PATENT DOCUMENT

Publication Number: US-9344720-B2
Application Number: US-201213715088-A
Country: US
Kind Code: B2

Title: Entropy coding techniques and protocol to support parallel processing with low latency

Abstract:
In a communication system, parallel encoding and decoding of serially-coded data occurs in a manner that supports low latency communication. A plurality of data items may be coded as serially-coded data sequences and a transmission sequence may be built from them. An index table may be built having a plurality of entries representing respective start points of the serially-coded data sequences within the transmission sequence. The transmission sequence may be transmitted to a channel and, thereafter, the index table may be transmitted. Latencies otherwise involved in inserting an index table into the beginning of a transmission sequence may be avoided.

Claims:
We claim: 
     
       1. A method, comprising:
 coding a plurality of data items as serially-coded data sequences, the coded data sequences having lower bit rates than the data items, 
 building a transmission sequence from the serially-coded data sequences, 
 building an index table having a plurality of entries representing respective start points of the serially-coded data sequences within the transmission sequence, and 
 transmitting the transmission sequence and the index table in a channel, wherein the transmission sequence precedes the index table in transmission order. 
 
     
     
       2. The method of  claim 1 , wherein the serially-coded data sequences each represent entropy-coded video data. 
     
     
       3. The method of  claim 1 , wherein the serially-coded data sequences each represent entropy-coded audio data. 
     
     
       4. The method of  claim 1 , wherein the serially-coded data sequences each represent entropy-coded video encryption data. 
     
     
       5. The method of  claim 1 , wherein the transmission sequence and the index table and transmitted in a common NAL unit. 
     
     
       6. The method of  claim 1 , wherein the transmission sequence and the index table and transmitted in different NAL units. 
     
     
       7. The method of  claim 1 , further comprising transmitting a back pointer to the channel, following transmission of the index table, that identifies a location of the index table. 
     
     
       8. A method, comprising:
 entropy coding input data and generating a plurality of coded strings therefrom, wherein a context of at least one coded string may be derived from a prior coded string, 
 building a transmission unit that includes, in series:
 a header region identifying the transmission unit, 
 a payload region including the coded strings, 
 an index table identifying locations of the coded strings within the payload region and 
 a back pointer, and 
 
 transmitting the transmission unit to a decoder. 
 
     
     
       9. The method of  claim 8 , wherein the transmitting of the coded slices occurs prior to the building of the index table. 
     
     
       10. The method of  claim 8 , wherein the back pointer is a variable length coded field provided in reverse order to other bits in the transmission unit. 
     
     
       11. The method of  claim 8 , wherein the transmission unit is a slice for video coding. 
     
     
       12. The method of  claim 8 , wherein the index table includes fields representing positions of the coded strings as offsets from a common location of the transmission unit. 
     
     
       13. The method of  claim 8 , wherein the index table includes fields representing positions of the coded strings as offsets from start positions of other strings. 
     
     
       14. The method of  claim 8 , wherein the index table includes fields representing positions of the coded strings as a difference in offsets of start positions of other strings. 
     
     
       15. The method of  claim 8 , wherein the header region includes a flag indicating the presence of the back pointer in the transmission unit. 
     
     
       16. A method, comprising:
 coding a plurality of data items as serially-coded data sequences, 
 building a transmission sequence from the serially-coded data sequences, 
 writing coding selections associated with the serially-coded data sequences into the transmission sequence in a transmission position following the serially-coded data sequences, and 
 transmitting the transmission sequence and the coding selections in a channel, wherein the transmission sequence precedes the coding selections in transmission order. 
 
     
     
       17. A method, comprising:
 deriving a length of a transmission unit received as serial data from a channel, 
 reading a back pointer from an end of the transmission unit, 
 determining, from the back pointer, a location of an index table, and 
 parsing the transmission unit into a plurality of entropy-coded strings according to fields of the index table, and 
 entropy decoding the strings in a plurality of parallel processing systems. 
 
     
     
       18. The method of  claim 17 , further comprising, for a given string, reading context data from a first processing system to a second processing system, the context data providing a decoding context for entropy-decoding a string assigned to the second processing system. 
     
     
       19. The method of  claim 17 , wherein the entropy-coded strings precede the index table in the received serial data. 
     
     
       20. The method of  claim 17 , wherein the back pointer is a variable length coded field provided in reverse order to other bits in the transmission unit. 
     
     
       21. The method of  claim 17 , wherein the transmission unit is a slice for video coding. 
     
     
       22. The method of  claim 17 , wherein the index table includes fields representing positions of the coded strings as offsets from a common location of the transmission unit. 
     
     
       23. The method of  claim 17 , wherein the index table includes fields representing positions of the coded strings as offsets from start positions of other strings. 
     
     
       24. The method of  claim 17 , wherein the index table includes fields representing positions of the coded strings as a difference in offsets of start positions of other strings. 
     
     
       25. The method of  claim 17 , wherein the header region includes a flag indicating the presence of the back pointer in the transmission unit. 
     
