Adaptive loop filter signalling

Example techniques are described for coding video data by obtaining a block of video data, obtaining an adaptive parameter set, determining a set of adaptive loop filter parameters for a plurality of filters for the block of video data based on the adaptive parameter set, wherein a plurality of adaptive loop parameters of the set of adaptive loop filter parameters are signaled using the same signaling parameter for each of the plurality of filters of the adaptive parameter set, and coding the block of video data using the set of adaptive loop filter parameters. The example techniques can be performed as part of an encoding or decoding process and/or by an encoder or a decoder.

BACKGROUND

Video coding techniques may include filtering techniques that can enhance the quality of a decoded video signal. The filtering techniques can be applied post-filter, where filtered frames are not used for prediction of future frames, and/or can be applied in-loop, where the filtered frames are available to be used to predict future frames.

SUMMARY

In general, the techniques of this disclosure are related to improvements to filtering techniques in video coding. More specifically, the techniques are related to improvements to Adaptive Loop Filter (ALF) signaling.

Methods, devices, apparatus, and computer-readable media for coding video data are described herein. The methods can include obtaining a block of video data, obtaining an adaptive parameter set, determining, using a processor, a set of adaptive loop filter parameters for a plurality of filters for the block of video data based on the adaptive parameter set, wherein a plurality of adaptive loop parameters of the set of adaptive loop filter parameters are signaled using the same signaling parameter for each of the plurality of filters of the adaptive parameter set, and coding the block of video data using the set of adaptive loop filter parameters.

In some embodiments, the plurality of adaptive loop parameters can include filter coefficients that are signaled using the same signaling parameter for each of the plurality of filters.

In further embodiments, the plurality of adaptive loop parameters can include filter coefficient positions that are signaled using the same signaling parameter for each of the plurality of filters.

In additional embodiments, the same signaling parameter can be an exponential-Golomb code order, the order can be a 0thorder, the order can be signaled, the order can be a default value (e.g., determined to be a default value based on a flag or based on not being signaled), and the like.

In some embodiments, the plurality of filters can include all of the filters signaled in the adaptive parameter set.

In additional embodiments, filter information from the adaptive parameter set can be used to form one or more new adaptive parameter sets.

In additional embodiments, the coding is in-loop coding.

In further embodiments, the coded block of video data is used for prediction of future frames of the video data.

In some embodiments, the coding is post-processing.

The device can be any type of computing device, such as a wireless communication device, and can include a memory configured to store video data and one or more processors configured to perform a method that includes obtaining a block of video data, obtaining an adaptive parameter set, determining a set of adaptive loop filter parameters for a plurality of filters for the block of video data based on the adaptive parameter set, wherein a plurality of adaptive loop parameters of the set of adaptive loop filter parameters are signaled using the same signaling parameter for each of the plurality of filters of the adaptive parameter set, and coding the block of video data using the set of adaptive loop filter parameters.

The non-transitory computer-readable medium can be for storing a program containing instructions that, when executed by a processor of a device, cause the device to perform a method that includes obtaining a block of video data, obtaining an adaptive parameter set, determining a set of adaptive loop filter parameters for a plurality of filters for the block of video data based on the adaptive parameter set, wherein a plurality of adaptive loop parameters of the set of adaptive loop filter parameters are signaled using the same signaling parameter for each of the plurality of filters of the adaptive parameter set, and coding the block of video data using the set of adaptive loop filter parameters.

The apparatus can include means for obtaining a block of video data, means for obtaining an adaptive parameter set, means for determining a set of adaptive loop filter parameters for a plurality of filters for the block of video data based on the adaptive parameter set, wherein a plurality of adaptive loop parameters of the set of adaptive loop filter parameters are signaled using the same signaling parameter for each of the plurality of filters of the adaptive parameter set, and means for coding the block of video data using the set of adaptive loop filter parameters.

The details of one or more examples of the disclosure are set forth in the accompanying drawings and the description below. Other features, objects, and advantages will be apparent from the description, drawings, and claims.

DETAILED DESCRIPTION

In general, the techniques of this disclosure are related to improvements to Adaptive Loop Filter (ALF) signalling.

