GB2498982A - Encoding or Decoding an Image Composed of a Plurality of Colour Components - Google Patents
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
- H04N19/00—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals
- H04N19/10—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using adaptive coding
- H04N19/102—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using adaptive coding characterised by the element, parameter or selection affected or controlled by the adaptive coding
- H04N19/103—Selection of coding mode or of prediction mode
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
- H04N19/00—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals
- H04N19/10—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using adaptive coding
- H04N19/169—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using adaptive coding characterised by the coding unit, i.e. the structural portion or semantic portion of the video signal being the object or the subject of the adaptive coding
- H04N19/17—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using adaptive coding characterised by the coding unit, i.e. the structural portion or semantic portion of the video signal being the object or the subject of the adaptive coding the unit being an image region, e.g. an object
- H04N19/176—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using adaptive coding characterised by the coding unit, i.e. the structural portion or semantic portion of the video signal being the object or the subject of the adaptive coding the unit being an image region, e.g. an object the region being a block, e.g. a macroblock
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
- H04N19/00—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals
- H04N19/10—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using adaptive coding
- H04N19/169—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using adaptive coding characterised by the coding unit, i.e. the structural portion or semantic portion of the video signal being the object or the subject of the adaptive coding
- H04N19/186—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using adaptive coding characterised by the coding unit, i.e. the structural portion or semantic portion of the video signal being the object or the subject of the adaptive coding the unit being a colour or a chrominance component
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
- H04N19/00—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals
- H04N19/60—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using transform coding
- H04N19/61—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using transform coding in combination with predictive coding
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Abstract
An image composed of colour components or channels (e.g. luma and chroma components), each component being composed of one or more colour component portions is encoded/decoded. Information relating to the usability of one or more cross channel prediction methods (S102) in which a portion is dependent on a portion of a different component for encoding is determined or received (e.g. receiving a syntax bit flag at the decoder), for example determining that the current processing load is excessive, decoder delay is too great or a profile limitation exists or decoder capabilities are insufficient. If the method(s) are unusable, a first set of usable methods are provided (S103) which excludes the cross channel prediction method(s) and includes as a replacement at least one intra channel prediction method in which the portion is dependent on another portion of the same component for encoding (S103). If however it is determined that the cross channel prediction method(s) are usable, a second set of usable prediction methods are provided (S104) including the cross channel prediction method(s) to select a prediction method for encoding the portion (S105). Coding/decoding can thus be adapted to network or processing conditions.
Description
1
METHOD AND DEVICE FOR ENCODING OR DECODING AN IMAGE
Field of the invention
5 The invention relates to a method and a device for encoding or decoding an image. Particularly, but not exclusively the invention relates more specifically to coding of image portions of an image in accordance with the High Efficiency Video Coding (HEVC) standard under development.
10 Description of the prior-art
Video applications are continuously moving towards higher resolution. A large quantity of video material is distributed in digital form over broadcast channels, digital networks and packaged media, with a continuous evolution 15 towards higher quality and resolution (e.g. higher number of pixels per frame, higher frame rate, higher bit-depth or extended color gamut). This technology evolution puts higher pressure on the distribution networks that are already facing difficulties in bringing HDTV resolution and high data rates economically to the end user. Consequently, any further data rate increases will put additional 20 pressure on the networks.
To handle this challenge, ITU-T and ISO/MPEG decided to launch in January 2010 a new video coding standard project, named High Efficiency Video Coding (HEVC).
The HEVC codec design is similar to that of previous so-called block-25 based hybrid transform codecs such as H.263, H.264, MPEG-1, MPEG-2, MPEG-4, SVC. Video compression algorithms such as those standardized by standardization bodies ITU, ISO and SMPTE use the spatial and temporal redundancies of the images in order to generate data bit streams of reduced size compared with the video sequences. Such compression techniques render 30 the transmission and/or storage of the video sequences more effective.
An original video sequence to be encoded or decoded generally comprises a succession of digital images which may be represented by one or
2
more matrices the coefficients of which represent pixels. The images can be divided into slices where a slice can form part of the image or the entire image. In HEVC the slices are divided into non-overlapping Largest Coding Units (LCUs) which are generally blocks of size 64 pixels x 64 pixels. Each LCU may 5 in its turn be iteratively divided into smaller variable size Coding Units using a quadtree decomposition.
Each image may be made up of one or more image components, also called colour components or channels. The colour components are sets of two-dimensional arrays of sample values, each entry of which represents the 10 intensity of a colour component such as a measure of luma brightness and chroma colour deviations from neutral grayscale colour toward blue or red (YUV) or as a measure of red, green, or blue light component intensity (RGB).
A YUV model generally defines a colour space in terms of one luma (Y) and two chrominance (UV) components. Generally Y stands for the luma 15 component (the brightness) and U and V are the chrominance (colour) or chroma components.
A 4:2:0 YUV image, for example, is made up of one luma component plus two chroma components having a quarter of the spatial resolution (half width and half height) of the luma component. Specific encoding or decoding 20 methods employing cross-component processes, often referred to as cross-channel processes are used in various video coding technologies, such as in the recent HEVC standard under development.
For example, luma-based chroma prediction methods use a linear model in a block to link luma components to chroma components. The parameters of 25 the model may be determined, using samples located on the outer neighbouring borders of the block to be encoded or decoded and a least-mean square method. Decoded luma and chroma samples of a block being encoded or decoded is thus used to generate the prediction for the block as illustrated in Figure 1A. As illustrated in Figure 1A cycles 4 to 6 of the processing method 30 involving chroma prediction, residual decoding and reconstructions cannot be initiated until cycle 3 involving reconstruction of luma samples has been completed.
