Patent Application: US-72780103-A

Abstract:
a method and arrangement for arithmetic encoding / decoding is described , wherein the probability estimation is performed by a finite state machine fsm , wherein the generation of n representative states of the fsm is performed offline . corresponding transition rules are filed in the form of tables . in addition , a pre - quantization of the interval width r to a number of k pre - defined quantization values is carried out . with suitable dimensioning of k and n , this allows the generation of a table containing all k × n combinations of pre - calculated product values r × p lps for a multiplication - free determination of r lps . overall , the result is a good compromise between high coding efficiency and low calculation effort .

Description:
first of all , however , the theoretical background is to be explained in more detail : as it was already mentioned above , the effect of the arithmetic coding relies on an estimation of the occurrence probability of the symbols to be coded which is to be as good as possible . in order to enable an adaptation to non - stationary source statistics , this estimation needs to be updated in the course of the coding process . generally , usually methods are used for this which operate using scaled frequency counters of the coded results [ 17 ]. if c lps and c mps designates counters for the occurrence frequencies of lps and mps , then using these counters the estimation may be performed and then the operation outlined in fig1 of the interval separation may be carried out . for practical purposes the division required in equation ( 1 ) is disadvantageous . it is often convenient and required , however , to perform a resealing of the counter readings when a predetermined threshold value c max of the overall counter c total = c mps + c lps is exceeded . ( in this context it is to be noted that with a b - bit representation of l and r the smallest probability which may be indicated correctly is 2 − b + 2 , so that for preventing that this lower limit is fallen short of , if necessary a resealing of the counter readings is required .) with a suitable selection of c max the reciprocal values of c total may be tabulated , so that the division required in equation ( 1 ) may be replaced by a table access and by a multiplication and shift operation . in order to prevent also these arithmetic operations , however , in the present invention a completely table - aided method is used for the probability estimation . for this purpose in a training phase representative probability states { p k | 0 ≦ k & lt ; n max } are preselected , wherein the selection of the states is on the one hand dependent on the statistics of the data to be coded and on the other hand on the side conditions of the default maximum number n max of states . additionally , transition rules are defined which indicate which new state is to be used for the next symbol to be coded based on the currently coded symbol . these transition rules are provided in the form of two tables : { next_state_lps k | 0 ≦ k & lt ; n max } and { next_state_mps k | 0 ≦ k & lt ; n max }, wherein the tables provide the index m of the new probability state p m when an lps or mps occurs , respectively , for the index n of the currently given probability state . it is to be noted here , that for a probability estimation in the arithmetic encoder or decoder , respectively , as it is proposed herein , no explicit tabulation of the probability states is required . rather , the states are only implicitly addressed using their respective indices , as it is described in the following section . in addition to the transition rules it needs to be specified at which probability states the value of the lps and mps needs to be exchanged . generally , there will only be one such excellent state which may be identified using its index p_state . fig2 shows the modified scheme for a table - aided arithmetic coding , as it is proposed herein . after the determination of the lps , first of all the given interval width r is mapped to a quantized value q using a tabulated mapping qtab and a suitable shift operation ( by q bit ) alternatively , the quantization may in special cases also be performed without the use of a tabulated mapping qtab only with the help of a combination of shift and masking operations . generally , here a relatively coarse quantization to k = 2 . . . 8 representative values is performed . also here , similar to the case of the probability estimation , no explicit determination of q is performed ; rather , only an index q_index is transferred to q . this index is now used together with the index p_state for a characterization of the current probability state for the determination of the interval width r lps . for this , now the corresponding entry of the table rtab is used . there , the k · n max product values r × p lps , that correspond to all k quantized values of r and the n max different from the probability states , are entered as integer values with an accuracy of generally b − 2 bits . for practical implementations a possibility is given here to weigh up between the storage requirements for the table size and the arithmetic accuracy which finally also determines the efficiency of the coding . both target variables are determined by the granularity of the representation of r and p lps . in the forth step of fig2 it is shown , how the updating of the probability state p_state is performed depending on the above coded event bit . here , the transition tables next_state_lps and next_state_mps are