The techniques of this disclosure may be applied to any existing video codec, such as those conforming to ITU-T H.264/AVC (Advanced Video Coding) or High Efficiency Video Coding (HEVC), also referred to as ITU-T H.265. H.264 is described in International Telecommunication Union, “Advanced video coding for generic audiovisual services,” SERIES H: AUDIOVISUAL AND MULTIMEDIA SYSTEMS, Infrastructure of audiovisual services—Coding of moving video, H.264, June 2011, and H.265 is described in International Telecommunication Union, “High efficiency video coding,” SERIES H: AUDIOVISUAL AND MULTIMEDIA SYSTEMS, Infrastructure of audiovisual services—Coding of moving video, April 2015. The techniques of this disclosure may also be applied to any other previous, current, or future video coding standards as an efficient coding tool.

Other video coding standards include ITU-T H.261, ISO/IEC MPEG-1 Visual, ITU-T H.262 or ISO/IEC MPEG-2 Visual, ITU-T H.263, ISO/IEC MPEG-4 Visual and the Scalable Video Coding (SVC) and Multiview Video Coding (MVC) extensions of H.264, as well as the extensions of HEVC, such as the range extension, multiview extension (MV-HEVC) and scalable extension (SHVC).

There currently exists a need for standardization of video coding technology with a compression capability that exceeds that of the HEVC standard (including its current extensions).

Certain techniques of this disclosure may be described with reference to H.264 and/or HEVC to aid in understanding, but the techniques described are not limited to H.264 or HEVC and can be used in conjunction with other coding standards and other coding tools.

FIG.1is a block diagram illustrating an example video encoding and decoding system10that may utilize the techniques for adaptive loop filtering signaling described in this disclosure. As shown inFIG.1, system10includes a source device12that generates encoded video data to be decoded at a later time by a destination device14. Source device12and destination device14may be any of a wide range of devices, including desktop computers, notebook (i.e., laptop) computers, tablet computers, set-top boxes, telephone handsets such as so-called “smart” phones, so-called “smart” pads, televisions, cameras, display devices, digital media players, video gaming consoles, video streaming device, head-mounted displays (HMDs), wearable technology devices (e.g., so-called “smart” watches), or the like. In some cases, source device12and destination device14may be equipped for wireless communication.

In another example, encoded video data may be output from output interface22to a storage device26. Similarly, encoded video data may be accessed from storage device26by input interface28. Storage device26may include any of a variety of distributed or locally accessed data storage media such as a hard drive, Blu-ray™ discs, DVDs, CD-ROMs, flash memory, volatile or non-volatile memory, or any other suitable digital storage media for storing encoded video data. In a further example, storage device26may correspond to a file server or another intermediate storage device that may hold the encoded video generated by source device12. Destination device14may access stored video data from storage device26via streaming or download. The file server may be any type of server capable of storing encoded video data and transmitting that encoded video data to the destination device14. Example file servers include a web server (e.g., for a website), a file transfer protocol (FTP) server, network attached storage (NAS) devices, or a local disk drive. Destination device14may access the encoded video data through any standard data connection, including an Internet connection. This may include a wireless connection (e.g., a Wi-Fi™ connection), a wired connection (e.g., DSL, cable modem, etc.), or a combination of both that is suitable for accessing encoded video data stored on a file server. The transmission of encoded video data from storage device26may be a streaming transmission, a download transmission, or a combination of both.

In the example ofFIG.1, source device12includes a video source18, a video encoder20and an output interface22. In some cases, output interface22may include a modulator/demodulator (modem) and/or a transmitter. In source device12, video source18may include a source such as a video capture device, e.g., a video camera, a video archive containing previously captured video, a video feed interface to receive video from a video content provider, and/or a computer graphics system for generating computer graphics data as the source video, or a combination of such sources. In general, capturing video data may include any technique for recording, generating, and/or sensing video data. As one example, if video source18is a video camera, source device12and destination device14may form so-called camera phones or video phones. However, the techniques described in this disclosure may be applicable to video coding in general, and may be applied to wireless and/or wired applications.

The captured, pre-captured, or computer-generated video may be encoded by video encoder20. The encoded video data may be transmitted directly to destination device14via output interface22of source device12. The encoded video data may also (or alternatively) be stored onto storage device26for later access by destination device14or other devices, for decoding and/or playback.

Destination device14includes an input interface28, a video decoder30, and a display device32. In some cases, input interface28may include a receiver and/or a modem. Input interface28of destination device14can receive the encoded video data over link16. The encoded video data communicated over link16, or provided on storage device26, may include a variety of syntax elements generated by video encoder20for use by a video decoder, such as video decoder30, in decoding the video data. Such syntax elements may be included with the encoded video data transmitted on a communication medium, stored on a storage medium, or stored on a file server.