3
As illustrated in Figure 1B Luma and chroma blocks are predicted according to predetermined prediction modes, generating residuals for each component. Luma residuals are then used to predict the chroma residuals. Cycles 4 and 5 of Figure 1B cannot be initiated until luma samples have been 5 reconstructed in cycle 3.
As illustrated in Figure 1C decoded residuals from U chroma components are used to predict residual V Chroma components. Cycles 4 and 5 of Figure 1C cannot be initiated until V samples have been predicted in cycle 3 have been reconstructed in cycle 3.
10 Such processes lead to added complexity and delays in processing since channels cannot be processed in parallel.
The present invention has been devised to address one or more of the foregoing concerns.
According to a first aspect of the invention there is provided a method of 15 encoding an image, the image being composed of a plurality of colour components, each colour component being composed of one or more colour component portions, the method comprising:
determining, based on at least one criterion, the usability of one or more cross channel prediction methods in which a colour component portion of a 20 colour component is dependent on a colour component portion of a different colour component of the plurality of colour components for encoding, and in the case where it is determined that said one or more cross channel prediction methods are unusable, providing, for selection of a prediction method for encoding of a colour component portion, a first set of usable prediction 25 methods, wherein the first set of usable prediction methods excludes the said one or more cross channel prediction methods and includes for replacement of at least one of said excluded one or more cross channel prediction methods, at least one intra channel prediction method in which the colour component portion is dependent on another colour component portion of the same colour 30 component for encoding of the said colour component portion;
otherwise, in the case where it is determined that said one or more cross channel prediction methods are usable, providing, for selection of a
4
prediction method for encoding of the colour component portion, a second set of usable prediction methods including said one or more cross channel prediction methods.
Thus, in accordance with network and processing conditions one or more 5 cross channel prediction methods may be deactivated and replaced by intra channel prediction methods enabling parallel processing to be applied where necessary, thereby improving processing efficiency when necessary.
In an embodiment, the second set of usable prediction methods further includes one or more intra channel prediction methods. Thus, intra channel 10 prediction methods can be selected for processing from the second set of usable prediction methods where appropriate.
In an embodiment for the first set of usable prediction methods, all said unusable cross channel prediction methods are replaced by at least one intra channel prediction method. In some embodiments each said unusable cross 15 channel prediction method is replaced by a respective intra channel prediction method.
In an embodiment, the at least one criterion is based on at least one of a current coding or decoding efficiency, a current processing load, a network bandwidth, and a current coding or decoding delay.
20 Accordingly, in encoding or decoding processing to network and/or processing conditions may be taken into account
In an embodiment, the prediction methods included in the first or second set of usable prediction methods is dependent upon at least one of the size of the colour component portion being encoded, the position of the colour 25 component portion being encoded within a predetermined processing order and the content of the image.
Accordingly, the prediction methods used can be adapted accordingly.
In an embodiment, the at least one criterion is based on signalled criterion data indicative of the usability of one or more cross channel prediction 30 methods.
For example a command may be received indicating that one or more cross channel prediction methods are not to be used.
5
In one or more embodiments indication data indicative of the usability of one or more cross channel prediction methods is used.
An encoder or decoder can access an indication when can be used to decide whether or not one or more cross channel prediction methods are 5 usable.
In some embodiments the indication data is signalled to at least one decoder. In this way the encoder can inform the decoder of what prediction methods are usable.
In an embodiment, the indication data is provided in the form of a 10 syntax bit flag.
In an embodiment, the indication data indicates which set from among the first set and the second set of prediction methods is usable.
In an embodiment, the indication data is further representative of the content of the first set of usable prediction methods in the case where said one 15 or more cross channel prediction methods are unusable and the indication data is further representative of the content of the second set of usable prediction methods in the case where said one or more cross channel prediction methods are usable
In an embodiment, each usable prediction method is assigned a 20 corresponding codeword.
In an embodiment, the codeword is interpreted in dependence upon the syntax flag. The same codeword may be used to refer to different prediction methods depending on the set of usable prediction methods.
In an embodiment, the size of the codeword is dependent on the 25 probability of use of the corresponding prediction method. In this way prediction methods which are more commonly used may be attributed shorter code words.
In some embodiments of the invention one colour component of the image is a luma component and another colour component of the image is a chroma component.
30 According to a second aspect of the invention there is provided of decoding an image, the image being composed of a plurality of colour
6
components, each colour component being composed of one or more colour component portions, the method comprising:
determining, based on at least one criterion, the usability of one or more cross channel prediction methods in which a colour component portion of a 5 colour component is dependent on a colour component portion of a different colour component of the plurality of colour components for decoding, and in the case where it is determined that said one or more cross channel prediction methods are usable, providing, for selection of a prediction method for decoding of a colour component portion, a first set of usable prediction 10 methods, wherein the first set of usable prediction methods excludes the said one or more cross channel prediction methods and includes for replacement of at least one of said excluded one or more cross channel prediction methods, at least one intra channel prediction method in which the colour component portion of a colour component is dependent on another colour component portion of the 15 same colour component for decoding of the said colour component portion;
otherwise, in the case where it is determined that said one or more cross channel prediction methods are unusable, providing, for selection of a prediction method for decoding of the colour component portion, a second set of usable prediction methods including said one or more cross channel prediction 20 methods.
