Patent Application: US-96043110-A

Abstract:
method and device for processing pixels contained in a video sequence including processing the pixel amplitude of at least one image pixel contained in a current frame of a video sequence . the method includes the steps of constructing an individual motion trajectory including motion - shifted versions of the image pixel over a plurality of neighboring frames , and processing the pixel amplitude based on the individual motion trajectory . the step of the constructing the individual motion trajectory includes the steps of choosing at least one image pixel of the current frame as a start pixel of the individual motion trajectory , and adding motion - shifted versions of the image pixel of preceding find / or subsequent frames to the individual motion trajectory .

Description:
the preferred embodiments of the present invention will be best understood by reference to the drawings , wherein identical or comparable parts are designated by the same reference signs throughout . it will be readily understood that the present invention , as generally described herein , could vary in a wide range . thus , the following more detailed description of the exemplary embodiments of the present invention , is not intended to limit the scope of the invention , as claimed , but is merely representative of presently preferred embodiments of the invention . in an exemplary fashion , an inloop filter based on temporal pixel trajectories is described . a pixel trajectory can be defined as 2d - locations through which a certain image point moves from frame to frame . it will be shown that the temporal trajectory filter ( ttf ) performs well when integrated into the h . 264 / avc baseline profile , providing significant bit rate savings for a wide range of sequences . in block - based motion compensated video codecs such as the h . 264 / avc , there exist two main blocks that introduce noise at the encoder . one of these is the motion compensation itself , the other is the quantization of block - based integer discrete cosine transform ( dct ) coefficients . in order to improve the rate distortion , rd , performance of the h . 264 / avc codec , the remaining coding noise is reduced in the reconstructed frames at encoder and decoder . since these reconstructed frames are successively used for prediction of future frames , every degree of noise reduction will increase the performance of the codec . the applied strategy involves identification and tracking of identical image content over the past coded and noisy frames . for each pixel in frame f 1 an individual motion trajectory is constructed that identifies n pixel amplitudes in the past n frames . it is assumed that image content in the pixels identified along the motion trajectory does not change and that any deviation of these pixel amplitudes is caused by additive independent ( white ) coding noise . statistical signal theory implies that averaging the n pixel amplitudes along the trajectory potentially reduces the noise variance by a factor of n , resulting in a coding gain . hereinafter , y i ( x , y ), u i ( x , y ) and v i ( x , y ) shall denote the luminance and chrominance components of the i - th frame of a video sequence at pixel position ( x , y ) t respectively . since in h . 264 / avc motion vectors are assigned not to individual pixels but rather to blocks of various sizes , all pixels within such a block are assumed to have identical motion , resulting in a field of motion vectors with components dx i ( x , y ) and dy i ( x , y ). these give the motion vector needed to retrieve the pixel ( x , y ) t in frame f i from its respective reference frame . hereinafter , in an exemplary fashion , the h . 264 baseline profile with an ippp - structure ( i = intra - mode , p = predictive - mode ) is considered , thus there exists always only one reference frame , which is the previously encoded frame fi - 1 . starting with frame fi currently to be encoded , the trajectory is retrieved for a pixel ( x0 , y0 ) t inside that frame . the first luminance and chrominance components along the trajectory y t 0 , u t 0 and v t 0 are identical to the current image values y i ( x 0 , y 0 ) , u i ( x 0 , y 0 ) and v i ( x 0 , y 0 ). the location occupied by the pixel ( x 0 , y 0 ) t in the previous frame can be calculated by adding the motion vector associated with ( x 0 , y 0 ) t to that location as given in since ( x 1 , y 1 ) t generally points not to an integer pixel position but rather has quarterpel resolution , a rounding operation is helpful to retrieve the motion vector from the next motion vector field . the resulting pixel location in frame f i - 2 is subsequently given by the associated luma component is y t 2 = y i - 2 ( x 2 , y 2 ). thus it is possible to formulate a general recursive formula for determining the j - th position of a pixel from frame f i along the trajectory for any given frame f j as notated in equation 3 : equivalently the trajectory &# 39 ; s j - th luminance sample is y t j = y i - j ( x j , y j ), the same holds for both u - and v - channels , too . the averaging along the pixel trajectory as described above can be integrated into the h . 264 encoder depicted in fig1 . firstly , all predicted frames that have not yet been deblocked are stored in a separate queue . additionally , all motion vectors that have been used to predict these frames are also stored . in order to minimize memory usage only eight past frames and motion vector fields are kept . the new inloop filter is inserted in the encoder in front of the deblocking filter , which filters according to remaining block edges and does not reverse ttf - induced improvements . for every pixel inside a frame to be deblocked a trajectory is calculated as described above . the luma components , that have to be obtained with quarterpel accuracy according to equation 3 , may be calculated using the standard h . 