     
       26. Computer readable storage device to store entropy-coded data having stored thereon a serial datastream comprising in order:
 a header, 
 a payload with a plurality of entropy-coded strings, 
 an index table having entries identifying locations of the strings within the datastream and 
 a back pointer. 
 
     
     
       27. The storage device of  claim 26 , wherein the entropy-coded strings precede the index table in the received serial data. 
     
     
       28. The storage device of  claim 26 , wherein the back pointer is a variable length coded field provided in reverse order to other bits in the datastream. 
     
     
       29. The storage device of  claim 26 , wherein the index table includes fields representing positions of the coded strings as offsets from a common location of the datastream. 
     
     
       30. The storage device of  claim 26 , wherein the index table includes fields representing positions of the coded strings as offsets from start positions of other strings. 
     
     
       31. The storage device of  claim 26 , wherein the index table includes fields representing positions of the coded strings as a difference in offsets of start positions of other strings. 
     
     
       32. The storage device of  claim 26 , wherein the header includes a flag indicating the presence of the back pointer in the datastream. 
     
     
       33. A method, comprising:
 receiving data from a channel including a transmission sequence having a plurality of serially-coded data sequences contained therein and an index table, wherein the transmission sequence precedes the index table in reception order, 
 parsing the index table to identify respective start points of the serially-coded data sequences within the transmission sequence, and 
 decoding at least two of the data sequences using parallel processing threads, the decoding generating decoded data sequences having higher bit rates than the coded data sequences. 
 
     
     
       34. The method of  claim 33 , wherein the serially-coded data sequences each represent entropy-coded video data. 
     
     
       35. The method of  claim 33 , wherein the serially-coded data sequences each represent entropy-coded audio data. 
     
     
       36. The method of  claim 35 , wherein entropy decoding of a first data sequence develops an entropy decoding context for decoding of a second data sequence. 
     
     
       37. The method of  claim 33 , wherein the serially-coded data sequences each represent entropy-coded video encryption data. 
     
     
       38. The method of  claim 33 , wherein the transmission sequence and the index table are received in a common NAL unit. 
     
     
       39. The method of  claim 33 , wherein the transmission sequence and the index table are received in different NAL units.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present invention benefits from priority of U.S. Provisional Application Ser. No. 61/680,590, filed Aug. 7, 2012 and entitled “Entropy Coding Techniques and Protocol to Support Parallel Processing with Low Latency,” the disclosure of which is incorporated herein in its entirety. 
    