According to a third aspect of the invention there is provided a method of decoding an image, the image being composed of a plurality of colour components, each colour component being composed of one or more colour component portions, the method comprising:
25 receiving information indicative of the usability of one or more cross channel prediction methods in which a colour component portion of a colour component is dependent on a colour component portion of a different colour component of the plurality of colour components for decoding, and in the case where the information indicates that said one or more cross 30 channel prediction methods are usable, selecting a prediction method for decoding of a colour component portion, from a first set of usable prediction methods, wherein the first set of usable prediction methods excludes the said
7
one or more cross channel prediction methods and includes for replacement of at least one of said excluded one or more cross channel prediction methods, at least one intra channel prediction method in which the colour component portion of a colour component is dependent on another colour component portion of the 5 same colour component for decoding of the said colour component portion;
otherwise, in the case in the case where the information indicates that said one or more cross channel prediction methods are unusable, selecting a prediction method for decoding of the colour component portion, from a second set of usable prediction methods including said one or more cross channel 10 prediction methods.
In an embodiment the second set of usable prediction methods further includes one or more intra channel prediction methods.
In an embodiment for the first set of usable prediction methods, all said unusable cross channel prediction methods are replaced by at least one intra 15 channel prediction method. In some embodiments each said unusable cross channel prediction method is replaced by a respective intra channel prediction method.
In an embodiment the at least one criterion is based on at least one of a current coding or decoding efficiency, a current processing load, a network 20 bandwidth, and a current coding or decoding delay .
In an embodiment the prediction methods included in the first or second set of usable prediction methods is dependent upon at least one of the size of the colour component portion being encoded, the position of the colour component portion being encoded within a predetermined processing order and 25 the content of the image.
In an embodiment the information is provided in the form of a syntax bit flag.
In an embodiment the indication data indicates which set from among the first set and the second set of prediction methods is usable. 30 In an embodiment the indication data is further representative of the content of the first set of usable prediction methods in the case where said one or more cross channel prediction methods are unusable and the indication data
8
is further representative of the content of the second set of usable prediction methods in the case where said one or more cross channel prediction methods are usable
In an embodiment each usable prediction method is represented by a 5 corresponding codeword.
In an embodiment, the codeword is interpreted in dependence upon the syntax flag.
In an embodiment, the size of the codeword is dependent on the probability of use of the corresponding prediction method.
10 In an embodiment one colour component of the image is a luma component and another colour component of the image is a chroma component.
According to a fourth aspect of the invention there is provided an encoding device for encoding an image, the image being composed of a 15 plurality of colour components, each colour component being composed of one or more colour component portions, the device comprising:
means for determining, based on at least one criterion, the usability of one or more cross channel prediction methods in which a colour component portion of a colour component is dependent on a colour component portion of a 20 different colour component of the plurality of colour components for encoding, and means for providing, for selection of a prediction method for encoding of a colour component portion in the case where it is determined that said one or more cross channel prediction methods are unusable, providing, for selection of 25 a prediction method for encoding of a colour component portion, a first set of usable prediction methods, wherein the first set of usable prediction methods excludes the said one or more cross channel prediction methods, and means for replacing at least one of said excluded one or more cross channel prediction methods by at least one intra channel prediction method in 30 which the colour component portion is dependent on another colour component portion of the same colour component for encoding of the said colour component portion; and
9
means for providing, for selection of a prediction method for encoding of the colour component portion, a second set of usable prediction methods including said one or more cross channel prediction methods in the case where it is determined that said one or more cross channel prediction methods are 5 usable.
According to a fifth aspect of the invention there is provided decoding device for decoding an image, the image being composed of a plurality of colour components, each colour component being composed of one or more colour component portions, the device comprising:
10 means for determining, based on at least one criterion, the usability of one or more cross channel prediction methods in which a colour component portion of a colour component is dependent on a colour component portion of a different colour component of the plurality of colour components for decoding, and
15 means for providing, for selection of a prediction method for decoding of a colour component portion in the case where it is determined that said one or more cross channel prediction methods are unusable, providing, for selection of a prediction method for decoding of a colour component portion, a first set of usable prediction methods, wherein the first set of usable prediction methods 20 excludes the said one or more cross channel prediction methods, and means for replacing at least one of said excluded one or more cross channel prediction methods by at least one intra channel prediction method in which the colour component portion is dependent on another colour component portion of the same colour component for decoding of the said colour 25 component portion; and means for providing, for selection of a prediction method for decoding of the colour component portion, a second set of usable prediction methods including said one or more cross channel prediction methods in the case where it is determined that said one or more cross channel prediction methods are 30 usable.
According to a sixth aspect of the invention there is provided decoding device for decoding an image, the image being composed of a plurality of colour
10
components, each colour component being composed of one or more colour component portions, the device comprising:
means for receiving information indicative of the usability of one or more cross channel prediction methods in which a colour component portion of a 5 colour component is dependent on a colour component portion of a different colour component of the plurality of colour components for decoding, and selection means for selecting, in the case where the information indicates that said one or more cross channel prediction methods are usable, a prediction method for decoding of a colour component portion from a first set of 10 usable prediction methods, wherein the first set of usable prediction methods excludes the said one or more cross channel prediction methods and includes for replacement of at least one of said excluded one or more cross channel prediction methods, at least one intra channel prediction method in which the colour component portion of a colour component is dependent on another 15 colour component portion of the same colour component for decoding of the said colour component portion;
the selection means being configured to select a prediction method for decoding of the colour component portion, from a second set of usable prediction methods including said one or more cross channel prediction 20 methods, in the case in the case where the information indicates that said one or more cross channel prediction methods are unusable.
