in the following description , a preferred embodiment of the invention is described with regard to preferred process steps and data structures . those skilled in the art would recognize after perusal of this application that embodiments of the invention can be implemented using dsp circuits adapted to particular process steps and data structures described herein , and that implementation of the process steps and data structures described herein would not require undue experimentation or further invention . in a memory space for processing picture data , a frame comprises a plurality of horizontal lines , and each horizontal line comprises a plurality of pixels . for instance , one rgb display format in common use comprises 640 × 480 pixels , with each pixel represented with three pixel values ( red , green , blue ). fig1 shows one embodiment of a pixel group 102 consisting of a set of four pixels selected from an entire frame 100 of pixels . in an uncompressed format , a set of four pixels , each represented by three pixel values would require twelve eight - bit bytes of memory for storage . however , in a compressed format such as that described herein , each set of four pixel values ( 32 bits total ) is reformatted as a unit and requires only three eight - bit bytes ( 24 bits total ) for storage . thus , four pixels which previously used twelve bytes for storage now on average requires eight , a 25 % decrease in storage requirements . reformatted units of pixel values will be termed a “ codeword ” for the purposes of this specification . fig2 shows two different arrangements of pixels . pixel grouping 104 depicts four pixels chosen such that each pixel is immediately adjacent to all other pixels in its group , whether diagonally , horizontally , or vertically . pixel grouping 106 depicts four pixels which are linearly arranged such that each successive pixel is adjacent to and linearly arranged with the previous pixel . although any of the pixel relationships shown would be acceptable for this algorithm , grouping 104 provides the best arrangement of primary and dependent pixels because pixels r 1 , r 2 , and r 3 are likely to contain information which has a high degree of similarity to one another and pixel r 0 due to their relative closeness to one another in the frame . should the location of r 0 be such that r 1 , r 2 , or r 3 would fall outside the display area , the difference values associated with those pixels would be assigned a value of zero . for example , if r 2 and r 3 fall outside the viewing area , diff 12 = r 1 − r 2 is set to zero , and diff 23 = r 2 − r 3 is set to zero . this situation will never occur when using data decoded from mpeg standards , since the block size for mpeg data is 16 × 16 , resulting in a block which is always evenly divisible by 4 . in this case , four 4 - pixel groups will always be available . pixel value p 0 , chosen from pixel r 0 , might represent a saturation for a particular hue , perhaps red , as seen in an rgb format , or a value representing a luminance or chrominance value ( such as a y value , a cr value , or a cb value ), as seen in a yuv format . similarly , pixel values p 1 , p 2 , and p 3 , might represent the saturation level for the color red for pixels r 1 , r 2 , and r 3 . referring to the flow chart depicted in fig3 the process begins at block 201 where the difference between pixel values is computed in order to determine their relative similarity . next , at a block 203 the resulting data is examined to decided if the absolute value of the difference between them is less than 16 . if so , a block 204 generates the codeword using pixel value p 0 and the four least significant bits of the difference between values p 0 and p 1 , p 1 and p 2 , p 2 and p 3 , followed by a tag comprising a trailing tag bit “ 1 ”. it is the location of this tag bit “ 1 ”, in bit position “ 0 ”, “ 1 ”, “ 2 ”, or “ 3 ”, that tells a restoration technique which method was used in constructing the codeword so that the proper restoration steps may be used . if , at the block 203 , the resulting differences are greater than 16 , the method then examines the three most significant bits . this is accomplished , as shown at a block 205 , when the differences between the pixel values are examined to decide if they are less than 32 . if so , the codeword is generated , in a block 206 , as the seven most significant bits of the base value p 0 and p 1 , p 1 and p 2 , p 2 and p 3 , followed by two trailing tags bits “ 10 ”. if , in block 205 , the resulting differences are greater than 32 , the algorithm then examines the two most significant bits . this is accomplished , as shown in block 207 , when the differences between the pixel values are examined to determine if they are less than 64 . if yes , the codeword is generated , in block 208 , as the six most significant bits of the base value p 0 , followed by bits 2 through 6 of