Display device32may be integrated with, or external to, destination device14. In some examples, destination device14may include an integrated display device and also be configured to interface with an external display device. In other examples, destination device14may be a display device. In general, display device32displays the decoded video data to a user, and may be any of a variety of display devices such as a liquid crystal display (LCD), a plasma display, an organic light emitting diode (OLED) display, or another type of display device.

Video encoder20and video decoder30may operate according to a video compression standard, such as HEVC, and may conform to the HEVC Test Model (HM). Video encoder20and video decoder30may additionally operate according to an HEVC extension, such as the range extension, MV-HEVC, or SHVC which have been developed by the Joint Collaboration Team on Video Coding (JCT-VC) as well as Joint Collaboration Team on 3D Video Coding Extension Development (JCT-3V) of ITU-T Video Coding Experts Group (VCEG) and ISO/IBC Motion Picture Experts Group (MPEG). Alternatively, video encoder20and video decoder30may operate according to other proprietary or industry standards, such as the ITU-T H.264 standard, alternatively referred to as ISO/IEC MPEG-4, Part 10, Advanced Video Coding (AVC), or extensions of such standards, such as the Scalable Video Coding (SVC) and Multiview Video Coding (MVC) extensions. The techniques of this disclosure, however, are not limited to any particular coding standard. Other examples of video compression standards include ITU-T H.261, ISO/IEC MPEG-1 Visual, ITU-T H.262 or ISO/IEC MPEG-2 Visual, ITU-T H.263, ISO/IEC MPEG-4 Visual, ISO/IEC MPEG-5 Essential Video Coding (EVC) and Low Complexity Enhancement Video Coding.

ITU-T VCEG (Q6/16) and ISO/IEC MPEG (JTC 1/SC 29/WG 11) are now developing future video coding technology with a compression capability that potentially exceeds that of the current HEVC standard (including its current extensions and near-term extensions for screen content coding and high-dynamic-range coding). The new standard is called H.266/VVC (Versatile Video coding). The groups are working together on this exploration activity in a joint collaboration effort known as the Joint Video Exploration Team (JVET) to evaluate compression technology designs. The techniques of this disclosure, however, are not limited to any particular coding standard.

This disclosure describes techniques related to filtering operations which could be used in a post-processing stage, as part of in-loop coding, or in the prediction stage of video coding. The techniques of this disclosure may be implemented into existing video codecs, such as HEVC, or be an efficient coding tool for a future video coding standard, such as the H.266/VVC standard presently under development.

Video coding typically involves predicting a block of video data from either an already coded block of video data in the same picture (e.g., intra prediction) or an already coded block of video data in a different picture (e.g., inter prediction). In some instances, the video encoder20also calculates residual data by comparing the predictive block to the original block. Thus, the residual data represents a difference between the predictive block and the original block.

Video encoder20transforms and quantizes the residual data and signals the transformed and quantized residual data in the encoded bitstream. Video decoder30adds the residual data to the predictive block to produce a reconstructed video block that matches the original video block more closely than the predictive block alone. To further improve the quality of decoded video, video encoder20and video decoder30can perform one or more filtering operations on the reconstructed video blocks. Examples of these filtering operations include deblocking filtering, sample adaptive offset (SAO) filtering, and adaptive loop filtering (ALF). As used herein ALF, can refer to the process of adaptive loop filtering and/or the adaptive loop filter itself. Parameters for these filtering operations may be determined by video encoder20and explicitly signaled in the encoded video bitstream or may be implicitly determined by video decoder30without needing the parameters to be explicitly signaled in the encoded video bitstream.

This disclosure describes techniques related to ALF. An ALF may be used in a post-processing stage, for in-loop coding, or in a prediction process. ALF may be applied to any of various existing video codec technologies, such as HEVC-compliant codec technology, or be an efficient coding tool in any future video coding standards.

As used in this disclosure, the term video coding generically refers to either video encoding or video decoding. Similarly, the term video coder may generically refer to a video encoder or a video decoder. Moreover, certain techniques described in this disclosure with respect to video decoding may also apply to video encoding, and vice versa. For example, often video encoders and video decoders are configured to perform the same process, or reciprocal processes. Also, video encoder20may perform video decoding as part of the processes of determining how to encode video data.

As will be explained in more detail below, in accordance with the techniques of this disclosure video encoder20and video decoder30may be configured to utilize adaptive parameter sets (APSs) to signal information associated with adaptive loop filters (e.g., ALF parameters, such as ALF coefficients). Various techniques are described below for efficiently encoding ALF information and decoding ALF information using APSs, thus improving coding efficiency and/or picture quality.

