Patent Application: US-45500906-A

Abstract:
method , systems and software are proposed for obtaining for blocks of a first image similar blocks of a second image . the blocks of the first image are processed sequentially , for each block trying out a number of candidate locations in the second image and evaluating a cost function for each . each candidate location in the second image is displaced by a respective motion vector from the block of the first image . in a first aspect of the invention the cost function is a function of a predicted motion vector for future blocks of the first image . in a second aspect of the invention the motion vectors are given by location values which are not all whole pixel spacings , halves of the pixel spacing , or quarters of the pixel spacing .

Description:
the first embodiment of the invention employs many features of the pmvfast algorithm , but improves upon pmvfast ( and other existing algorithms ) by considering a few neighboring blocks instead of just one block . eqn . ( 2 ) and ( 3 ) show that the choice of the mv of the current block directly affects the rd cost of the neighboring blocks , including the right block ( or the ( i , j + 1 ) th block ), the lower - left block ( or the ( i + 1 , j − 1 ) th block ), and the lower block ( or the ( i + 1 , j ) th block ). this is because the current mv would affect the predicted mv of these neighboring blocks and thus in turn affect the optimal motion vectors for those blocks . these are “ future ” blocks as motion estimation has not been performed on them when the current block is processed . we cannot compute the optimal motion vector of these future blocks concurrently with the current block because this we would require computing the optimal motion vectors for all the blocks in the whole frame simultaneously as in eqn . ( 2 ), which would incur excessive complexity . instead , to assess the implication of the choice of the current mv for the current block on the right block or the ( i , j + 1 ) th block , we might add to the rd cost function of the current block of eqn . ( 1 ) a term where m i , j + 1 is the optimal motion vector of the right block based on the current mv of the current block , and p i , j + 1 is the median mv predictor for the right block based on the current mv of the current block , i . e . however , m i , j + 1 in eqn . ( 4 ) is unknown as the motion estimation for the right block ( a future block ) has not been performed yet . however , we observe that  m i , j + 1 - p i , j + 1   m i , j - p i , j + 1  .  m i , j + 1 - p i , j + 1  -  m i , j - p i , j + 1  . we performed experiments on many video test sequences and studied the distribution of diff . the typical result is heavily biased towards zero as shown in fig2 , which shows the probability density function ( pdf ) for the foreman sequence . this implies that the two quantities are almost identical in most of the cases ( identical in about 70 % of the cases and differ by only 1 in about 23 %). this suggests that  m i , j - p i , j + 1   m i , j + 1 - p i , j + 1  . as a result , r (| m i , j + 1 − p i , j + 1 |) can be approximated by w * r (| m i , j − p i , j + 1 |), where w & gt ; 0 . likewise , we may add additional terms to the cost for the lower - left and lower blocks . for the ( i , j ) th block , let medianmv denote the median mv predictor given by eqn . 3 . let fmedianmv denote the future median mv predictor ( median mv predictor for the right block ) given by eqn . ( 5 ). consequently , fmedianmv is a function of the mv candidate . the first embodiment is here referred to as the “ enhanced predictive motion vector field adaptive search technique ” ( e - pmvfast ) for the ( i , j )- th block . the steps of the embodiment are follows . they are also shown in fig7 . 1 . compute cost for three motion vector predictors : ( i ) the median mv predictor (“ medianmv ”), ( ii ) the estimated motion vector for the right block (“ futuremv ”) defined as and ( iii ) the mv predictor from a past block (“ pastmv ”), which is the one of the previous co - located mv (“ premv ”) and previous - bottom - right mv (“ prebottomrightmv ”) that is farther away from medianmv , i . e . note that item ( ii ) can be supplemented or replaced by an estimated motion vector for another neighbouring future block , such as the bottom - left , bottom , and / or bottom - right . note also that in item ( iii ), the previous - bottom - right mv can be supplemented or replaced with by a previous mv predictor for another neighbouring block . note that items ( ii ) and ( iii ) form independent aspects of the invention . if any of the above mv predictors is not available ( e . g . at frame boundaries ), then skip that predictor . 