as reconstructed from the data already transmitted . then , provided | i - i ′| is less than the wf threshold t , wf law implies that most human observers will not be able to sense the difference between displaying i ′ instead of i at the target pixel . in view of this limitation of human visual perception , we should expect a reasonable perceptual metric for video coding to achieve zero distortion numerically even though some ( i , i ′) pairs are not exactly the same . a perceptual metric d ( z ) based on the weber - fechner logarithmical relationship has been presented in y . j . chiu et al ., “ perceptual rate - distortion theory ,” proceedings of the 1996 ieee international symposium on information theory and its applications , victoria , canada , sep . 17 - 20 , 1996 . the perceptual metric is represented as : d  ( z ) = { 0 , when    z  ≤ t k   log  (  z  / t ) , when    z  ≥ t ( 5 ) where z = u − v represents the distortion of the input source u approximated by output source v . recent psychology experiments have shown that the determination of t is very complicated for video because video involves temporal masking and spatial masking at the same time . this is discussed in a . n . netravali et al ., “ digital pictures : representation , compression , and standards ,” 2 nd edition , at & amp ; t bell laboratories , 1995 . the threshold t is sometimes referred to as the “ just - noticeable - distortion ” ( jnd ) in literature on perception , e . g ., y . j . chiu et al ., “ perceptual rate - distortion theory ,” proceedings of the 1996 ieee international symposium on information theory and its applications , victoria , canada , sep . 17 - 20 , 1996 ; and n . s . jayant et al ., “ signal compression based on models of human perception ,” proceedings of ieee , volume 10 , october 1993 . the ideal jnd provides each pixel being coded with a threshold level below which discrepancies are perceptually distortion - free . the minimum rate required for perceptual lossless compression is called “ perceptual entropy .” also of interest is the analysis of noticeable , above - threshold distortions that inevitably result at low bit rates . a term called “ maximally noticeable distortion ” ( mnd ) characterizes distortion profiles above the perception threshold . in such scenarios , information - theoretic considerations from rate - distortion theory are coupled with research on perceptual metrics in order to adjust thresholds locally in an optimal fashion . a framework of rate - distortion consideration on perceptual metric can be found in y . j . chiu et al ., “ perceptual rate - distortion theory ,” proceedings of the 1996 ieee international symposium on information theory and its applications , victoria , canada , sep . 17 - 20 , 1996 . in accordance with weber - fechner theory , the previous displayed frame can be treated as the perceived stimulus background . this permits an update of those pixel sites whose intensity differences exceed the visibility threshold specified by the background pixels . as described above , jnd provides each signal being coded with a threshold level below which reconstruction error is imperceptible in the sense that a human can not perceive any impairment in the video if the discrepancies are below the jnd value . the information for updating such pixels can be considered to be perceptual redundancy . perceptual redundancy may be removed in the pixel domain via a procedure referred to as “ bush - hogging ,” as described below and in u . s . pat . no . 5 , 740 , 278 , issued to t . berger and y . j . chiu et al . on apr . 14 , 1998 , entitled : “ facsimile - based video compression method and system ,” the disclosure of which is incorporated by reference herein . although the same threshold value can be used for each pixel , perceptual considerations suggest that only those prediction residuals that exceed a perceptually predetermined threshold need to be corrected . the selection of this perceptual threshold should be based not only on the difference intensity values in the difference frame domain but also on the intensity values of the background pixels . the correct choice of the bush - hog threshold t for a particular pixel depends on many factors . if only achromatic images are considered , the principal factors are : ( i ) average spatial and temporal background luminance level against which the pixel is presented ; ( ii ) supra - threshold luminance changes adjacent in space and time to the test pixel ; and ( iii ) spatio - temporal edge effects . these factors are further discussed in a . n . netravali et al ., “ digital pictures : representation , compression , and standards ,” 2 nd edition , at & amp ; t bell laboratories , 1995 . in real video , the three factors are correlated with one another . a complete study on the determination of pixel - by - pixel threshold t based on the intensity values of the background pixels and local masking effects is described in detail in the above - referenced u . s . pat . no . 