WO2011088593A1

WO2011088593A1 - Method and device for encoding of an image block and method and device for decoding of an image block

Info

Publication number: WO2011088593A1
Application number: PCT/CN2010/000107
Authority: WO
Inventors: Xiaozhong Xu; Zhibo Chen
Original assignee: Thomson Licensing
Priority date: 2010-01-25
Filing date: 2010-01-25
Publication date: 2011-07-28

Abstract

A method for encoding a current image block preceded by previous block coding modes selected of one or more preceding image blocks is proposed. Said method comprises using the current image block for selecting a current block coding mode from a predefined set of coding modes, statistically analysing, at least, the previous block coding mode selections for determining selection frequencies of at least the previous block coding modes, determining a current mapping between those block coding modes, for which selection frequencies are determined, and codes of variable length, which are ordered according to increasing length, by ordering the block coding modes according to decreasing selection frequency, encoding the current image block using the current block coding mode and writing current code onto which the current block coding mode is mapped and the encoded current image block to the bit stream which comprises additional information allowing for determining the current mapping.

Description

METHOD AND DEVICE FOR ENCODING OF AN IMAGE BLOCK AND METHOD AND DEVICE FOR DECODING OF AN IMAGE BLOCK

TECHNICAL FIELD The invention is made in the technical field of image block encoding .

BACKGROUND OF THE INVENTION

For encoding, images are commonly partitioned into image blocks which are encoded separately. Some encoding

frameworks allow for selection of a block coding mode for each image block, separately. If block coding mode

selection is such, that a current block coding mode is adapted to a current image block to-be-encoded, this allows for very efficient encoding. But, further encoding of code allowing for determination of the selected block coding mode is required.

An example of such block coding modes are partitioning masks used for separating image blocks into two or more partitions which are then transformed, for instance

according to a partition-based transform scheme,

separately. If a current partitioning mask is selected according to a current texture pattern present in the current image block, the partitioning allows for separating parts of the texture which are dissimilar to each other but show some self-similarity. Transform of each of these parts separately allows for more efficient removal of redundancy than transformation of all parts together. Binary examples of partitioning masks are depicted in Fig. 1.

Another example of such block coding modes are block archetypes. For encoding a current block, a block archetype is selected from a set of predefined block archetypes wherein the block archetype is selected such that it a residual of blocks with respect to the archetype is

minimal. Then, only the residual is encoded.

Yet another example is intra prediction as used in H.264. The intra prediction of H.264 selects one prediction direction out from 9 possible directions to adapt to different textural orientation in image. Block coding modes can also refer to different block sizes of the current block.

If the selected block coding mode is from an ordered list of predefined block coding modes, the codes can be mapped on predefined block coding modes by being variable length codes representing ordinal numbers of positions of the block coding modes in the ordered list. For determining the mapping, the ordered list is present at encoder side as well as decoder side. In order to keep bit rate low

globally resulting from the codes, block coding modes are ordered in the list according to global selection

frequencies .

SUMMARY OF THE INVENTION There is an ongoing effort in the art for providing

alternative encoding methods.

Local selection frequencies of block coding modes may differ significantly from global ones. The inventors realized that using local selection frequencies for

ordering instead of global ones can result in improved encoding efficiency of the codes.

Therefore, a method for encoding a current image block preceded in an encoding order of a bit stream by one or more previous block coding modes selected for encoding one or more preceding image blocks is proposed, said method for encoding being in accordance with claim 1. And a

corresponding decoding method according to claim 7 is proposed. Further, an encoder according to claim 6 and a decoder according to claim 10 are proposed.

Said method comprises the steps of using the current image block for selecting a current block coding mode from a predefined set of block coding modes, statistically

analysing, at least, the previous block coding mode

selections for determining selection frequencies of at least the previous block coding modes, determining a current mapping between those block coding modes, for which selection frequencies are determined, and codes of variable length, which are ordered according to increasing length, by ordering the block coding modes according to decreasing selection frequency, encoding the current image block using the current block coding mode and writing current code onto which the current block coding mode is mapped and the encoded current image block to the bit stream which

comprises additional information allowing for determining the current mapping.