At least parts of the methods according to the invention may be computer implemented. Accordingly, the present invention may take the form of an entirely hardware embodiment, an entirely software embodiment (including 25 firmware, resident software, micro-code, etc.) or an embodiment combining software and hardware aspects that may all generally be referred to herein as a "circuit", "module" or "system". Furthermore, the present invention may take the form of a computer program product embodied in any tangible medium of expression having computer usable program code embodied in the medium.
Since the present invention can be implemented in software, the present invention can be embodied as computer readable code for provision to a programmable apparatus on any suitable carrier medium. A tangible carrier
11
medium may comprise a storage medium such as a floppy disk, a CD-ROM, a hard disk drive, a magnetic tape device or a solid state memory device and the like. A transient carrier medium may include a signal such as an electrical signal, an electronic signal, an optical signal, an acoustic signal, a magnetic signal or an electromagnetic signal, e.g. a microwave or RF signal.
Embodiments of the invention will now be described, by way of example only, and with reference to the following drawings in which:
5 Figures 1A-1C are schematic block diagrams of examples of cross channel prediction process;
Figure 2 is a schematic diagram of a wireless communication network in which one or more embodiments of the invention may be implemented;
10
Figure 3 is a schematic block diagram of a wireless communication device according to at least one embodiment of the invention; and
Figure 4 is a flow chart of a method of encoding or decoding an image portion in 15 accordance with one or more embodiments of the invention.
Figure 2 illustrates a data communication system in which one or more embodiments of the invention may be implemented. Although a streaming scenario is considered here, data communication can be performed using for example a media storage device such as an optical disc. The data communication system comprises a transmission device, in this case a server 1001, which is operable to transmit data packets of a data stream to a receiving device, in this case a client terminal 1002, via a data communication network 1000. The data communication network 1000 may be a Wide Area Network (WAN) or a Local Area Network (LAN). Such a network may be for example a wireless network (Wifi / 802.11a or b or g), an Ethernet network, an Internet network or a mixed network composed of several different networks. In a
12
particular embodiment of the invention the data communication system may be a digital television broadcast system in which the server 1001 sends the same data content to multiple clients.
The data stream 1004 provided by the server 1001 may be composed of a bitstream representing multimedia data such as video and/or audio data. Audio and video data streams may, in some embodiments of the invention, be captured by the server 1001 using a microphone and a camera respectively. In some embodiments data streams may be stored on the server 1001 or received by the server 1001 from another data provider, or generated at the server 1001. The server 1001 is provided with an encoder for encoding video and audio streams in particular to provide a compressed bitstream for transmission that is a more compact representation of the data input to the encoder.
In order to obtain a better ratio of the quality of transmitted data to quantity of transmitted data, the compression of video data may be for example in accordance with the HEVC format or H.264/AVC format.
The client 1002 receives the transmitted bitstream and decodes the reconstructed bitstream to reproduce video images on a display device and audio data by a loud speaker.
In one or more embodiments of the invention an encoded video image is transmitted with a brightness component (luma) and two colour components (chroma). The digital representation of the video signal thus includes a luma component (Y), representative of brightness, and colour difference (or chroma) components U and V.
It will be appreciated that while the detailed examples related to a YUV model the invention is not limited thereto, and may be applied to other models such as an RGB, or for encoding any image composed of several colour components, at least one colour component being considered as a reference colour component, the other colour components being dependency coded based on this reference colour component.
Figure 3 schematically illustrates a processing device 300 configured to implement at least one embodiment of the present invention. The processing device 300 may be a device such as a micro-computer, a workstation or a light
13
portable device such as a smart phone and portable computer. The device 300 comprises a communication bus 313 connected to:
-a central processing unit 311, such as a microprocessor, denoted
CPU;
5 -a read only memory 307, denoted ROM, for storing computer programs for implementing embodiments of the invention;
-a random access memory 312, denoted RAM, which may be used for storing the executable code of the method of embodiments of the invention as well as the registers adapted to record variables and parameters necessary 10 for implementing the method of encoding a sequence of digital images and/or the method of decoding a bitstream according to embodiments of the invention; and
-a communication interface 302 connected to a communication network 303 over which data to be processed is transmitted or received. 15 Optionally, the apparatus 300 may also include the following components:
-a data storage means 304 such as a hard disk, for storing computer programs for implementing methods of one or more embodiments of the invention and data used or produced during the implementation of one or more 20 embodiments of the invention;
-a disk drive 305 for a disk 306, the disk drive being adapted to read data from the disk 306 or to write data onto said disk;
-a screen 309 for displaying data and/or serving as a graphical interface with the user, by means of a keyboard 310 or any other pointing 25 means.
The apparatus 300 can be connected to various peripherals, such as for example a digital camera 320 or a microphone 308, each being connected to an input/output card (not shown) so as to supply multimedia data to the apparatus 300.
30 The communication bus 313 provides communication and interoperability between the various elements included in the apparatus 300 or connected to it. The representation of the communication bus is not limiting and
14
in particular the central processing unit is operable to communicate instructions to any element of the apparatus 300 directly or by means of another element of the apparatus 300.
The disk 306 can be replaced by any information medium such as for 5 example a compact disk (CD-ROM), rewritable or not, a ZIP disk or a memory card and, in general terms, by an information storage means that can be read by a microcomputer or by a microprocessor, integrated or not into the apparatus, possibly removable and adapted to store one or more programs whose execution enables the method of encoding a sequence of digital images 10 and/or the method of decoding a bitstream according to the invention to be implemented.