5 , 740 , 278 to berger et al . referring now to fig2 an exemplary video encoder 100 in accordance with an illustrative embodiment of the invention is shown . as is evident , the exemplary configuration of the video encoder 100 includes similar components as the video encoder 10 ( fig1 ), with the important exception of the novel inclusion of a perceptual preprocessor 110 in the encoder 100 of the invention . as such , a description of the functions of the similar components of fig2 ( with their reference designations incremented by 100 as compared to fig1 ) will not be repeated as they were explained in detail above and are known in the art . as shown , the video encoder 100 includes the perceptual preprocessor 110 coupled to the input of the encoder . the encoder 100 also includes a motion estimator 112 and a signal subtractor 114 , both coupled to the input of the encoder . the motion estimator 112 is also coupled to the perceptual preprocessor 110 . the encoder 100 also includes a transformer ( e . g ., dct generator ) 116 coupled to the perceptual preprocessor 110 and the signal subtractor 114 , a quantizer 118 coupled to the transformer 116 , and an entropy encoder 120 coupled to the quantizer 118 and the output of the encoder 100 . an inverse transformer ( e . g ., an inverse dct generator ) 122 is also included and coupled between the quantizer 118 and the entropy encoder 120 . the encoder 100 also includes a signal combiner 124 coupled to the inverse transformer 122 , a delay 126 coupled to the signal combiner 124 and the perceptual preprocessor 110 , and a motion compensator 128 coupled to the delay 126 , the signal subtractor 114 , the signal combiner 124 , and the motion estimator 112 . from implication of the weber - fechner theory , we note that the values of residuals that do not exceed the perceptual threshold t may not need to be corrected . if the residuals of the pixels inside a macroblock are small enough , then we can continue to use the previously displayed macroblock without updating and at no loss of perceived quality . accordingly , the computational effort of motion estimation / compensation for such macroblock is saved or can be preserved for other macroblocks requiring more motion searches . it is to be appreciated that the perceptual preprocessor 110 of the invention provides such advantages by making use of the insensitivity of the human visual system ( hvs ) to the mild changes in pixel intensity in order to segment video into regions according to perceptibility of picture changes . with the information of segmentation , we then determine which macroblocks require motion estimation . thus , as shown in fig2 the current picture ( i . e ., frame f k ) and the previously reconstructed picture ( i . e ., frame { circumflex over ( f )} k − 1 ) are input to the perceptual preprocessor 110 . the following description is an explanation of an embodiment of a perceptual preprocessing method according to the invention to be performed by the perceptual preprocessor 110 based on such input signals . referring to fig3 a flow chart of a perceptual preprocessing method 300 according to the invention is shown . in step 302 , the current frame f k and the previously reconstructed frame { circumflex over ( f )} k − 1 are input . then , in step 304 , a δ - function test , also known as the “ bush - hogging ” thresholding process , is performed on every pixel x such that : δ  ( x ) = { 1 if    f k  ( x ) - f ^ k - 1  ( x )  & gt ; t  ( f ^ k - 1  ( n  ( x ) ) , f k  ( x ) ) 0 otherwise ( 6 ) where n ( x ) is the neighborhood of x . the term “ f k ( x )−{ circumflex over ( f )} k − 1 ( x ) ” is known in the art as a difference frame . it is to be appreciated that the size of the neighborhood n ( x ) for x involves temporal masking and perhaps spatial masking . this is due to the consideration that sometimes two successive pixels on the same line in the previously reconstructed frame { circumflex over ( f )} k − 1 , say ( i , j − 1 ) and ( i , j ), where each pixel x in the frame is characterized by a ( row , column ) coordinate , both have intensity difference magnitudes , with respect to their counterparts in the current frame f k , that may be above a certain perceptual threshold . in such case , rather than consider an independent pixel in setting threshold t , the intensities of neighboring pixels may be considered since it is known that human visual perception may be more sensitive to intensity change in a given neighborhood rather than in any single pixel . as such , it is to be appreciated that the model to determine the threshold function t ({ circumflex over ( f )} k − 1 ( n ( x )), f k ( x )) is non - trivial . accordingly , a preferred approach which is practical and effective is to use the pixel at the same location in the previously reconstructed frame to represent n ( x ) and use only this pixel as a reference to model the threshold as t ({ circumflex over ( f )} k − 1 ( n ( x ))). thus , each pixel x is tested and , if the intensity difference between pixel x in the current frame and the previously reconstructed frame is greater than the threshold function , then δ ( x ) equals 1 for that pixel , otherwise , δ ( x ) equals 0 . by way of further example , threshold t may be set to a single intensity value that accommodates human visual perception levels based on