In an embodiment of the encoding method, only previous block coding mode selections are analysed and selection frequencies are determined for all block coding modes of the predefined set, the additional information comprises preceding codes onto which the previous block coding modes are mapped. As the preceding codes are already comprised in the bit stream no additional bit rate is generated by the

additional information.

Thus, a method of efficiently representing mode information in Image/Video signal is disclosed. In an embodiment, the present invention includes essentially: the order of possible modes are presented in a given syntax level (i.e. slice, picture, GOP, sequence, etc.) and the entropy encoding/decoding method of the reordered mode information. The features of further advantageous embodiments are specified in the dependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments of the invention are illustrated in the drawings and are explained in more detail in the following description. The exemplary embodiments are explained only for elucidating the invention, but not limiting the invention's disclosure, scope or spirit defined in the claims. In the figures:

Fig. 1 depicts examples of binary partition masks;

Fig. 2 depicts a flowchart of a first exemplary image block encoding;

Fig. 3 depicts a flowchart of a first exemplary image block decoding,

Fig. 4 depicts a flowchart of a second exemplary image block encoding;

Fig. 5 depicts a flowchart of a second exemplary image block decoding; and Fig. 6 depicts a flow chart of exemplary updating of

code-mode mapping.

EXEMPLARY EMBODIMENTS OF THE INVENTION

The invention may be realized on any electronic device comprising a processing device correspondingly adapted. For instance, the invention may be realized in a television, a mobile phone, a personal computer, a digital still camera, a digital video camera, an mp3-player, a navigation system or a car audio system. In particular, the invention is exemplarily embodied in a video or still image encoder or in a video or still image decoder .

In such encoder or decoder, a module is comprised which allows for using a current image block C for selecting a block coding mode to be used or being used for coding of said current block C. The coding modes are defined by a codec and existing in both encoder and decoder memory.

For instance, the module is adapted for analysing the image block's texture and an indexed partitioning mask is

selected from an indexed partitioning mask list.

In the encoder an encoding module is comprised which is adapted for using the selected block coding mode for encoding the current image block.

The encoding module is further adapted for encoding the selected block coding mode using a mode index table and variable length codes of the ordinal numbers. The mode index table maps ordinal numbers of an index onto

predefined block coding modes, implicitly or explicitly. The decoder comprises a corresponding decoding module.

The mode index table used for encoding the current image block must be used for decoding, also.

Thus, either a global and invariant mode index is predefined in the codec and stored in both, encoder memory as well as decoder memory. An example of such pre-defined index, also called original mode index, is shown in table I wherein only the last column of table I need to be stored:

Code Ordinal number Mode

1 0 Mode a

010 1 Mode b Oil 2 Mode c

00100 3 Mode d

0...1... N Mode x

Table I

For ensuring encoding efficiency the modes are ordered in the table such that more frequent modes are provided with shorter codes. Thus, mode "a" is selected, at a global level, at least as frequent as mode "b" which is selected at least as frequent as mode "c" and so on.

Or, as proposed by the current invention the mode index is locally adaptive and variant, as for modes, which really depend on the content, occurrence frequency can vary greatly for different content.

Then, a current mode index table, also called reordered mode index, providing a current mapping between coding modes and codes, the current mapping being used at encoder side, must be determinable at decoder side. In a first exemplary embodiment this is achieved by writing additional updating information to the bit stream wherein the updating information can be used for determining a current mapping, the updating information being used as is or together with a previous mapping. The writing of updating information can be based on a certain syntax level (e.g. at Group of Pictures (GoP) level, at scene cut level or at sequence level) . After encoding a current GoP, updating information (the new mapping or an update of a previous mapping) is written to the bit stream before encoding the next GoP, in order to update the relation between mode information and code. This allows for analyzing the current GoP statistically regarding frequency of occurrence of the different coding modes and for determining a current mapping matched to the statistical properties of the current GoP. Using such current mapping for encoding coding modes used for encoding of blocks of the current GoP then provides optimal code for the current GoP. Thus, the current mapping is determined at encoder side using not yet encoded information also, therefore additional bit rate is required for the updating information .

But, the current mapping needs to provide a mapping only for those coding modes which are actually used in the current GoP. And an update mapping from previous ordinal numbers to current ordinal numbers is sufficient for updating a previous mapping.