In HEVC, VVC, and other video coding specifications, a video sequence typically includes a series of pictures. Pictures may also be referred to as “frames.” In one example approach, a picture may include three sample arrays, denoted SL, SCb, and SCr. In such an example approach, SLis a two-dimensional array (i.e., a block) of luma samples. SCbis a two-dimensional array of Cb chrominance samples. SCris a two-dimensional array of Cr chrominance samples. Chrominance samples may also be referred to herein as “chroma” samples. In other instances, a picture may be monochrome and may only include an array of luma samples.

To generate an encoded representation of a picture, video encoder20may generate a set of coding tree units (CTUs). In some embodiments, CTUs can be further divided into coding tree blocks (CTBs) (e.g., the two-dimensional arrays of luma, Cb chrominance, and Cr chrominance samples)). In additional embodiments, CTBs can be further divided into coding units (CUs).

In further embodiments, video encoder20may additionally or alternatively generate a set of tiles of a picture. A tile may include one or more CTUs of a picture. A tile may define vertical and/or horizontal lines that divide the picture (e.g., into rectangles). The components of a tile (e.g., CTUs) can be decoded in raster scan order inside each tile and the tiles can be decoded in the raster scan order inside a picture. The tiles may affect the availability of the neighboring CTUs, CTBs, or CUs for prediction and may or may not include resetting any entropy coding.

In still further embodiments, video encoder20may further segment a tile into slices. In some instances, the slices are designed to be independently decodable, enabling parallel processing.

Each of the CTUs may include a CTB of luma samples, two corresponding CTBs of chroma samples, and syntax structures used to code the samples of the CTBs. In monochrome pictures or pictures having three separate color planes, a CTU may include a single CTB and syntax structures used to code the samples of the CTB. A CTB may be an N×N block of samples. A CTU may also be referred to as a “tree block” or a “largest coding unit” (LCU). The CTUs of HEVC may be broadly analogous to the macroblocks of other standards, such as H.264/AVC. However, a CTU is not necessarily limited to a particular size and may include one or more CUs. A slice may include an integer number of CTUs ordered consecutively in a raster scan order.

In one example, to generate a coded CTU, video encoder20may recursively perform quad-tree partitioning on the CTBs of a CTU to divide the CTBs into coding blocks, hence the name “coding tree units.” A coding block may be an N×N block of samples. A CU may include a coding block of luma samples and two corresponding coding blocks of chroma samples of a picture that has a luma sample array, a Cb sample array, and a Cr sample array, and syntax structures used to code the samples of the coding blocks. In monochrome pictures or pictures having three separate color planes, a CU may include a single coding block and syntax structures used to code the samples of the coding block.

Video encoder20may partition a coding block of a CU into one or more prediction blocks. A prediction block is a rectangular (i.e., square or non-square) block of samples on which the same prediction is applied. A prediction unit (PU) of a CU may include a prediction block of luma samples, two corresponding prediction blocks of chroma samples, and syntax structures used to predict the prediction blocks. In monochrome pictures or pictures having three separate color planes, a PU may include a single prediction block and syntax structures used to predict the prediction block. Video encoder20may generate predictive luma, Cb, and Cr blocks for luma, Cb, and Cr prediction blocks of each PU of the CU.

As another example, video encoder20and video decoder30may be configured to operate according to JEM or VVC. According to JEM or VVC, a video coder (such as video encoder20) partitions a picture into a coding tree units (CTUs). Video encoder20may partition a CTU according to a tree structure, such as a quadtree-binary tree (QTBT) structure or Multi-Type Tree (MTT) structure. The QTBT structure removes the concepts of multiple partition types, such as the separation between CUs, PUs, and TUs (transform units) of HEVC. A QTBT structure includes two levels: a first level partitioned according to quadtree partitioning, and a second level partitioned according to binary tree partitioning. A root node of the QTBT structure corresponds to a CTU. Leaf nodes of the binary trees correspond to coding units (CUs).

In an MTT partitioning structure, blocks may be partitioned using a quadtree (QT) partition, a binary tree (BT) partition, and one or more types of triple tree (TT) partitions. A triple tree partition is a partition where a block is split into three sub-blocks. In some examples, a triple tree partition divides a block into three sub-blocks without dividing the original block through the center. The partitioning types in MTT (e.g., QT, BT, and TT), may be symmetrical or asymmetrical.