2 . if the smallest cost of the motion vector predictors is less than a threshold t1 , then stop the search and goto step 7 . otherwise , choose the motion vector with the smallest cost as currentmv ( current mv ) and goto the next step . note that , the cost of the 3 motion vector predictors can be computed in some predefined order ( e . g . medianmv , followed by futuremv , followed by pastmv ), and at any moment , if the cost of any motion vector predictor is less than certain threshold , then the search may stop and goto step 7 . 3 . perform 1 iteration of directional small diamond search around currentmv . the concept of a directional small diamond search is explained below . 4 . if the smallest cost is less than a threshold t2 , stop the search and goto step 7 . otherwise , choose the motion vector with the smallest cost as currentmv and goto the next step . 5 . if ( currentmv = medianmv ) and the current smallest cost is less than a threshold t3 , perform small diamond search and go to step 7 . 6 . if the video is not interlaced , perform large diamond search , as shown in fig3 ( a ); otherwise , perform modified large diamond search as shown in fig3 ( b ). in each of these steps the cost function is evaluated for each of the marked points of the diamond . 7 . select the mv with smallest cost . in our experiments , a value for w of about 0 . 8 was found to be effective . the steps of the directional small diamond search are now explained . suppose that centermv is the current search center and mv1 , mv2 , mv3 and mv4 are four surrounding search points as shown in fig4 . compute r ( mvi − medianmv ) for each mvi . if r ( mvi − medianmv )& lt ; r ( centermv − medianmv ), then compute the sad and the cost for mvi . otherwise , ignore the mvi . select the mv with the lowest cost as the currentmv . note that the concept of a directional diamond search is believed to be new , and constitutes an independent aspect of the invention , which need not be performed in combination with the concept of using a futuremv . the steps of a large diamond search and modified large diamond search are the same , but the search is done for all the set of points shown respectively in fig3 ( a ) and 3 ( b ). we now consider a number of possible variations to the embodiment within the scope of the invention . firstly , note that the weighting factor w in the cost function can be different for different blocks . furthermore , optionally the w may be different for different mv candidates . in particular , the definition of w may depend on situations such as whether the mv candidate is close to the medianmv and / or the futuremv , or whether the x - component or y - component of the mv candidate is the same as the x - component or y - component of the fmedianmv . furthermore , the cost function may not be restricted to the form of eqn . ( 6 ). it can be any function which includes a distortion measurement term ( e . g . sad , sum of square distortion ( ssd ), mean absolute distortion ( mad ), msd , etc ) and a term which takes into account the bits required to encode the motion vector of the current block and those of some neighboring blocks ( e . g . the right block , the bottom block , the bottom left block , etc ). furthermore , in step 1 , the definition of futuremv is not restricted to the form given in step 1 above . two possible alternative definitions for futuremv are : furthermore , in step 1 as expressed above , pastmv is selected as the one out of a list collection of possible mvs ( premv and prebottomrightmv ) in the previous frame which is farthest from medianmv . however , the list of mv to be considered can contain more than two possibles mv ( e . g . premv , preleftmv , prerightmv , pretopmv , pretopleftmv , pretoprightmv , prebottommb , prebottomleftmv , prebottomrightmv , etc ). in addition , mv from more than one previously encoded frame can be included in the list ( e . g . if current frame is frame n , the list can contain frame n - 1 , n - 2 , n - 3 , . . . ). if the current frame is a b - frame , the list of previously encoded frames can include future p - frames . furthermore , in step 1 , pastmv is chosen to be the possible mv that is farthest from a reference mv ( medianmv in step 1 ). other reference mvs are possible , including the leftmv , or topmv , or toprightmv , or some combination . other ways of choosing from the list of possible mv are also possible . in step 2 , the cost of the 3 motion vector predictors are derived in some predefined order . possible predefined orders include a ) medianmv , followed by futuremv , followed by pastmv b ) medianmv , followed by pastmv , followed by futuremv c ) futuremv , followed by medianmv , followed by pastmv d ) futuremv , followed by pastmv , followed by medianmv e ) pastmv , followed by medianmv , followed by futuremv f ) pastmv , followed by futuremv , followed by medianmv furthermore , while one iteration of the directional small diamond search is performed in step 3 as expressed above , more than one iteration can be applied . we now present simulation results for the embodiment e - pmvfast . the embodiment was embedded into h . 