The executable code may be stored either in read only memory 307, on the hard disk 304 or on a removable digital medium such as for example a disk 306 as described previously. Moreover in some embodiments, the 15 executable code of the programs can be received by means of the communication network 303, via the interface 302, in order to be stored in one of the storage means of the apparatus 300 before being executed, such as the hard disk 304.
The central processing unit 311 is adapted to control and direct the 20 execution of the instructions or portions of software code of the program or programs for running methods such as encoding or decoding according to embodiments of the invention, instructions that are stored in one of the aforementioned storage means. On powering up, the program or programs that are stored in a non-volatile memory, for example on the hard disk 304 or in the 25 read only memory 307, are transferred into the random access memory 312, which then contains the executable code of the program or programs, as well as registers for storing the variables and parameters necessary for implementing embodiments of the invention.
In this embodiment, the apparatus is a programmable apparatus 30 which uses software to implement the invention. However, alternatively, the present invention may be implemented in hardware (for example, in the form of an Application Specific Integrated Circuit or ASIC).
15
A method of encoding or decoding an image according to an embodiment of the invention will now be described with reference to Figure 4. The image being encoded is composed of a first type of image component or channel: a luma component and at least one second type of channel - chroma 5 component. In some applicable prediction methods for encoding or decoding of the image according to embodiments of the invention, chroma samples are predictable from luma samples using a prediction model linking luma components to corresponding chroma components.
The process illustrated in Figure 4 is applicable to both a decoder and an 10 encoder, and valid for any portion of image data (macroblock, LCU or image).
In step S101 the encoding (or decoding) process is started. Any data or information relevant to the encoding/decoding process is prepared. This may for example involve preparation of the image data to be encoded or decoded or analysis of the status of the system such as determining the current network 15 bandwidth or load, the current processing load or processing capacity, coding/decoding efficiency, coding/decoding delays or any other process relevant to one or more subsequent steps of the method.
In step S102, the allowability of one or more cross channel prediction methods for coding or decoding of the current image portion is determined 20 based on one or more criterions. For an encoder, this may involve for example: determining if the current processing load is too high, for example by comparing the current CPU load with a predetermined threshold or as to whether or not the CPU is processing data quickly enough (e.g. determining if there are too many frames pending),
25 - analysing information received from the network to determine for example if a network decoder delay exceeds a threshold, if the network bandwidth is decreasing and that consequently increased coding efficiency is required, etc,
determining if a command has been given to not produce data 30 dependent on cross-channel methods because of a profile limitation or decoder capabilities being signalled.
16
In some particular embodiments of the invention, the encoder may signal explicitly to one or more decoders whether all or part of a predetermined set of cross-channel prediction methods are in use. The signalled data may take the form of a syntax bit flag indicative of whether all or some predetermined cross-5 channel prediction methods are usable. The information provided by the syntax flag may be indicative of a profile associated with a predetermined subset of prediction methods.
In some embodiments, the writing of the signaling information into the bitstream may be conditioned by said profile information. If a profile disallows 10 the use of all or part of a predetermined subset of methods, no explicit signalization is needed. If the respective profile allows the prediction methods to be used, then explicit signalization is needed, and thus written by the encoder into the bitstream.
In another embodiment of the invention the encoder may provide an 15 indication in the form of a signalled syntax enabling several subsets of the cross-channel methods to be selectively deactivated. For example the signalling may comprise a bit field in which each bit flag, when set to 0, deactivates one or more such cross-channel methods. Again, the profile information may adjust the length of the bit field (down to 1, i.e. a bit flag only, or 0, i.e. no bit field present), 20 and/or the meaning of each of its bit flags (one such flag may deactivate 1 prediction mode in a profile, and 2 in another). For example one bit of a bit-field could be used to deactivate cross-channel luma—>chroma prediction methods, another bit could be used for deactivation of cross-channel residual—^residual prediction methods, and another for cross-channel chroma^chroma prediction 25 methods.
Consequently, in the case of a decoder, step S101 may involve reading the signalling information (said profile information and/or bit flag) received from the encoder
Based on the determination of the usability of cross channel prediction 30 methods in step S102, all or part of the cross-channel prediction methods and any related processing or signalling may be deactivated. In the case where
17
some or all of the cross channel prediction processes are unusable, a list of intra channel prediction modes is provided in step S103.
Intra channel prediction methods correspond to prediction methods in which the prediction process for encoding or decoding of an image block is 5 dependent only on samples of the same component or channel.
Such intra channel prediction methods include, for example bilinear angular prediction methods. Assuming borders around a block to be encoded or decoded and a particular angle, any point inside the block may be projected onto the borders around that block. The value of the pixel at the projection 10 position is then used to set the value of the pixel at said point position. The exact position is easy to determine, but may be fractional and thus not exactly match an existing pixel value. In such a case, linear interpolation is performed. Each angle (or direction is associated with an index value which may be coded and, in some embodiments, signalled in a bitstream.
15 Other intra channel prediction methods include planar prediction methods. Planar prediction methods employ decoded samples on borders surrounding the block to be encoded or decoded. Two estimated pixel values are averaged together:
horizontal prediction uses the weighted average of the projected 20 pixel on the vertical left border and the top right pixel, the weights being the distance of the predicted pixel to those 2 pixels,
vertical prediction using the projected pixel on the horizontal top border and the bottom left pixel.