An example of such update mapping is shown in table II assuming that only M modes are used for encoding current GoP. Only the last column of table II needs to be

represented in the bit stream:

Table II If the updating is based on the predefined index as

previous mapping, update mapping is a queue of parts of the predefined index re-sorted with a given order. If length of the queue can differ from length of the predefined index, because modes not selected for encoding the current picture or segment are not presented in the queue, the length of the queue (e.g. M is Table II) is also written to the bitstream to inform the decoder about the length of the queue. Then the queue will be written in the bit stream to indicate the update mapping.

Use of the first exemplary embodiment of the proposed invention requires a change in syntax at a syntax level which can be image level, GoP level, sequence level or the like. Exemplarily assumed that the modes are used in block level, the mode reordering is done in picture level and Exp-Golomb code is used for index coding. Then, syntax can be as shown in Table III:

Table III Fig. 2 depicts an example of the encoding process according to the first embodiment. Images of an image sequence IS are analysed in a pre-processing step PREP using original mode index OMI . Pre-processing results in reordered mode index RMI. Further, image block data from the image sequence IS is analysed for texture patterns in step TPA which results in a current coding mode MOD to be used for encoding of the image block in step BCD. The current mode MOD is further used for determining a current code CCD wherein reordered mode index RMI and a code table CTB stored in memory MEM are used. Memory MEM also stores original mode index OMI which is used for determining update mapping UPM. The update mapping, the encoded image block and the current code are written to output bit stream BS. Fig. 3 depicts an example of the decoding process according to the first embodiment. From bit stream BS, update mapping UPM, current code CCD and block code BCD are read. Update mapping UPM is used together with original mode index OMI retrieved from a memory MEM for determining reordered mode index RMI. Reordered mode index RMI is used together with code table CTB retrieved from memory MEM for determining mode MOD. Mode MOD is then used for decoding image block BLK from block code BCD. Block BLK is outputted as part of image sequence IS. In a second exemplary embodiment, the current mapping is determined at encoder side using already encoded

information, only. Then, the current mapping can be

determined in the same at decoder side using already decoded information and no updating information need to be sent.

For instance, a given set of data can be divided into several sub-divisions (e.g. a scene cut can be divided in several GoPs, or a GoP can be divided into series of pictures) . After encoding/decoding current coding modes used for encoding/decoding blocks of a current sub-division wherein the current coding modes are encoded/decoded using a current mapping, the actual mode distribution for the current sub-division can be calculated at both encoder and decoder. The calculated mode distribution information can then be used to establish a next mapping, by updating or replacing the current mapping, the next mapping then being used for encoding/decoding next coding modes used for encoding/decoding of blocks of a next sub-division.

The actual mode distribution can be determined using statistical analysis, structural analysis of image content for most dominant modes or can result from offline

training .

Fig. 4 depicts an example of the encoding process according to the second embodiment. Preceding images in image

sequence IS are analysed in a pre-processing step PREP using original mode index OMI stored in memory MEM. Preprocessing results in reordered mode index RMI. Further, image block data from the image sequence IS is analysed for texture patterns in step TPA which results in a current coding mode MOD to be used for encoding of the image block in step BCD. The current mode MOD is further used for determining a current code CCD wherein reordered mode index RMI and a code table CTB stored in memory MEM are used. Then, the encoded image block and the current code are written to output bit stream BS.

Fig. 5 depicts an example of the decoding process according to the second embodiment. Previous coding modes are

determined from bit stream BS and pre-processed PREP using one or more previous reordered mode index RMI' and/or original mode index OMI. This results in current reordered mode index RMI which is stored in memory MEM. Reordered mode index RMI is used together with code table CTB

retrieved from memory MEM for determining mode MOD

corresponding to current code CCD. Mode MOD is then used for decoding image block BLK from block code BCD. Block BLK is outputted as part of image sequence IS.

Fig. 6 depicts an exemplary embodiment of code-mode mapping in detail. The code shown in table I and table II is exemplary Exp- Golomb code of order 0. Any entropy encoding is applicable for encoding ordinal numbers, for instance Exp-Golomb coding of higher order or Arithmetic coding can be applied ' instead wherein both, encoder and decoder, use the same entropy encoding/decoding method.