psycho physical experimentation . it is to be understood that , given the teachings of the invention , one of skill in the art will contemplate other methods of modeling the threshold function . in any case , the δ - function tests result in an intensity difference value being obtained for each comparison of a pixel in f k ( x ) with a corresponding pixel in { circumflex over ( f )} k − 1 ( x ). if the intensity difference value is greater than the perceptual threshold t , then a value of one ( 1 ) is assigned to the comparison , else a value of zero ( 0 ) is assigned . it is to be appreciated that the δ - function tests may be performed on an entire frame or on one or more macroblocks at a time . next , in step 306 , for the ith macroblock b i of the current frame , the results of the δ - function test on every pixel x εb i are collected such that if : ∑ x  ∈  bi   δ  ( x ) = { & gt ; n , then   b i   will   be   motion   compensated ≤ n , then   b i   will   be   skipped . ( 7 ) thus , the δ ( x ) results are summed for a given macroblock and , if the sum is greater than a decision value n , then the macroblock is motion compensated by the remainder of the encoder , for example , as explained above . however , if the sum is less than or equal to the decision value n , then the macroblock is skipped , that is , not motion compensated . advantageously , if a current frame macroblock under evaluation contains pixels that , after comparison to similarly situated pixels in a previously reconstructed frame macroblock , do not result in a sum greater than the decision value , then that macroblock is not subjected to motion estimation / compensation . this results in a segmentation of a frame into macroblocks that have changed a visually perceptible amount from the previous frame and macroblocks that have not changed a visually perceptible amount from the previous frame . as a result , the computational effort otherwise associated with motion estimation / compensation of that macroblock is saved or may be allocated to motion estimation / compensation of another macroblock or some other encoder processing function . it is to be appreciated that the decision value n for the number of pixels surviving from bush - hogging or thresholding ( δ - function tests ) may be set to a default value of zero in order to provide a perceptually distortion - free macroblock . however , we have observed that a video signal generated with a small , non - zero value n can be indistinguishable from a video signal generated with n equal to zero . it is to be appreciated that using a decision value greater than zero permits the encoder 100 to skip more macroblocks so that there is a greater realization of savings with respect to computational effort without sacrificing picture quality . lastly , in step 308 , an output signal is sent to the motion estimator 112 to inform the motion estimator whether or not each macroblock should be motion compensated . the output signal is also provided to the transformer 116 , which then informs the other affected components of the encoder 100 ( i . e ., quantizer 118 , entropy encoder 120 , etc . ), that a particular macroblock is not to be processed thereby . that is , once a macroblock is identified as one to be skipped , the macroblock is not motion estimated / compensated and therefore the transformer 116 , the quantizer 118 , and the entropy encoder 120 do not process data associated with that particular macroblock . it should be understood that an encoder implementing these inventive compression techniques then need only transmit or store motion vectors for those macroblocks that are motion compensated , that is , those macroblocks that have been identified as having changed a visually perceptible degree from a corresponding macroblock of the previous frame . advantageously , a corresponding video decoder need only inverse motion compensate these motion compensated macroblocks and repeat the corresponding macroblocks from the previous frame for the macroblocks of the current frame that were not motion compensated . thus , improved computational efficiency is realized in a video decoder according to the coding techniques of the invention . it is to be appreciated that the teachings of the present invention may be implemented in a wide variety of video applications and , as such , the examples discussed herein are not intended to limit the invention . by way of example , the encoding techniques of the invention may be employed to provide a low complexity motion compensation solution in order to realize a software - only low bit rate video conferencing system . therefore , experiments were conducted on an h . 263 system with a quantization step size of 12 . four qcif ( 176 * 144 pixels ) sequences were used to compare the compression efficiency and complexity of standard motion estimation techniques a feature - based method . the sequences were coded at 7 . 