    
     BACKGROUND 
     Various video coding systems can be designed to support parallel entropy coding and entropy decoding processes for different segments of video, for example, slices, tiles or blocks. As one of the final stages of video coding, coded data from different spatial areas of a frame is formed into data sequences and entropy-coded as a string of bits. Early entropy coding techniques had been serial. A coding context carried from bit to bit, down each sequence, and then to the beginning of a next sequence. Until the entropy coding was undone serially, a decoder could not perform any parallel processing of constructs within the entropy-coded sequence. 
     Wavefront Parallel Processing (“WPP”) introduced the idea of selecting some or all of the sequences to get their entropy context from an initial portion of a previous sequence rather than from an end portion of the previous sequence. By developing the context of a given sequence from the start of the preceding sequence, parallel entropy decoding of the second sequence could be performed once decoding of the first sequence developed a decoding context for the second sequence. Thus, WPP supports parallel processing of the sequences to some degree. 
     The WPP technique, however, has certain consequences. Parallel decoding of sequences cannot be performed until sequence start points have been identified and an appropriate context has been developed for each sequence. Because the context of a current sequence is developed by entropy decoding a relevant portion of a previously-coded sequence, the WPP introduces dependencies among the sequences. Moreover, because the entropy-coded data is a serially coded bitstream, positions of the various sequences must be identified by an index field table that specifies start points of the sequences. 
     In the current design of the forthcoming HEVC coding standard, it has been proposed to provide an index in front of the entropy-coded data that identifies the bit-positions of these start points. This causes significant delay, however, because an encoder must buffer all coded video data to be represented by the table, build the table and add it to a coded bitstream as a position that precedes the coded data itself. Essentially, an encoder may start transmitting coded video data of a segment to which the table applies only after the segment is coded in its entirety. 
     The inventors perceive a need in the art for an entropy coding protocol that supports parallel-processing and yet avoids the latencies associated with prior solutions. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a system suitable for use with embodiments of the present invention. 
         FIG. 2  is a simplified block diagram of a video coding system according to an embodiment of the present invention. 
         FIG. 3  schematically illustrates an exemplary entropy coding process. 
         FIG. 4  illustrates syntax of a slice according to an embodiment of the present invention. 
         FIG. 5  illustrates a coding method according to an embodiment of the present invention. 
         FIG. 6  illustrates a method according to another embodiment of the present invention. 
         FIG. 7  is a simplified block diagram of a decoder according to an embodiment of the present invention. 
         FIG. 8  illustrates an example of a multi-thread processing system suitable for use with entropy decoding according to an embodiment of the present invention. 
         FIG. 9  illustrates an entropy decoding method according to an embodiment of the present invention. 
         FIG. 10  illustrates a syntax according to an embodiment of the present invention. 
         FIG. 11  illustrates a method according to another embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present invention provide techniques to support parallel encoding and decoding of serially-coded data in a manner that supports low latency communication. The techniques involve coding a plurality of data items as serially-coded data sequences and building a transmission sequence from them. An index table may be built having a plurality of entries representing respective start points of the serially-coded data sequences within the transmission sequence. The transmission sequence may be transmitted to a channel and, thereafter, the index table may be transmitted. Thus, latencies involved in inserting an index table into the beginning of the transmission sequence may be avoided. 
     The following discussion presents the embodiments of the present invention in the context of a video coding system but the principles of the present invention are not so limited. The present invention may find application in a variety of coding environments, such as audio coding systems, encryption systems and the like, where entropy coding of strings may provide benefits. 
       FIG. 1  illustrates a system  100  suitable for use with embodiments of the present invention. The system  100  may include at least two terminals  110 - 120  interconnected via a channel  150 . For unidirectional transmission of data, a first terminal  110  may code video data at a local location for transmission to the other terminal  120  via the channel  150 . The second terminal  120  may receive the coded video data of the other terminal from the channel  150 , decode the coded data and display the recovered video data. Unidirectional data transmission is common in media streaming applications and the like. 
       FIG. 1  illustrates a second pair of terminals  130 ,  140  provided to support bidirectional transmission of coded video that may occur, for example, during videoconferencing. For bidirectional transmission of data, each terminal  130 ,  140  may code video data captured at a local location for transmission to the other terminal via the channel  150 . Each terminal  130 ,  140  also may receive the coded video data transmitted by the other terminal, may decode the coded data and may display the recovered video data at a local display device. 
     In  FIG. 1 , the terminals  110 - 140  are illustrated as servers, personal computers and smart phones but the principles of the present invention are not so limited. Embodiments of the present invention find application with laptop computers, tablet computers, media players and/or dedicated video conferencing equipment. The channel  150  represents any number of networks that convey coded video data among the terminals  110 - 140 , including for example wireline and/or wireless communication networks. A communication network may exchange data in circuit-switched and/or packet-switched channels. Representative networks include telecommunications networks, local area networks, wide area networks and/or the Internet. In another embodiment, the channel  150  may be provided as a storage device, for example, an electrical, optical or magnetic storage device. For the purposes of the present discussion, the architecture and topology of the channel  150  is immaterial to the operation of the present invention unless explained hereinbelow. 
       FIG. 2  is a simplified block diagram of a video coding system  200  according to an embodiment of the present invention. The coding system  200  may include a video source  210 , a pre-processor  220 , a coding engine  230 , a format buffer  240 , a transmitter  250  and a controller  260 . The video source  210  may supply source video data to the rest of the system  200 . Common video sources  210  include cameras that capture video data representing local image data and storage units that store video data generated by some other system (not shown). Typically, the video data is organized into frames of image content. 
     The pre-processor  220  may perform various analytical and signal conditioning operations on video data. For example, the pre-processor  220  also may apply various filtering operations to the frame data to improve efficiency of coding operations applied by a video coder  230 . The pre-processor  220  also may perform analytical operations on the source video data to derive statistics of the video, which may be provided to the controller  260  to otherwise manage operations of the video coding system  200 . 