In some examples, video encoder20and video decoder30may use a single QTBT or MTT structure to represent each of the luminance and chrominance components, while in other examples, video encoder20and video decoder30may use two or more QTBT or MTT structures, such as one QTBT/MTT structure for the luminance component and another QTBT/MTT structure for both chrominance components (or two QTBT/MTT structures for respective chrominance components).

Video encoder20and video decoder30may be configured to use quadtree partitioning, QTBT partitioning, MTT partitioning, or other partitioning structures.

Video encoder20may use intra prediction or inter prediction to generate the predictive blocks for a block (e.g., a PU). If video encoder20uses intra prediction to generate the predictive blocks of a PU, video encoder20may generate the predictive blocks of the PU based on decoded samples of the picture associated with the PU. If video encoder20uses inter prediction to generate the predictive blocks of a PU, video encoder20may generate the predictive blocks of the PU based on decoded samples of one or more pictures other than the picture associated with the PU.

In some examples, video encoder20may use quad-tree partitioning to decompose the luma, Cb, and Cr residual blocks of a CU into one or more luma, Cb, and Cr transform blocks. In other examples, the transform block is the same size as the prediction block. A transform block is a rectangular (e.g., square or non-square) block of samples on which the same transform is applied. A transform unit (TU) of a CU may include a transform block of luma samples, two corresponding transform blocks of chroma samples, and syntax structures used to transform the transform block samples. Thus, each TU of a CU may be associated with a luma transform block, a Cb transform block, and a Cr transform block. The luma transform block associated with the TU may be a sub-block of the CU's luma residual block. The Cb transform block may be a sub-block of the CU's Cb residual block. The Cr transform block may be a sub-block of the CU's Cr residual block. In monochrome pictures or pictures having three separate color planes, a TU may include a single transform block and syntax structures used to transform the samples of the transform block.

Video encoder20may apply one or more transforms to a luma transform block of a TU to generate a luma coefficient block for the TU. A coefficient block may be a two-dimensional array of transform coefficients. A transform coefficient may be a scalar quantity. Video encoder20may apply one or more transforms to a Cb transform block of a TU to generate a Cb coefficient block for the TU. Video encoder20may apply one or more transforms to a Cr transform block of a TU to generate a Cr coefficient block for the TU.

The above block structure with CTUs, CUs, PUs, and TUs generally describes the block structure that can be used in HEVC. Other video coding standards, however, may use different block structures. As one example, although HEVC allows PUs and TUs to have different sizes or shapes, other video coding standards may require predictive blocks and transform blocks to have a same size. The techniques of this disclosure are not limited to the block structure of HEVC or any other standard and may be compatible with other block structures.

After generating a coefficient block (e.g., a luma coefficient block, a Cb coefficient block or a Cr coefficient block), video encoder20may quantize the coefficient block. Quantization generally refers to a process in which transform coefficients are quantized to possibly reduce the amount of data used to represent the transform coefficients, providing further compression. After video encoder20quantizes a coefficient block, video encoder20may entropy encode syntax elements indicating the quantized transform coefficients. For example, video encoder20may perform Context-Adaptive Binary Arithmetic Coding (CABAC) on the syntax elements indicating the quantized transform coefficients.

Video encoder20may output a bitstream that includes a sequence of bits that forms a representation of coded pictures and associated data. The bitstream may include a sequence of Network Abstraction Layer (NAL) units. A NAL unit is a syntax structure containing an indication of the type of data in the NAL unit and bytes containing that data in the form of a raw byte sequence payload (RBSP) interspersed as necessary with emulation prevention bits. Each of the NAL units includes a NAL unit header and encapsulates a RBSP. The NAL unit header may include a syntax element that indicates a NAL unit type code. The NAL unit type code specified by the NAL unit header of a NAL unit indicates the type of the NAL unit. A RBSP may be a syntax structure containing an integer number of bytes that is encapsulated within a NAL unit. In some instances, an RBSP includes zero bits.

Different types of NAL units may encapsulate different types of RB SP s. For example, a first type of NAL unit may encapsulate an RBSP for a video parameter set (VPS), a second type of NAL unit may encapsulate an RB SP for a sequence parameter set (SPS), a third type of NAL unit may encapsulate an RBSP for a picture parameter set (PPS), a fourth type of NAL unit may encapsulate an RBSP for an adaptive parameter set (APS), a fifth type of NAL unit may encapsulate an RBSP for a coded slice, a sixth type of NAL unit may encapsulate an RBSP for supplemental enhancement information (SEI) messages, and so on. NAL units that encapsulate RBSPs for video coding data (as opposed to RBSPs for parameter sets and SEI messages) may be referred to as video coding layer (VCL) NAL units.