While in some embodiments of the invention all the unusable cross 25 channel prediction methods are replaced by some intra channel prediction methods, in other embodiments of the invention only some (one or more) of the cross channel prediction methods may be replaced by intra channel prediction method.
In the case where it is determined that one or more cross channel 30 prediction methods are usable a list of usable prediction methods for encoding or decoding of the image block can be built in step S104. This list of usable prediction modes may include any usable cross-channel prediction process. In
18
some embodiments of the invention the list of usable prediction modes provided in step S104 may further include intra channel prediction processes.
The entropy coder of the encoder may be configured for the list of prediction methods included in the list of step S103 or step S104 accordingly.
5 In one particular embodiment, each chroma prediction method is assigned a corresponding codeword for signaling that chroma prediction method. Table 1 presents a list of intra chroma prediction methods and their corresponding codewords.
Chroma prediction mode
Codewords
DM
0
LM
10
Planar
110
Vertical
11110
Horizontal
11111
DC
11111
Table 1
10 DM refers to luma method (and thus either an angular mode, the DC mode or the planar mode). Other alternatives are possible, depending on the number of cross-channel prediction modes. For example, LM1 and LM2 are two cross-channel methods:
Chroma prediction mode
Codewords
DM
0
LM1
10
LM2
110
DC
11110
Planar
11111
Vertical
11111
Table 2
15 It will be appreciated that any cross-channel prediction method and regular prediction may be included in the list. This list can of course be longer, and the associated entropy coding adapted to the frequency (known to the
19
skilled person in the field for building Huffman canonical codewords) or conditional occurrences observed (i.e. contextual coding or use of predictive means to code the mode).
Once the list of allowed prediction modes has been provided, the process for 5 cross-channel methods can be performed in step S105. For an encoder, this step may include:
- writing any image-level syntax pertaining to the allowable prediction methods;
- applying associated cross-channel methods in addition to the prediction 10 ones, where appropriate;
- encoding a macroblock, LCU or image using a prediction method selected from the provided set of usable prediction methods.
For a decoder, Step S105 may include:
- reading any image-level syntax pertaining to the allowable prediction 15 methods;
- applying associated cross-channel methods in addition to the prediction processes;
- decoding a macroblock, LCU or image, using a prediction method selected from the provided set of usable prediction methods.
20 When it is determined in step S102 that cross channel prediction methods are not allowed then the list of intra channel prediction modes generated in step S103 typically uses prediction modes exemplified in Table 3:
Chroma intra prediction mode
Codewords
DM
0
Planar
10
Vertical
110
Horizontal
11110
DC
11111
Vertical+8
11111
Table 3
20
In some embodiments of the invention the list of allowable prediction modes may be determined dynamically. For example by the encoder building a list of allowable prediction modes and signalling the list in the bitstream for the decoder to use. The encoder may build the list for instance by: 5 • Performing image analysis to determine the most relevant prediction directions;
• Testing exhaustively all potential prediction modes in addition to the allowable modes, i.e. for each prediction mode, measuring one or more performance criterion values (such as, for example
10 the distortion between the original data and the prediction generated by using said mode) and keeping the best ones;
• By using predetermined lists of prediction modes dependent on properties of the data to be encoded or decoded (e.g., image type, block position, block size, quantizer step, ...)
15 In step S106 the image block is encoded or decoded using an appropriate usable prediction method selected from the list of usable prediction modes. In this case, according to a particular embodiment there is no need to write any syntax related to cross channel modes since such modes are not usable in that case.
20 Although the present invention has been described hereinabove with reference to specific embodiments, the present invention is not limited to the specific embodiments, and modifications will be apparent to a skilled person in the art which lie within the scope of the present invention.
For instance, while in the foregoing examples have been described with
25 respect to luma and chroma components it will be appreciated that the invention may be applied to other component types and cross channel prediction processes.
Many further modifications and variations will suggest themselves to those versed in the art upon making reference to the foregoing illustrative
30 embodiments, which are given by way of example only and which are not intended to limit the scope of the invention, that being determined solely by the
21
appended claims. In particular the different features from different embodiments may be interchanged, where appropriate.
In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. The 5 mere fact that different features are recited in mutually different dependent claims does not indicate that a combination of these features cannot be advantageously used.
22
Claims (1)
1. A method of encoding an image, the image being composed of a plurality of colour components, each colour component being composed of one 5 or more colour component portions, the method comprising:
determining, based on at least one criterion, the usability of one or more cross channel prediction methods in which a colour component portion of a colour component is dependent on a colour component portion of a different colour component of the plurality of colour components for encoding, and 10 in the case where it is determined that said one or more cross channel prediction methods are unusable, providing, for selection of a prediction method for encoding of a colour component portion, a first set of usable prediction methods, wherein the first set of usable prediction methods excludes the said one or more cross channel prediction methods and includes for replacement of 15 at least one of said excluded one or more cross channel prediction methods, at least one intra channel prediction method in which the colour component portion is dependent on another colour component portion of the same colour component for encoding of the said colour component portion;
otherwise, in the case where it is determined that said one or more 20 cross channel prediction methods are usable, providing, for selection of a prediction method for encoding of the colour component portion, a second set of usable prediction methods including said one or more cross channel prediction methods.
25 2. A method according to claim 1 wherein the second set of usable prediction methods further includes one or more intra channel prediction methods.
3. A method according to claim 1 or 2 wherein for the first set of usable 30 prediction methods, all said unusable cross channel prediction methods are replaced by at least one intra channel prediction method.