264 interpolation filter . for the chroma components an eighthpel interpolation is used equivalently . this results in the formation of a list of luminance values y t 0 , . . . , y t 7 and two lists of chrominance values for every pixel in the current frame . before applying an averaging operation along the trajectory the validity of the predicted trajectory has to be verified . here two indicators for a badly predicted trajectory are used . a sudden change of the luminance value δy j =| y t j + 1 − y t j | can indicate that a retrieved motion vector no longer correctly describes a pixel &# 39 ; s trajectory . ( this is for instance the case when foreground objects are partly inside a background block thus interrupting the trajectory .) in order to avoid the inclusion of wrong luminance values the trajectory is only continued if δy j ≦ t y for a given threshold t y . the derivation of an optimal t y per frame will be described in further detail below . another indicator for a badly predicted trajectory are strong variations in the motion vector field . during the retrieval of the motion vector for a specified pixel as given in equation 3 , the retrieved motion vector may be compared against the vectors for the eight neighboring 4 × 4 blocks as illustrated by fig2 . the block vote metric bv j ( x j , y j ) denotes the number of 4 × 4 blocks surrounding location ( x j , y j ) t whose motion vectors differ from the one in which ( x j , y j ) t lies . in fig2 the bv j ( x j , y j ) value is 4 . based on this metric a second threshold t bv can be used to control the length of the predicted trajectory based on the confidence in the motion vector accuracy as given in the bv metric effectively checks for every pixel along the trajectory if a number of surrounding 4 × 4 blocks has identical motion to the current trajectory &# 39 ; s motion . as an example t bv = 0 would not restrict the motion at all , while t bv = 8 only continues a trajectory if all surrounding blocks have identical motion . in this context 4 × 4 blocks lying outside the frame are not considered at all , so that blocks at the edge of the frame are only checked against a smaller number of reference blocks . intra - coded blocks are assumed to have zero motion . there are two further cases in which a trajectory may be interrupted : the first being intra - coded blocks . should a trajectory reach such a block , which has no associated motion vector , it is terminated . the second case occurs when the predicted block retrieved from the previous frame lies partly outside the frame as shown in fig3 . in this case no motion information is available for a number of pixels whose trajectory is also interrupted . both thresholds introduced above may now be optimized for each frame . for all possible combinations of 1 ≦ t y ≦ 8 and 0 ≦ t bv & lt ; 4 the trajectories for each pixel in the frame to be deblocked are calculated and the luma and chroma components of every pixel are then replaced by the average along its respective trajectory of length k ≦ 8 as given by the current implementation only applies the filter to the luma component , leaving both chroma components in their original state . each combination can now be assigned a mean squared error ( mse ) compared to the original frame to be encoded . the optimum thresholds yielding the minimum mse are then chosen and encoded in the bitstream requiring five additional bits per frame . instead of computing all 32 different versions of the filtered frame it is easily possible to calculate all mses simultaneously only having to process every pixel in the current frame once . the first advantage of this approach is that no additional side information is required on the macroblock level . secondly , every pixel is filtered with an individual filter length which helps to break up blocking artifacts . the proposed filter has been implemented in c and integrated into the jm 1l ( kta 2 . 2 ) software using the h . 264 / avc baseline profile . the resulting bit rates and peak signal - to - noise ratio ( psnr ) values are compared against the same software without the additional inloop filter , which behaves exactly as h . 264 / avc . the implementation has been tested on a large variety of sequences including five sequences in tv resolution and 15 mpeg test sequences . exemplary rd - curves for the bqsquare sequence are shown in fig4 . from the underlying data it becomes instantly apparent , that the filter is less effective for high quantization parameter , qp . this is due to the fact that at very low quality reference blocks themselves are of such poor quality that averaging along the trajectory cannot significantly improve the current frame . in the final implementation the filter was therefore switched off automatically for qp higher than 45 . for all sequences average psnr gain and bit rate reduction were measured using the bjøntegaard metric [ 7 ]. the average values are given in the tables shown in fig6 and 7 . these values show , that the filter performs well for sequences of resolutions ranging from 416 × 240 ( bqsquare ) to 2560 × 1600 ( traffic ). additionally , the actual frame rate also seems to have no significant impact on the resulting quality , as significant bit rate reduction is present both for sequences with 24 hz ( parkscene ) and with 60 hz frame rates ( bqterrace ). since the filter makes a pixel - wise decision concerning the filter length , the visual quality of the decoded bit stream is also improved as is illustrated by the segments from the bqterrace sequence shown in fig5 . the performance gain illustrated in fig5 , however , still does not explore the full potential of the proposed filter . the mse minimization is , for instance , not necessarily the best criterion for determining the transmitted thresholds . ideally , an rd - optimization scheme should be adopted to derive both t y and t bv for each frame . this is especially important since there is no linear dependency between the transmitted error residual and the amount of bits needed to encode it . equivalently , maximizing the mse of any reference picture does not result in a minimization of the total bit rate for the entire video sequence . nevertheless , the proposed algorithm has shown to improve the compression efficiency of the h . 264 / avc baseline profile for all tested sequences fully justifying the introduced overhead of five bits per frame . in summary , the present invention provides a new inloop filtering approach that uses temporal information in the form of pixel trajectories to improve the quality of reference pictures used in h . 264 / avc . the algorithm based on individual temporal pixel trajectories as described provides bit rate savings for all tested sequences , reducing the bit rate by more than 4 % on average . this is achieved without significantly increasing computational complexity or memory usage of the encoder . moreover , even better results could be achieved by adopting an rd - optimization scheme . p . list , a . joch , j . lainema , g . bjøntegaard , and m . karczewicz , “ adaptive deblocking filter ,” ieee transactions on circuits and systems for video technology ( tcsvt ), 2003 . e . dubois and s . sabri , “ noise reduction in image sequences using motion - compensated temporal filtering ,” ieee transactions on communications , vol . 32 , no . 7 , pp . 826 - 831 , jul . 1984 . 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