5 fps and we chose head - and - shoulder sequences commonly used in the itu and iso standard bodies . the sequences are “ claire ,” “ foreman ,” “ grandmom ,” and “ mom - daughter .” an optimal , full - search algorithm is used as a benchmark to evaluate the prediction error of the algorithm of the invention . fig4 a shows the “ mom - daughter ” data rate comparison between exhaustive motion search with and without the use of a perceptual preprocessor of the invention . data rate means the bit rate generated using the h . 263 algorithm with the two different motion estimation schemes . the peak signal to noise ratio ( psnr ) is shown in fig4 b in order to compare the objective quality of the “ mom - daughter ” coded sequences . we have also found that informal subjective tests indicate that the two schemes provide very similar quality . fig4 c shows the number of macroblocks which have been selected to have zero motion vectors by the perceptual preprocessor of the invention . this number is used to indicate the savings in computational effort . on average , it has been shown that the perceptual preprocessor of the invention provides lower data rate ( about 13 . 6 % reduction ), lower computational complexity ( about 45 . 3 %) with substantially similar picture quality for the “ mom - daughter ” video sequence . referring to fig5 a list of the average bit rates , psnrs and the percentage of non - motion compensated macroblocks to total macroblocks for all four video sequences . pmae represents the results with an embodiment of the perceptual preprocessor of the invention . these results show various degrees of improvement in data rate and improvement in computational complexity according to the picture content . it is evident that the degree of reduction in data rate is proportional to the percentage of non - motion compensated macroblocks . this is related to the selection of the minimum - mae motion vector instead of choosing the zero - displaced motion vector for the smooth , low - motion backgrounds above . to justify the feasibility of the teachings of the invention to real - time software - only applications , the perceptual preprocessor of the invention was tested with fast motion search techniques such as cross search and three - step search schemes . such fast search techniques are known in the art , for instance , as described in a . n . netravali et al ., “ digital pictures : representation , compression , and standards ,” 2 nd edition , at & amp ; t bell laboratories , 1995 . fig6 a through 6c show the “ grandmom ” sequence results for the three - step search with and without perceptual preprocessing . note that the results of the cross search scheme are similar . again , it is found that perceptual preprocessing according to the invention achieves lower data rate ( about 21 . 4 % lower ) and lower computational complexity ( about 68 . 2 % lower ) at the same time providing a picture quality which is indistinguishable with pictures processed without perceptual preprocessing techniques . fig7 a and 7b illustrate average bit rates , psnrs , and the percentage of non - motion compensated macroblocks for all video sequences based on the cross search scheme and the three - step search , respectively . based on the simulation results presented above , it is evident that the spatio - temporal perceptual preprocessor attains an improved compression ratio without a perceived loss in picture quality , and at the same time reducing computational complexity . these are results that are advantageous to any application that has limited computing capability and restricted transmission bandwidth . further , perceptual preprocessing techniques of the invention indicate which macroblocks have undergone large changes from the previous frame . these blocks , which usually involve high motion or scene changes , require more motion searches . we performed simulations for a fixed computing budget and use the savings of computational effort in non - motion compensated blocks to compute the motion vectors in blocks which have high motion . higher compression ratio and sharper picture quality can be attained , especially in the high motion periods . it should be noted that the elements of the encoder 100 may be implemented using a central processing unit , microprocessor , application - specific integrated circuit or other data processing device in a computer , video transmitter , cable system headend , television set - top box or other type of video signal processor . the central processing unit , microprocessor , application - specific integrated circuit or other data processing device may also have memory associated therewith for storing data and results associated with each elements function when necessary . the invention may be utilized in conjunction with numerous types of video processing or transmission systems , including but not limited to global computer networks such as the internet , wide area networks , local area networks , cable systems , satellite systems and standard wired telephone networks . although illustrative embodiments of the present invention have been described herein with reference to the accompanying drawings , it is to be understood that the invention is not limited to those precise embodiments , and that various other changes and modifications may be affected therein by one skilled in the art without departing from the scope or spirit of the invention .