264 reference software jm9 . 3 [ 13 ], and simulated using various qp , video sequence , resolution , and search range . tables 1 ( a - c ) and 2 ( a - c ) show some typical simulation results . the psnr ( peak - signal - to - noise ratio ) change and br ( bit - rate ) change are the changes of psnr and bit rate with respect to full search ( fs ). the simulation results suggest that the bit rate and psnr of the proposed e - pmvfast tend to be similar to that of full search and pmvfast but e - pmvfast tends to be about 40 % faster than pmvfast , across a wide range of video sequences and bit rates . one important feature of e - pmvfast is that its motion vector field tends to be very smooth , so that the motion vectors can represent the objects &# 39 ; movement more accurately than other fast motion estimation algorithms . in each of fig5 ( a ) and 5 ( b ), the left - hand image shows ( as the short lines ) the motion vector fields obtained by the pmvfast algorithm , and the right - hand image shows the motions vectors for the same image obtained by the embodiment . the motion vector field of e - pmvfast is significantly smoother than that of pmvfast , especially in the circled regions . the smooth motion field can be very useful for classifying the motion content of a video in perceptual trans - coding , rate control , multiple block size motion estimation , multiple reference frame motion estimation , and so on . we now turn to a second embodiment of the invention , which illustrates the second aspect of the invention . as described above , conventional full integer pixel allows motion vectors to take on location values in each direction of − 2 . 0 , − 1 . 0 , 0 , 1 . 0 , 2 . 0 , etc . in the second embodiment of the invention , the possible location values are selected to be close to the predictor . for the location value which is nearest to 0 , we can use ( instead of 1 . 0 ) another location value such as 0 . 85 such that the allowable location values would include − 2 . 0 , − 0 . 85 , 0 , 0 . 85 , 2 . 0 , etc . the advantage of this is that statistically motion vectors tend to be close to 0 . and thus by choosing the location closer to 0 , we would be closer to the true motion vector and thus can give better motion compensation that can lead to higher compression efficiency . similarly , the other location values can be changed . as an example , the location value of 2 . 0 can be changed to 1 . 9 such that the allowable location values would include − 1 . 9 , − 0 . 85 , 0 , 0 . 85 , 1 . 9 , etc . the beauty of the proposed change is that the same motion vector code can be used , except that an encoded motion vector location of 1 . 0 should be interpreted as 0 . 85 , and 2 . 0 as 1 . 9 , etc . half pixel precision allows motion vector to take on location values such as 0 . 0 , 0 . 5 , 1 . 0 , 1 . 5 , 2 . 0 , etc . we propose to modify these location values , especially those close to the predictor . for the location value of 0 . 5 that is very close to 0 , we propose to use a different value . for example , one possibility is to use 0 . 4 instead of 0 . 5 . in other words , the location values would include 0 . 0 , 0 . 4 , 1 . 0 , 1 . 5 , 2 . 0 . similarly , other location values can be modified . for example , the location value of 1 . 0 can be changed to 0 . 95 so that the new set of location values would include 0 . 0 , 0 . 4 , 0 . 95 , 1 . 5 , 2 . 0 , etc . again this can help to increase the compression efficiency . similarly , the other location values can be modified to increase the compression efficiency . however , changing such locations can lead to significantly higher computation efficiency both at the encoder and the decoder . usually , most of the compression efficiency gain comes from changing the location values close to the predictor . quarter pixel precision allows motion vector to take on location values such as 0 . 00 , 0 . 25 , 0 . 50 , 0 . 75 , 1 . 00 , etc . we can modify the location values , especially those close to the predictor . as an example , we can modify them to be 0 . 00 , 0 . 20 , 0 . 47 , 0 . 73 , 0 . 99 , etc . note that the proposed method allows us to choose an arbitrary number n of location values between each integer location values . for example , between the location values of 0 and 1 , half - pixel precision uses 1 location value { 0 . 5 }, quarter - pixel precision uses 3 location values { 0 . 25 , 0 . 