     The coding engine  230  may perform coding operations on the video sequence to reduce the sequence&#39;s bit rate. The coding engine  230  may parse each frame into sub-units, such as slices and coding units (“CUs”), and may code the sub-units according to motion compensated predictive coding techniques that exploit spatial and/or temporal redundancies therein. For purposes of the present discussion, it is sufficient to note that, as part of its operation, the coding engine may include a CU-based coder that includes a transform unit  232 , a quantizer  234  and an entropy coder  236 . The coding engine  230  may select and apply a coding mode to the CU. Thereafter, pixels of the CUs (which may be expressed as pixel residuals, depending on the selected coding mode) may be subject to a transform, for example a discrete cosine transform or a wavelet transform. Transform coefficients obtained from the transform unit  232  may be quantized by a quantization parameter (Qp) in the quantizer  234 . The coding mode and the quantized coefficients may be entropy coded by the entropy coder  236 . 
       FIG. 3  schematically illustrates an exemplary entropy coding process.  FIG. 3( a )  illustrates a matrix  310  of quantized coefficients that may be output from a quantizer  234  ( FIG. 2 ). Entropy coding typically involves arranging the matrix  310  into a serial string  320  of coefficients ( FIG. 3( b ) ) and coding the serial string  320  according to an entropy coding technique such as Context-Adaptive Binary Arithmetic Coding (CABAC), Huffman coding, arithmetic coding, Elias gamma coding, Fibonacci coding, Golomb coding and Golomb-Rice coding.  FIG. 3( c )  is a graphical representation of an entropy-coded string  330  of data. 
     In an embodiment, shown in  FIG. 3( d ) , where a context adaptive entropy coding technique is used, entropy coding of a first string  340  may develop a coding context for another string  350  that follows it. Further, entropy coding of the second string  350  may develop a coding context for a third string  360  that follows the second string  350 . Thus, the coding techniques applied by the entropy coder  236  define prediction dependencies among the strings  340 - 360 . The coding context of a given string (say,  360 ) may be developed from a portion  352  of the preceding string  350  and therefore, it is not necessary to code the preceding string  350  in its entirety before beginning coding of the subsequent string  360 . 
     In another embodiment, strings may be entropy coded independently of each other, by using, for example, entropy slices or tile representations. This can further streamline the decoding process by eliminating the entropy decoding dependencies between strings. In such an embodiment, the coding context of each string may reset to a predetermined state at the onset of each string and, therefore, threads need not pass coding contexts among one another. 
     Returning to  FIG. 2 , coded data may be output from the block coder  230  to a format buffer  240 . The format buffer  240  may store the output data from the coding engine  230  and build a datastream therefrom that adheres to a syntax of a coding protocol that governs communication among the terminals  110 - 140  ( FIG. 1 ). For example, the format buffer  240  may build a CU datastream that includes syntactic elements that satisfies the protocol&#39;s requirements for CUs. The format buffer  240  may build a slice data stream from corresponding CU datastreams that satisfies the protocols requirements for slices. The format buffer  240  further may pack slice transmission data into other artifacts required by the protocol, for example, Network Adaptation Layer units (NAL units). The format buffer  240  also may accept data from other sources, such as audio coders and metadata sources (not shown). The format buffer  240  may output a serial datastream representing the system&#39;s output to a transmitter  250 , which may format the datastream for transmission to the channel and output the transmission data from the coding system  200 . 
     During operation, the coding system  200  may accept the input video sequence as a stream of video data, which may be coded and output from the system  200  on a running basis. Thus, at a time when the video source  210  provides a new frame to the system  200  for coding, the format buffer  240  and transmitter  250  may be outputting coded video data of earlier-received frames. Indeed, the format buffer  240  may output coded video data of early portions of a slice from a given frame while the coding engine  230  is generating coded video data later portions of the same slice. To provide high throughput, operations of the components illustrated in  FIG. 2  may be distributed across parallel processing systems (not shown), subject to dependencies of the coding operations (for example, the entropy coding processes described above in  FIG. 3( d ) ). 
       FIG. 4  schematically illustrates syntax of slices  400 ,  450  according to various embodiments of the present invention. In a first embodiment, shown in  FIG. 4( a ) , a slice  400  may include a slice header  410 , a slice payload  420 , a backpointer  430  and a string index  440 . The slice header  410  may include a data pattern indicating the onset of the slice  400 , which may include metadata (not shown) defining coding parameters that have been applied to the slice. In an embodiment, the slice header  410  may include a flag  412  to indicate whether the slice  400  includes a backpointer  430  or not. The payload  420  may include coded video data of the slice, including one or more entropy-coded strings  472 - 478 . The backpointer  430  may be provided at the end of the slice  400  and may identify a location of a string index  440  within the slice. The string index  440  may indicate locations of the strings  472 - 478  within the slice payload  420 . In the embodiment illustrated in  FIG. 4( a ) , the slice  400  may be provided in a common NAL unit. 
     As indicated, the slice header  410  may include a data pattern that indicates the start of a slice within the serial data stream and a flag  412  that indicates whether the slice  400  includes a backpointer  430 . In an embodiment, the slice header  410  may include fields to provide the index table within the slice header  410  itself (not shown). Thus, embodiments of the present invention permit an encoder to place signaling for the index table  440  either at the beginning of a slice  400  within the slice header  410  or at the end of a slice whose location is identified by the backpointer  420  based on local coding decisions made by the encoder. 
       FIG. 4( b )  schematically illustrates a syntax that may be used in accordance with another embodiment of the present invention. In the embodiment of  FIG. 4( b ) , a slice  450  may be provided in a NAL unit  460  that is different from a NAL unit  470  in which the string index table  480  is provided. The slice may include a slice header  410  and payload  420  as in the  FIG. 4( a )  embodiment. The slice header  410  may include a flag  412  to indicate whether an index table  480  is used and the payload  420  may include entropy-coded strings  422 - 428 . The index table  480  may include entries to indicate start points of the strings  422 - 428  within the slice  450 . 
     As indicated, the embodiment of  FIG. 4( b )  may provide the string index table  480  in a NAL unit  470  that is separate from the NAL unit  460  in which the slice  450  is provided. A back pointer need not be used in the embodiment of  FIG. 4( b ) . In this embodiment, the location of the index table may be provided expressly or impliedly within metadata of the second NAL unit  470  or it may be provided as a field within the slice header  410 . In another embodiment, the string index table  480  may be provided within a supplemental enhancement information (“SEI”) message within the coded bit stream. To maximize performance, often it will be most convenient to provide the string index table  480  in a NAL unit  470  that immediately follows the NAL unit  460  to which the table  480  refers. 
     The following tables illustrate a syntax of a slice in an embodiment consistent with  FIG. 4( a ) . Table 1 illustrates a syntax of a slice  400  according to this embodiment. 
                                 TABLE 1                       slice_layer_rbsp( ) {   Descriptor                                                slice_header( )           slice_data( )           slice_extension( )           rbsp_slice_trailing_bits( )                         }                        
where slice_header( ) represents content of the slice header  410 , slice_data( ) represents content of the slice payload  420 , slice_extension( ) represents content of the index table  440  and the back pointer  430 . The field rbsp_slice_trailing_bits( ) may represent a process for forming the transmission bitstream.
 