23
4. A method according to any preceding claim wherein the at least one criterion is based on at least one of a current coding or decoding efficiency, a
5 current processing load, a network bandwidth, and a current coding or decoding delay .
5. A method according to any preceding claim, wherein the prediction methods included in the first or second set of usable prediction methods is
10 dependent upon at least one of the size of the colour component portion being encoded, the position of the colour component portion being encoded within a predetermined processing order and the content of the image.
6. A method according to any preceding claim, wherein the at least one 15 criterion is based on signalled criterion data indicative of the usability of one or more cross channel prediction methods.
7. A method according to any preceding claim further comprising using indication data indicative of the usability of one or more cross channel prediction
20 methods.
8. A method according to claim 7 wherein the indication data is signalled to at least one decoder
25 9. A method according to claim 7 or 8 wherein the indication data is provided in the form of a syntax bit flag.
10. A method according to any one of claims 7 to 9, wherein the indication data indicates which set from among the first set and the second set of 30 prediction methods is usable.
24
11. A method according to any one of claims 7 to 10. wherein the indication data is further representative of the content of the first set of usable prediction methods in the case where said one or more cross channel prediction methods are unusable and the indication data is further representative of the content of
5 the second set of usable prediction methods in the case where said one or more cross channel prediction methods are usable
12. A method according to any preceding claim wherein each usable prediction method is assigned a corresponding codeword.
10
13. A method according to claim 12 when appended to claim 9 wherein the codeword is interpreted in dependence upon the syntax flag.
14. A method according to claim 12 or 13 wherein the size of the codeword 15 is dependent on the probability of use of the corresponding prediction method.
15. A method according to any preceding claim wherein one colour component of the image is a luma component and another colour component of the image is a chroma component.
20
16. A method of decoding an image, the image being composed of a plurality of colour components, each colour component being composed of one or more colour component portions, the method comprising:
determining, based on at least one criterion, the usability of one or more 25 cross channel prediction methods in which a colour component portion of a colour component is dependent on a colour component portion of a different colour component of the plurality of colour components for decoding, and in the case where it is determined that said one or more cross channel prediction methods are usable, providing, for selection of a prediction method 30 for decoding of a colour component portion, a first set of usable prediction methods, wherein the first set of usable prediction methods excludes the said one or more cross channel prediction methods and includes for replacement of
25
at least one of said excluded one or more cross channel prediction methods, at least one intra channel prediction method in which the colour component portion of a colour component is dependent on another colour component portion of the same colour component for decoding of the said colour component portion;
5 otherwise, in the case where it is determined that said one or more cross channel prediction methods are unusable, providing, for selection of a prediction method for decoding of the colour component portion, a second set of usable prediction methods including said one or more cross channel prediction methods.
10
17. A method of decoding an image, the image being composed of a plurality of colour components, each colour component being composed of one or more colour component portions, the method comprising:
receiving information indicative of the usability of one or more cross 15 channel prediction methods in which a colour component portion of a colour component is dependent on a colour component portion of a different colour component of the plurality of colour components for decoding, and in the case where the information indicates that said one or more cross channel prediction methods are usable, selecting a prediction method for 20 decoding of a colour component portion, from a first set of usable prediction methods, wherein the first set of usable prediction methods excludes the said one or more cross channel prediction methods and includes for replacement of at least one of said excluded one or more cross channel prediction methods, at least one intra channel prediction method in which the colour component portion 25 of a colour component is dependent on another colour component portion of the same colour component for decoding of the said colour component portion;
otherwise, in the case in the case where the information indicates that said one or more cross channel prediction methods are unusable, selecting a prediction method for decoding of the colour component portion, from a second 30 set of usable prediction methods including said one or more cross channel prediction methods.
26
18. A method according to claim 16 or 17 wherein the second set of usable prediction methods further includes one or more intra channel prediction methods.
5 19. A method according to any one of claims 16 to 18 wherein for the first set of usable prediction methods, all said unusable cross channel prediction methods are replaced by at least one intra channel prediction method.
20. A method according to any one of claims 16 to 19, wherein the at least 10 one criterion is based on at least one of a current coding or decoding efficiency,
a current processing load, a network bandwidth, and a current coding or decoding delay .
21. A method according to any one of claims 16 to 20, wherein the 15 prediction methods included in the first or second set of usable prediction methods is dependent upon at least one of the size of the colour component portion being encoded, the position of the colour component portion being encoded within a predetermined processing order and the content of the image.
20 22. A method according to any one of claims 17 to 21, wherein the information is provided in the form of a syntax bit flag.
23. A method according to any one of claims 17 to 22, wherein the indication data indicates which set from among the first set and the second set
25 of prediction methods is usable.
24. A method according to any one of claims 17 to 23, wherein the indication data is further representative of the content of the first set of usable prediction methods in the case where said one or more cross channel prediction
30 methods are unusable and the indication data is further representative of the content of the second set of usable prediction methods in the case where said one or more cross channel prediction methods are usable
27
25. A method according to any to any one of claims 17 to 24 wherein each usable prediction method is represented by a corresponding codeword.
5 26. A method according to claim 25 when appended to claim 22. wherein the codeword is interpreted in dependence upon the syntax flag.
27. A method according to claim 25 or 26 wherein the size of the codeword is dependent on the probability of use of the corresponding prediction method.
10
28. A method according to any one of claims 16 to 27 wherein one colour component of the image is a luma component and another colour component of the image is a chroma component.