used which were already mentioned above in the section “ table - aided probability estimation ”. these operations correspond to the updating process indicated in fig1 in step 4 which is not explained in more detail . fig3 shows the corresponding flow chart of the table - aided arithmetic decoding . for characterizing the current partial interval in the decoder the interval width r and a value v is used . the latter is present within the partial interval and is refined successively with every read - out bit . as it may be seen from fig3 , the operations for the probability estimation and the determination of the interval width r are performed according to those of the encoder . in applications in which e . g . signed values are to be coded whose probability distribution is arranged symmetrically around zero , for coding the sign information generally an equal distribution may be assumed . as this information is one the one hand to be embedded in the arithmetic bit stream , while it is on the other hand not sensible to use a relatively compact apparatus of the table - aided probability estimation and interval separation for the case of a probability of p ≈ 0 . 5 , it is for this special case proposed to optionally use a special encoder / decoder procedure which may be illustrated as follows . in this special case the interval width of the new partial interval may be determined in the encoder by a simple shift operation corresponding to a bisection of the width of the original interval r . depending on the value of the bit to be coded , the upper or lower half of r , respectively , is then selected as a new partial interval ( see fig4 ). the subsequent renormalization and output of bits is performed as in the above case of the table - aided solution . in the corresponding decoder the required operations are reduced to determining the bit to be decoded using the value of v relatively to the current interval width r by a simple comparison operation . in the case that the decoded bit is set , v is to be reduced by the amount of r . as it is illustrated in fig4 , the decoding is ended by the renormalization and updating of v using the bit to be read in next . an alternative realization of the coding of events with a uniform probability distribution is illustrated in fig5 . in this exemplary implementation the current interval width r is not modified . instead , v is first doubled by a shift operation in the encoder . depending on the value of the bit to be coded , then , similar to the above example , the upper or lower half , respectively , of r is selected as a new partial interval ( see fig5 ). the subsequent renormalization and output of bits is performed as in the above case of the table - aided solution with the difference that the doubling of r and l is not performed and that the corresponding comparison operations are performed with doubled threshold values . in the corresponding decoder of the alternative realization first of all a bit is read out and v is updated . the second step is performed in the same way as step 1 in fig4 , i . e . the bit to be decoded is determined using the value of v relative to the current interval width r by a simple comparison operation , and in the case in which the decoded bit is set , v is to be reduced by the amount of r ( see fig5 ). every probability model , as it is used in the proposed invention , is indicated using two parameters : 1 ) the index p_state that characterizes the probability state of the lps , and 2 ) the value valmps of the mps . each of these two variables needs to be initialized at the beginning of the encoding or decoding , respectively , of a completed coding unit ( in applications of video coding about one slice ). the initialization values may thereby be derived from control information , like e . g . the quantization parameter ( of a slice ), as it is illustrated as an example in fig6 . a further possibility of adaptation of the starting distributions of the models is provided by the following method . in order to guarantee a better adaptation of the initializations of the models , in the encoder a selection of predetermined starting values of the models may be provided . these models may be combined into groups of starting distributions and may be addressed using indices , so that in the encoder the adaptive selection of a group of starting values is performed and is transmitted to the decoder in the form of an index as page information . this method is referred to as a forward - controlled initialization process . while this invention has been described in terms of several preferred embodiments , there are alterations , permutations , and equivalents which fall within the scope of this invention . it should also be noted that there are many alternative ways of implementing the methods and compositions of the present invention . it is therefore intended that the following appended claims be interpreted as including all such alterations , permutations , and equivalents as fall within the true spirit and scope of the present invention . t . wiegand , g . sullivan , “ draft text of final draft international standard ( fdis ) of joint video specification ( itu - t rec . h . 264 / iso / iec 14496 - 10 avc )”, jvt - g050 , march 2003 . d . a . huffman , “ a method for construction of minimum redundancy code ”, proc . ire , vol . 40 , pp . 1098 - 1101 , 1952 . i . h . witten , r . m . neal , j . g . cleary , “ arithmetic coding for data compression ”, communication of the acm , vol . 30 , no . 6 , pp . 520 - 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