     Table 2 illustrates an exemplary syntax that may be used within a slice header  410  according to these embodiments: 
                         TABLE 2               slice_header( ) {   Descriptor                                            • • •                         if( tiles_or_entropy_coding_sync_idc = = 1 | |                             tiles_or_entropy_coding_sync_idc = = 2 ) {               num_entry_point_offsets   ue(v)           if( num_entry_point_offsets &gt; 0 ) {                             offset_len_minus1   ue(v)           for( i = 0; i &lt; num_entry_point_offsets; i++ )                             entry_point_offset[ i ]   u(v)                         }                         }                        
In the foregoing, the field num_entry_point_offsets may represent a number of strings included within the payload field  420  and, by consequence, the number of entries within the table. In this embodiment, the num_entry_point_offsets field may double as a flag  412  to identify the presence of a back pointer  430 . A value of zero may indicate there are no table entries within the slice header  410  and may indicate impliedly that the slice  400  includes a back pointer  430 . A non-zero value may identify a number of entries provided within the slice header. The entry_point_offset[i] fields may represent respective locations within the payload field  420  of the start points of the strings  472 - 478 . For i&gt;0, the field entry_point_offset[i] may be calculated as entry_point_offset[i]=entry_point_offset[i−1]+entry_point_offset_delta[i], where the entry_point_offset_delta[i] field represents a change in length among coded successively-coded strings.
 