the differences between pixels p 0 and p 1 , p 1 and p 2 , p 2 and p 3 , followed by three trailing tag bits “ 100 ”. if , in block 207 , the resulting differences are greater than 64 , the worst case has been identified , and in blocks 209 and 211 , the algorithm then forms the codeword using the four most significant bits of pixel value p 0 , followed by bits 3 through 7 of the differences between p 0 and p 1 , p 1 and p 2 , p 2 and p 3 , followed by the sum of the two least significant bits , bit 0 and bit 1 , of values p 0 , p 1 , p 2 , and p 3 , followed by three trailing tag bits “ 000 ”. in sum , compression according to the invention can be performed by a method including the steps of accessing and selecting a plurality of pixels , identifying a first pixel and a second pixel in the plurality of pixels , determining a first difference between a value for the first pixel and a value for the second pixel , comparing the difference with a threshold , and storing a codeword responsive to the difference and a result of the comparison . preferably , the storing step includes storing exact values ( i . e ., all bits ) for the first value and for the difference when the difference is less than the selected threshold , and storing approximate values ( i . e ., less than all bits ) when the difference is greater than the selected threshold . the steps of the method can be expanded for application to more than a first and second pixel , for example four pixels at a time as discussed above . this method is performed for all pixel values representing the four pixel groups in succession in any order desired . its embodiment herein is unique in its efficiency and simplicity as well as its ability to perform random access to compressed pixels . the embodiment described herein typically follows the decoding of a formatted signal such as mpeg 1 or mpeg 2 . thus , i , b and p frames within the mpeg signal are decoded into raw picture data , compressed into dram or other video memory using the method described in this application , then decompressed for display on the view screen . in particular , the embodiment described herein provides particularly good random access to pixels by compressing very small blocks i . e . four pixels at a time . furthermore , the embodiment described herein is particularly suited to interlaced video and allows for example , every third or fourth line to be displayed . hence , scalability of output is enabled fast , simply and with the minimum logic . therefore , this embodiment allows flexibility of display in for example , a vga environment as well as in a tv environment . in one embodiment , compression according to the invention is performed by an mpeg decoder after that decoder decodes an mpeg formatted signal . d 12 [ 7 : 0 ]= p 1 [ 7 : 0 ]− p 2 [ 7 : 0 ] d 23 [ 7 : 0 ]= p 2 [ 7 : 0 ]− p 3 [ 7 : 0 ] if d 01 [ 7 : 5 ]== d 01 [ 4 ], d 01 [ 4 ], d 01 [ 4 ])& amp ;& amp ;( d 12 [ 7 : 5 ]== d 12 [ 4 ], d 12 [ 4 ], d 12 [ 4 ])& amp ;& amp ;( d 23 [ 7 : 5 ]== d 23 [ 4 ], d 23 [ 4 ], d 23 [ 4 ]) codeword [ 23 : 0 ]=( p 1 [ 7 : 0 ], d 01 [ 4 : 0 ], d 12 [ 4 : 0 ], d 23 [ 4 : 0 ], 1 ′ b 1 ). pp 2 [ 6 : 0 ]= p 2 [ 7 : 1 ], and dd 12 [ 6 : 0 ]= pp 1 [ 6 : 0 ]− pp 2 [ 6 : 0 ], dd 23 [ 6 : 0 ]= pp 2 [ 6 : 0 ]− pp 3 [ 6 : 0 ], if ( dd 01 [ 6 : 5 ]== dd 01 [ 4 ], dd 01 [ 4 ])& amp ;& amp ;( dd 12 [ 6 : 5 ]== dd 12 [ 4 ], dd 12 [ 4 ]& amp ;& amp ;( dd 23 [ 6 : 5 ]== dd 23 [ 4 ], dd 23 [ 4 ]) codeword [ 23 , 0 ]=( pp 1 [ 6 : 0 ], dd 01 [ 4 : 0 ], dd 12 [ 4 : 0 ], dd 23 [ 4 : 0 ], 2 ′ b 10 ). ddd 01 [ 5 : 0 ]= ppp 0 [ 6 : 0 ]− ppp 1 [ 6 : 0 ], ddd 12 [ 5 : 0 ]= ppp 1 [ 6 : 0 ]− ppp 2 [ 6 : 0 ], ddd 23 [ 5 : 0 ]= ppp 2 [ 6 : 0 ]− ppp 3 [ 6 : 0 ], if ( ddd 01 [ 5 ]== ddd 01 [ 4 ])& amp ;& amp ;( ddd 12 [ 5 ]== ddd 12 [ 4 ])& amp ;& amp ;( ddd 23 [ 5 ]==( dd 23 [ 4 ]), codeword [ 23 , 0 ]=( ppp 1 [ 5 : 0 ], ddd 01 [ 4 , 0 ], ddd 12 [ 4 , 0 ], ddd 23 [ 4 , 0 ], 3 ′ b 100 ). pppp 2 [ 4 : 0 ]= p 2 [ 7 : 3 ], and dddd 12 [ 4 : 0 ]= pppp 1 [ 4 : 0 ]− pppp 2 [ 4 : 0 ], dddd 23 [ 4 : 0 ]= pppp 2 [ 4 : 0 ]− pppp 3 [ 4 : 0 ], r [ 4 : 0 ]={ 2 ′ b 00 , p 0 [ 2 : 0 ]}+{ 2 ′ b 00 , p 1 [ 2 : 0 ]}+{ 2 ′ b 00 , p 2 [ 2 : 0 ]}+{ 2 ′ b 00 ), p 3 [ 2 : 0 ]} codeword [ 23 , 0 ]=( pppp 1 [ 4 : 0 ], dddd 01 [ 4 : 0 ], dddd 12 [ 4 : 0 ], dddd 23 [ 4 : 0 ], r [ 4 ], 3 ′ b 000 ) in order to expand the compressed file for display , the trailing tag bits for each block of pixel values is examined to determine what degree of compression was applied to that block . once the degree of compression is determined , the compression process which applied for that particular case may be reversed , and the values restored to viewable form . although preferred embodiments are disclosed herein , many variations are possible which remain within the concept , scope , and spirit of the invention , and these variations would become clear to those skilled in the art after perusal of this application .