The decoder first determines a current reordered mode index, either by doing the same type of statistical or structural analysis as the encoder does or by decoding the update mapping from the bit stream and determining the current reordered mode index using the decoded update mapping and the original mode index or a previous reordered mode index.

Then for each block for which current reordered mode index is valid, the decoder decodes the corresponding coding mode by entropy decoding a code word (known by both encoder and decoder) . The reordered mode index is used to convert the code word into an ordinal number which allows for

retrieving the corresponding coding mode, e.g. the

partitioning mask from a decoder memory.

Claims

CLAIMS :

1. Method for encoding of a current image block using a current block coding mode preceded in an encoding order of a bit stream by one or more previous block coding modes selected for encoding one or more preceding image blocks, said method comprises the steps of

- using the current image block for selecting a current block coding mode from a predefined set of block coding modes , - statistically analysing, at least, the previous block coding mode selections for determining selection

frequencies of at least the previous block coding modes,

- determining a current mapping between those block coding modes, for which selection frequencies are determined, and codes of variable length, which are ordered

according to increasing length, by ordering the block coding modes according to decreasing selection

frequency,

- encoding the current image block using the current block coding mode and

- writing current code onto which the current block coding mode is mapped and the encoded current image block to the bit stream which comprises additional information allowing for determining the current mapping.

2. Method according to claim 1, wherein only previous block coding mode selections are analysed and selection

frequencies are determined for all block coding modes of the predefined set, the additional information comprises preceding codes onto which the previous block coding modes are mapped.

3. Method according to claim 2, wherein the preceding image blocks are comprised in one or more preceding images preceding, in an image order, a current image comprising_ the current image block.

4. Method according to claim 1, wherein the current block coding mode is further taken into account for determining selection frequencies, which are only determined for the current and the previous coding modes, the method further comprising writing the current mapping, or a difference between the current mapping and an previous mapping, to the bit stream.

5. Method according to one of the preceding claims, wherein two or more coding modes having a same frequency of

selection are ordered according to a default order.

6. Encoder for encoding of a current image block comprised in a current image and being preceded in an encoding order by one or more preceding image blocks, said encoder

comprises

- selecting means adapted for using the current image

block for selecting a current block codin mode from predefined set of block coding modes,

- statistical analysis means for statistically analysing, at least, previous block coding mode selections for determining selection frequencies of at least the previous block coding modes, the previous block coding modes being selected for encoding the preceding image blocks, - encoding means for encoding the current image block and the current image block using the current block coding mode the encoding means being adapted for determining a current mapping between those block coding modes, for which selection frequencies are determined, and codes o variable length, which are ordered according to

increasing length, by ordering the block coding modes according to decreasing selection frequency, and

- means for writing, to a bit stream, which comprises

additional information allowing for determining the current mapping, current code onto which the current block coding mode is mapped and the encoded current image block.

7. Method for decoding of a current image block which is preceded in an encoding order of a bit stream by additional information allowing for determining a current mapping between codes and block coding modes, said method comprises the steps of - reading the additional information from the bit stream,

- using the additional information for determining the

current mapping,

- reading current code from the bit stream,

- using the current code and the current mapping for

selecting a current block coding mode,

- reading the encoded image block from the bit stream and

- using the current block coding mode for decoding the

current image block.

8. Method according to claim 7, wherein the decoded

information comprises preceding codes of coding modes selected for decoding of the preceding blocks, the method further comprising — using statistical analysis of the preceding codes for determining frequencies of selection of the coding modes comprised in a predefined set, and

- determining the current mapping by ordering the block coding modes according to decreasing selection

frequency.

9. Method according to claim 8, wherein two or more coding modes having a same frequency of selection are ordered according to a default order.

10. Method according to claim 7, wherein the additional information comprises the current mapping or a difference of the current mapping with respect to a previous mapping.

11. Device for decoding of a current image block which is preceded in an encoding order of a bit stream by additional information allowing for determining a current mapping between codes and block coding modes, said method comprises the steps of — reading means adapted for reading the additional

information, current code and the encoded image block from the bit stream,

- mapping means adapted for using the additional

information for determining the current mapping, - selection means adapted for using the current code and the current mapping for selecting a current block coding mode, and

— decoding means adapted for using the current block

coding mode for decoding the current image block.