     As indicated, the backpointer  430  may include data that identifies the location of an index table  440 . The backpointer  430  may include one or more variable length codes. As a series of variable length code, data of the backpointer  430  may be provided in reverse order within the slice  400 . That is, backpointer data may start with the last bit position of the slice and propagate from the last bit position forward toward the slice header  410 . 
     Table 3 illustrates an exemplary syntax that may be used for slice extension data according to these embodiments: 
                         TABLE 3               slice_extension( ) {   Descriptor                                            encoded_length = 0;           while (slice_data_remaining( ) &gt;                             ue_length_of( encoded_length)) {               slice_extension_tag   ue(v)           slice_extension_length   ue(v)           encoded_length += slice_extension_length +                         ue_length_of( slice_extension_tag ) +           ue_length_of( slice_extension_length );                             slice_extension_data   u(v)                             }               extension_back_pointer   rev-ue(v)                 }                    
In this example, the slice_extension_data field occupies slice_extension_length bits and has a structure indicated by the slice_extension_tag value. The value of extension_back_pointer is equal to encoded_length, and, as indicated, may be in the bitstream with the bits in reverse order. The function ue_length_of(x) returns the number of bits needed to encode the value x as a ue(v). The slice_extension_tag of the entry_point array may be defined to be 0 (which compactly codes as the bit ‘1’) and all other values may be reserved.
 
     Table 4 illustrates a embodiment for slice_extension_data when slice_extension_tag==0: 
                         TABLE 4               slice_extension_data( ) {   Descriptor                                            switch (slice_extension_tag) {                         case 0: /* entry points */                             offset_len_minus1   ue(v)           num_entry_point_offsets = 0;           while (extension_data_remaining( )) {                             entry_point_offset[ i ]   u(v)           num_entry_point_offsets++;                         }                          break;           }                 }                    
This structure resembles the table structure in the slice header above (Table 2).
 
     The foregoing discussion has presented the backpointer  430  and string index table  440  as the only metadata that is provided at the end of the slice  400 . The principles of the present invention do not foreclose use of metadata  460  provided by other sources (not shown). In embodiments where no other data is permitted in the end-of-slice structures, a backpointer  430  need not include an express pointer to the index table  440 . 
     String start points (shown as entry_point_offsets in Table 2 and Table 4) may be coded in a variety of ways. In a first embodiment, each string start point may be expressed as an offset from the end of the slice header. In a second embodiment, each string start point may be expressed as an offset from a start point of a preceding string (essentially, corresponding to prior string&#39;s length). In this embodiment, the start point of the first string may be taken to begin immediately following the end of the slice header. 
     In another embodiment, each string start point may be expressed as a difference in offsets between the current string&#39;s start point and the preceding string&#39;s start point (corresponding to a difference in lengths between the prior two strings). This is shown below in Table 5. 
     