50 , 0 . 75 }, and ⅛ - pixel precision uses 7 location values ( 0 . 125 , 0 . 250 , 0 . 375 , 0 . 500 , 0 . 625 , 0 . 750 , 0 . 875 ). the proposed method allows us to choose any n location values between 0 and 1 . for example , we can choose n = 2 values such as 0 . 3 , and 0 . 6 . the proposed non - uniformed subpixel motion estimation and compensation does not need to be applied to every region of every frame . instead , some bits can be introduced in the headers to indicate whether it is turned on or off for each region ( e . g . slice ) of the video frames . other than that , it can be directly applied to the existing standards without any change in syntax because the same motion vector code can be applied . the proposed non - uniform subpixel motion estimation and compensation was simulated using h . 264 jm82 software and the results are shown in the tables above , in which qp stands for quantization parameter . the simulation used the location values ( . . . − 1 , − 0 . 75 , − 0 . 5 , − 0 . 15 , 0 , 0 . 15 , 0 . 5 , 0 . 75 , 1 . . . ), in both x and y directions . that is , only the location values at − 0 . 25 and + 0 . 25 were modified as compared with the standard scheme using quarter - pixel spaced location values . apart from using the novel location values , the algorithm otherwise identical to the known h . 264 standard algorithm . as indicated in the tables , the second embodiment can achieve considerable reduction in bit rate while achieving similar psnr . no change in the syntax of h . 264 is necessary . although only a few embodiments of the invention are described above , many variations are possible within the scope of the invention . for example , the description of the invention given above is for blocks of fixed size in p - frames with one reference frame . however , this invention can be applied to blocks with multiple sub - block sizes , and the blocks need not necessarily be non - overlapping . there can be more than one reference frame , and the reference frame ( s ) can be any block in the past or in the future of the video sequence relative to the current frame . for the video , one picture element ( pixel ) may have one or more components such as the luminance component , the red , green , blue ( rgb ) components , the yuv components , the ycrcb components , the infra - red components , the x - ray or other components . each component of a picture element is a symbol that can be represented as a number , which may be a natural number , an integer , a real number or even a complex number . in the case of natural numbers , they may be 12 - bit , 8 - bit , or any other bit resolution . while the pixels in video are 2 - dimensional samples with rectangular sampling grid and uniform sampling period , the sampling grid does not need to be rectangular and the sampling period does not need to be uniform . each embodiment of the invention is suitable for implementation by fast , low - delay and low cost software and hardware implementation of mpeg - 1 , mpeg - 2 , mpeg - 4 , h . 261 , h . 263 , h . 264 , avs , or related video coding standards or methods , which may be modified to include it . possible applications include digital video broadcast ( terrestrial , satellite , cable ), digital cameras , digital camcorders , digital video recorders , set - top boxes , personal digital assistants ( pda ), multimedia - enabled cellular phones ( 2 . 5 g , 3 g , and beyond ), video conferencing systems , video - on - demand systems , wireless lan devices , bluetooth applications , web servers , video streaming server in low or high bandwidth applications , video transcoders ( converter from one format to another ), and other visual communication systems , etc . the disclosure of the following references is incorporated herein in its entirety : joint video team of itu - t and iso / iec jtc 1 , “ draft itu - t recommendation and final draft international standard of joint video specification ( itu - t rec . h . 264 | iso / iec 14496 - 10 avc ),” document jvt - g 050 r 1 , may 2003 . 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[ 10 ] “ mpeg - 4 optimization model version 1 . 0 ”, in iso / iec jtc 1 / sc 29 / wg 11 mpeg 2000 / n 3324 , noordwijkerhout , nl , march &# 39 ; 00 . [ 11 ] t . koga , k . iinuma , a . hirano , y . iijima , and t . ishiguro , “ motion compensated interframe coding for video conferencing ,” proc . nat . telecommun . conf ., new orleans , la ., pp . g5 . 3 . 1 - g5 . 3 . 5 , december &# 39 ; 81 . [ 12 ] j . r . jain and a . k . jain , “ displacement measurement and its application in interframe image coding ,” ieee trans . on communications , vol . com - 29 , pp . 1799 - 808 , december &# 39 ; 81 . [ 13 ] jvt reference software jm9 . 2 for jvt / h . 264 frext .