15 29. An encoding device for encoding an image, the image being composed of a plurality of colour components, each colour component being composed of one or more colour component portions, the device comprising:
means for determining, based on at least one criterion, the usability of one or more cross channel prediction methods in which a colour component 20 portion of a colour component is dependent on a colour component portion of a different colour component of the plurality of colour components for encoding, and means for providing, for selection of a prediction method for encoding of a colour component portion in the case where it is determined that said one or 25 more cross channel prediction methods are unusable, providing, for selection of a prediction method for encoding of a colour component portion,a first set of usable prediction methods, wherein the first set of usable prediction methods excludes the said one or more cross channel prediction methods, and means for replacing at least one of said excluded one or more cross 30 channel prediction methods by at least one intra channel prediction method in which the colour component portion is dependent on another colour component
28
portion of the same colour component for encoding of the said colour component portion: and means for providing, for selection of a prediction method for encoding of the colour component portion, a second set of usable prediction methods 5 including said one or more cross channel prediction methods in the case where it is determined that said one or more cross channel prediction methods are usable.
30. A decoding device for decoding an image, the image being composed 10 of a plurality of colour components, each colour component being composed of one or more colour component portions, the device comprising:
means for determining, based on at least one criterion, the usability of one or more cross channel prediction methods in which a colour component portion of a colour component is dependent on a colour component portion of a 15 different colour component of the plurality of colour components for decoding, and means for providing, for selection of a prediction method for decoding of a colour component portion in the case where it is determined that said one or more cross channel prediction methods are unusable, providing, for selection of 20 a prediction method for decoding of a colour component portion,a first set of usable prediction methods, wherein the first set of usable prediction methods excludes the said one or more cross channel prediction methods, and means for replacing at least one of said excluded one or more cross channel prediction methods by at least one intra channel prediction method in 25 which the colour component portion is dependent on another colour component portion of the same colour component for decoding of the said colour component portion; and means for providing, for selection of a prediction method for decoding of the colour component portion, a second set of usable prediction methods 30 including said one or more cross channel prediction methods in the case where it is determined that said one or more cross channel prediction methods are usable.
29
31. A decoding device for decoding an image, the image being composed of a plurality of colour components, each colour component being composed of one or more colour component portions, the device comprising: 5 means for receiving information indicative of the usability of one or more cross channel prediction methods in which a colour component portion of a colour component is dependent on a colour component portion of a different colour component of the plurality of colour components for decoding, and selection means for selecting, in the case where the information 10 indicates that said one or more cross channel prediction methods are usable, a prediction method for decoding of a colour component portion from a first set of usable prediction methods, wherein the first set of usable prediction methods excludes the said one or more cross channel prediction methods and includes for replacement of at least one of said excluded one or more cross channel 15 prediction methods, at least one intra channel prediction method in which the colour component portion of a colour component is dependent on another colour component portion of the same colour component for decoding of the said colour component portion;
the selection means being configured to select a prediction method for 20 decoding of the colour component portion, from a second set of usable prediction methods including said one or more cross channel prediction methods, in the case in the case where the information indicates that said one or more cross channel prediction methods are unusable.
25 32. A computer program product for a programmable apparatus, the computer program product comprising a sequence of instructions for implementing a method according to any one of claims 1 to 28 when loaded into and executed by the programmable apparatus.
30 33. A computer-readable storage medium storing instructions of a computer program for implementing a method, according to any one of claims 1 to 28.
30
34. A method for encoding or decoding an image substantially as hereinbefore described with reference to, and as shown in Figure 4.
35. A signal carrying data representative of the indication data according to 5 any one of claims 7 to 15, or 22 to 28.
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| CN106688237A (en) * | 2014-09-30 | 2017-05-17 | 凯迪迪爱通信技术有限公司 | Moving image coding device, moving image decoding device, moving image compression and transmission system, moving image coding method, moving image decoding method, and program |
| US11272186B2 (en) * | 2019-01-10 | 2022-03-08 | Guangdong Oppo Mobile Telecommunications Corp., Ltd. | Method for picture decoding, method for picture encoding, decoder, and encoder |
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| Publication number | Priority date | Publication date | Assignee | Title |
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| CN106688237A (en) * | 2014-09-30 | 2017-05-17 | 凯迪迪爱通信技术有限公司 | Moving image coding device, moving image decoding device, moving image compression and transmission system, moving image coding method, moving image decoding method, and program |
| EP3203746A4 (en) * | 2014-09-30 | 2017-09-13 | KDDI Corporation | Moving image coding device, moving image decoding device, moving image compression and transmission system, moving image coding method, moving image decoding method, and program |
| CN106688237B (en) * | 2014-09-30 | 2020-02-21 | 凯迪迪爱通信技术有限公司 | Video encoding device, video decoding device, video compression transmission system, video encoding method, video decoding method, and program |
| US10681366B2 (en) | 2014-09-30 | 2020-06-09 | Kddi Corporation | Video encoding apparatus, video decoding apparatus, video compression and transmission system, video encoding method, video decoding method, and computer readable storage medium |
| US11272186B2 (en) * | 2019-01-10 | 2022-03-08 | Guangdong Oppo Mobile Telecommunications Corp., Ltd. | Method for picture decoding, method for picture encoding, decoder, and encoder |
| US11785225B2 (en) | 2019-01-10 | 2023-10-10 | Guangdong Oppo Mobile Telecommunications Corp., Ltd. | Method for picture decoding, method for picture encoding, decoder, and encoder |
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