       
         
           
               
               
             
               
                   
                 TABLE 5 
               
               
                   
                   
               
             
            
               
                   
                 i = 0; 
               
            
           
           
               
               
            
               
                   
                 while (extension_data_remaining( )) { 
               
            
           
           
               
               
               
            
               
                   
                 if (i == 0) entry_point_offset[0] 
                 ue(v) 
               
            
           
           
               
               
               
            
               
                   
                 else { entry_point_offset_delta[ i ] 
                 se(v) 
               
            
           
           
               
               
            
               
                   
                 entry_point_offset[i] = 
               
            
           
           
               
               
            
               
                   
                 entry_point_offset[ i − 1] + 
               
            
           
           
               
               
            
               
                   
                 entry_point_offset_delta[ i ]; 
               
            
           
           
               
               
            
               
                   
                 i++; 
               
            
           
           
               
               
            
               
                   
                 } 
               
            
           
           
               
            
               
                 num_entry_point_offsets = i 
               
               
                   
               
            
           
         
       
     
       FIG. 5  illustrates a coding method  500  according to an embodiment of the present invention. The method  500  may be applied when coding video data in slices. The method  500  may begin by transmitting a slice header  510 . Thereafter, the method  500  may cause video data to be coded (box  515 ) and transmitted (box  520 ) on a running basis. As it codes the video data, the method  500  may determine when new entropy-coded strings are started (box  520 ) and, when they do, may record the position of those strings (box  530 ). Operations of boxes  515 - 530  may repeat during coding of the slice. 
     At some point, the method  500  will reach the end of a slice. The method  500  may determine when coding has reached the end of the slice (box  535 ) and, when it does, may build an index table representing string start locations within the slice (box  540 ). The method  500  may transmit the index table (box  545 ) and any other metadata that may be required to serve other decoding needs associated with the slice (box  550 ). As a final transmission associated with the slice, the method  500  may transmit data of the backpointer, which identifies the location of the index table and is transmitted in reverse bit order (box  555 ). 
     As indicated, operation of the method  500  advantageously allows coded data to be transmitted as it is generated, without having to build the index table first. The method  500  may mark location(s) of the entropy-coded strings as the video data is generated and transmitted. The method  500  may transmit the index table (box  545 ) and, finally, the backpointer (box  555 ) without incurring delays that would be associated with transmitting the index table as part of the slice header. In this manner, the method  500  contributes to reduced latency of transmission. 
       FIG. 6  illustrates a method  600  according to another embodiment of the present invention. The  FIG. 6  embodiment illustrates a method  600  that may accommodate dynamic selection of slice structure, whether to provide an index table at the end of a slice or the beginning of a slice. The method  600  may begin when coding of a new slice is to begin. At the outset, the method  600  may determine which slice structure is to be used (box  610 ). If the method  600  determines that the index table is to be provided at the end of the slice, the method  600  may invoke operations as described in  FIG. 5 . Specifically, the method  600  may cause video data to be coded and entropy-string locations to be recorded (box  615 ) and may cause the coded video data to be transmitted to decoder (not shown) as the coded video data is prepared (box  620 ). The method may repeat operations of boxes  615 - 620  until all video data associated with the slice has been coded (box  630 ). 
     When the method  600  determines that the slice has been completed (box  630 ), the method  600  may build the index table representing start positions of the strings (box  630 ). The method  600  may transmit the index table and, finally, the backpointer to the decoder (box  635 ). 
     If at box  610  the method  600  determines that the slice will have the index table at the beginning of the slice, operation may advance to box  640 . The method  600  may code video data of the slice and mark string locations within the slice (box  640 ). The method  600  may store the coded video data in a buffer for later transmission (box  645 ). The method  600  may repeat operations of boxes  640 - 645  until all video data associated with the slice has been coded (box  650 ). 
     When the method  600  determines that the slice has been completed (box  650 ), the method  600  may build the index table representing start positions of the strings (box  655 ). The method  600  may place the index table in the slice header (box  660 ) and, finally, transmit the entirety of the slice to a decoder (box  665 ). 
     The method of  600  finds application with a multi-modal coding system that supports use of index tables both at the beginning and at the end of slices. As indicated, transmitting an index table at the end of the slice can reduce latency because coded video data may be transmitted as it is created (boxes  615 - 620 ). The method  600  may prove to be a natural extension of some coders that already support coding protocols that provide index tables at the beginning of slices. Thus, although the operations of boxes  640 - 665  involve greater transmission latency than the operations of boxes  615 - 635  (because transmission does not occur until box  665 , when the entire slice has been coded), the embodiment of  FIG. 6  may prove to be a useful extension of those coders. 
       FIG. 7  is a simplified block diagram of a decoder  700  according to an embodiment of the present invention. The decoder  700  may include a receiver  710 , a datastream parser  720 , a decoding engine  730 , a post-processor  740  and a video renderer  750 . The decoder  700  may invert coding operations applied by a video coder ( FIG. 2 ). The receiver  710  may receive data from a channel and recover a serial datastream therefrom. The parser  720  may identify coding artifacts within the datastream and route such artifacts to appropriate decoding systems. For example, coded slice data may be output to the decoding engine  730  for processing. As part of this operation, the parser  720  may interpret slice headers or backpointers (as the case may be) to recover an index table and identify start points of entropy-coded strings within a received slice. The decoding engine  730  may invert coding operations performed by the coding engine of the video coder ( FIG. 2 ) and, therefore, may perform motion compensated predictive decoding. The post-processor  740  may perform filtering or other operations upon recovered data. Recovered video data obtained therefrom may be output to the video renderer  710  for display or storage. 
     As illustrated in  FIG. 7 , the decoding engine  730  may include an entropy decoder  732 , a dequantizer  734  and an inverse transform unit  732  that invert coding operations performed by their counterparts in the video coder ( FIG. 2 ). The entropy decoder  732  may recover quantized coefficient data from entropy-coded strings provided in the slices ( FIG. 4 ). The dequantizer  734  may scale dequantized coefficient data according to the quantization parameters used at the video coder. The inverse transform unit may perform an inverse transform on scaled coefficients output from the dequantizer  734  to generate pixel data therefrom. Thereafter, the recovered pixel data may be output to other stages (not shown) such as prediction units to generate final recovered pixel data. 
       FIG. 8  illustrates an example of a multi-thread processing system  800  suitable for use with entropy decoding according to an embodiment of the present invention. In an embodiment, a video decoder may employ parallel processing systems (herein, “threads”) to perform entropy decoding. In this example, the multi-thread system  800  may include three processors  810 - 830 , each to decode entropy-coded strings that may be found in a received slice ( FIG. 4 ). Once string start points are identified, a first string may be provided to a first processor  810  to begin entropy decoding. Once the first string has been decoded sufficiently to develop a context for decoding the second string, the second string and context data may be provided to a second processor  820 . The second processor  820  may decode the second string to generate context data in addition to the decoded data of the second string. Thereafter, data of a third string may be provided to a third processor  830  along with the context data provided by the second processor  820  and entropy-decoding of the third string may commence. Although in theory parallel decoding may be extended to additional threads indefinitely, in practice, the number of parallel threads likely will be limited to a finite number by system designers as tradeoff between the performance improvements to be obtained and the costs of additional resources that are required to provide such threads. 
     The principles of the present invention also find application with strings that are coded independently of each other. In such an embodiment, the coding context of each string may reset to a predetermined state at the onset of each string and, therefore, threads need not pass coding contexts among one another. Thus, the present invention may apply to entropy slices and tiles. 
       FIG. 9  illustrates an entropy decoding method  900  according to an embodiment of the present invention. The method  900  may begin when a new coded slice is available for decoding. According to the method  900 , slice length data may be received (box  910 ) to determine the length of a backpointer provided at the end of the slice. The slice length data may be derived from transmission data recovered by recovery processes within a receiver. In one embodiment, the length of a slice may be determined from the length of a NAL unit that contained the slice, less any padding bits that are indicated as provided within the NAL unit. In another embodiment, a boundary between slices may be determined from NAL unit start codes that indicate the beginning and ends of NAL units; the end of a slice within a current NAL unit may be taken as the bit that precedes a start code of a next-received NAL unit. Having identified the end of the slice, the method  900  may read data from the end of the slice in reverse bits order (box  920 ). The method  900  may decode the end-of-slice data as a backpointer (box  930 ) and identify the location of a string index therefrom (box  940 ). The back pointer may point to the index table directly or, alternatively, may point to a slice extension that includes data (such as a slice extension tag) that identifies the index table. The method  900  may read string index data to identify start locations of strings within the slice (box  950 ). Using the string start locations, the method  900  may parse payload data of the slice into the strings and may distribute the strings to parallel threads as shown in connection with  FIG. 8 . 
     Operation of the method  900  of  FIG. 9  and the end-of-slice structures illustrated in  FIG. 4  are believed to reduce decoder latency in many use cases. The performance consequences, however, may vary depending on data rates of the channel ( FIG. 1 ) and resources available at the decoder ( FIG. 7 ). Some use cases are informative: 
     For receivers that load from disk (e.g., the channel is a storage device) or otherwise get the whole NAL unit in an atomic unit, a decoder will have instant access to the entirety of a slice upon receipt. The decoder may estimate the position of the back-pointer immediately, retrieve the index table and parse the slice payload to begin parallel threads as illustrated in  FIG. 8 . 
     For receivers that receive coded slices incrementally, a decoder can perform single-threaded entropy decoding immediately upon reception. The decoder cannot perform parallel processing for entropy decoding, however, until the backpointer is received. Thus, if the decoder can perform single-thread entropy decoding at a rate faster than the data arrival rate, the decoder will never start a second thread, but this does not incur a performance loss because single-threaded entropy decoding likely is the most efficient decoding structure to employ in such cases. 
     If the data arrival rate is faster than the decoder&#39;s single-thread decode rate, the end-of-slice structure incurs a performance consequence. In this case, the decoder will perform single threaded entropy decoding until it receives and decodes the backpointer. Once the decoder decodes the backpointer, it may engage additional threads to decode whatever strings in the slice may remain for entropy decoding. Nevertheless, it is believed that the end-of-slice structure contributes to reduced latency overall because, as discussed in  FIG. 6 , the encoder is able to transmit a slice&#39;s payload at a point earlier than it would otherwise be able to transmit if index tables were forced to be included with slice headers at the beginning of such slices. 
     The principles of the present invention also accommodate uses of end-of-slice coding for other types of coded information. The structure in Table 1 permits any data that must be generated after encoding to be transmitted after encoding, not just WPP entry points. Such other data may include post-filtering instructions or hints, or other information that is coding-dependent. For example, many coding systems also provide deblocking information within slice headers representing post-filtering operations that can be performed at a decoder following video reconstruction operations. Again, providing such deblocking in the beginning of slices can incur latency because a video coder must buffer all coded video data as it makes decisions as to the types of deblocking filters to be applied to the video, then code and insert its selections of the deblocking filters into the slice headers, before it can transmit the slice. Alternatively, the encoder may select a deblocking filter to be applied before coding occurs, which might prove to be sub-optimal. Embodiments of the present invention, therefore, as illustrated in  FIG. 10 , accommodate use of end-of-slice indicators, such as backpointers  1030  and signaling structures  1040  that contain the encoder&#39;s parameter selections (such as deblocking filter selections) to reduce such latencies. In this embodiment, the slice  1000  may include a slice extension  1040  that includes the parameter indicators  1042  merged with other content. The backpointer  1030  may point to the start point of the slice extension  1040  and the slice extension may include data, such as a slice extension tag, that indicates the onset of the parameter indicators  1042 . 
       FIG. 11  illustrates an entropy decoding method  1100  according to an embodiment of the present invention. The method  1100  may begin when a new coded slice is available for decoding. According to the method  1100 , slice length data may be received (box  1110 ) to determine the length of a backpointer provided at the end of the slice. The slice length data may be derived from transmission data recovered by recovery processes within a receiver. As in prior embodiments, the length of a slice may be determined from the length of a NAL unit that contained the slice, less any padding bits that are indicated as provided within the NAL unit. Alternatively, a boundary between slices may be determined from NAL unit start codes that indicate the beginning and ends of NAL units; the end of a slice within a current NAL unit may be taken as the bit that precedes a start code of a next-received NAL unit. Having identified the end of the slice, the method  1100  may read data from the end of the slice in reverse bits order (box  1120 ). The method  1100  may decode the end-of-slice data as a backpointer (box  1130 ) and identify the location of a slice extension therefrom (box  1140 ). The method  1100  may decode the slice extension and parse the slice extension according to slice extension tags contained therein (boxes  1150 - 1160 ). For example, a first type of slice extension tag may indicate the presence of deblocking filter selections while another type of slice extension tag may indicate the presence of some other coding parameter. Thereafter, the method  1100  may cause received data to be processed according to the parameter selections identified by each slice extension tag (box  1170 ). 
     For ease of description, the preceding discussion has presented the entropy-coding and entropy-decoding processes in the context of a video coding/decoding system ( FIGS. 1 and 7 ). The principles of the present invention, however, are not so limited. The entropy-coding and decoding processes of the present invention find application to other types of coding systems in which source data is presented to an entropy-coder as a plurality of sequences that are coded as strings. For example, the principles of the present invention find application in audio coding/decoding systems and/or encryption systems. In both cases, sequences of coded source data (coded audio data or encrypted data, as the case may be) may be presented to an entropy coder, which may code the sequences as respective strings. The strings may develop coding and decoding contexts for other strings. The coded strings may be packaged into transmission units that include a header, payload, index table and back pointer as described above in  FIG. 4 . The transmission unit may be parsed by an entropy decoder, which interprets the header, back pointer and index table respectively to identify coded strings therein. The entropy decoder also may engage parallel decoding threads corresponding to the threads of  FIG. 5 , as discussed above. In this regard, the entropy coding and decoding processes described herein may apply to a wide variety of data types and content. 
     Several embodiments of the invention are specifically illustrated and/or described herein. However, it will be appreciated that modifications and variations of the invention are covered by the above teachings and within the purview of the appended claims without departing from the spirit and intended scope of the invention.

Metadata:
Filing Date: 20121214
Publication Date: 20160517
Grant Date: 20160517
Priority Date: 20120807
Inventors: SINGER DAVID W.
TOURAPIS ALEXANDROS
LEONTARIS ATHANASIOS
ZHOU XIAOSONG
Assignee: APPLE INC
CPC Classifications: [{"code": "H04N19/117", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04N19/436", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04N19/174", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04N19/70", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04N19/13", "inventive": true, "first": true, "tree": "[]"}, {"code": "H04N19/13", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04N19/117", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04N19/70", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04N19/436", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04N19/174", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04N19/00121", "inventive": true, "first": true, "tree": "[]"}]
Family ID: 50066181