A VPS may include data that is valid across multiple video sequences. An SPS may include data that is valid for an entire video sequence. A PPS may include data that is valid on a picture-by-picture basis. An APS may include picture-adaptive data that is also valid on a picture-by-picture basis but can change more frequently than the data in the PPS.

Video decoder30may receive a bitstream generated by video encoder20. In addition, video decoder30may parse the bitstream to obtain syntax elements from the bitstream. Video decoder30may reconstruct the pictures of the video data based at least in part on the syntax elements obtained from the bitstream. The process to reconstruct the video data may be generally reciprocal to the process performed by video encoder20. In addition, video decoder30may inverse quantize coefficient blocks associated with TUs of a current CU. Video decoder30may perform inverse transforms on the coefficient blocks to reconstruct transform blocks associated with the TUs of the current CU. Video decoder30may reconstruct the coding blocks of the current CU by adding the samples of the predictive blocks for PUs of the current CU to corresponding samples of the transform blocks of the TUs of the current CU. By reconstructing the coding blocks for each CU of a picture, video decoder30may reconstruct the picture.

Video encoder20and video decoder30may be configured to implement various adaptive loop filtering techniques set forth in JEM and/or working drafts of VVC. Aspects of some example JEM filtering techniques (e.g., ALF) will now be described. In addition to the modified de-blocking (DB) and HEVC SAO methods, JEM includes another filtering method called Geometry transformation-based Adaptive Loop Filtering (GALF). The input to an ALF/GALF may be the reconstructed image after the application of SAO. Aspects of GALF are described in Tsai, C. Y., Chen, C. Y., Yamakage, T., Chong, I. S., Huang, Y. W., Fu, C. M., Itoh, T., Watanabe, T., Chujoh, T., Karczewicz, M. and Lei, S. M., “Adaptive loop filtering for video coding”, IEEE Journal of Selected Topics in Signal Processing, 7(6), pp. 934-945, 2013 and in M. Karczewicz, L. Zhang, W.-J. Chien, and X Li, “Geometry transformation-based adaptive in-loop filter”, Picture Coding Symposium (PCS), 2016.

ALF techniques attempt to minimize the mean square error between the original samples and decoded/reconstructed samples by using an adaptive Wiener filter. In some embodiments, ALF can be implemented as described below.

An input image can be denoted as p, a source image as S, and a finite impulse response (FIR) filter as h. The following expression of the sum of squared errors (SSE) can be minimized, where (x, y) denotes any pixel position in p or S.
SSE=Σx,y(Σi,jh(i,j)p(x−i,y−j)−S(x,y))2

The optimal h, denoted as hopt, can be obtained by setting the partial derivative of SSE with respect to h(i, j) equal to 0 as follows:

∂S⁢S⁢E∂h⁡(i,j)=0
This leads to the Wiener-Hopf equation shown below, which gives the optimal filter hopt:
Σi,jhopt(i,j)(Σx,yp(x−i,y−j)p(x−m,y−n))=Σx,yS(x,y)p(x−m,y−n)

In some examples, instead of using one filter for the whole picture, video encoder20and/or video decoder30may be configured to classify samples in a picture into twenty-five (25) classes based on the local gradients. Video encoder20and/or video decoder30may derive separate optimal Wiener filters for the pixels in each class. Several techniques may be used to increase the effectiveness of ALF by reducing signaling overhead and computational complexity. Some of the techniques that can be used to increase ALF effectiveness by reducing signaling overhead and/or computational complexity are listed below:1. Pre diction from fixed filters: Optimal filter coefficients for each class are predicted using a prediction pool of fixed filters which include 16 candidate filters for each class. The best prediction candidate is selected for each class and only the prediction errors are transmitted.2. Class merging: Instead of using twenty-five (25) different filters (one for each class), pixels in multiple classes can share one filter in order to reduce the number of filter parameters to be coded. Merging two classes can lead to higher cumulative SSE but a lower Rate-Distortion (RD) cost.3. Variable number of taps: The number of filter taps is adaptive at the frame level. Filters with more taps may achieve lower SSE, but may not be a good choice in terms of RD cost because of the bit overhead associated with more filter coefficients.4. Block level on/off control: ALF can be turned on and off (enabled or disabled) on a block basis. The block size at which the on/off control flag is signaled is adaptively selected at the frame level. Filter coefficients may be recomputed using pixels from only those blocks for which an ALF is enabled (i.e., an ALF is used).5. Temporal prediction: Filters derived for previously coded frames are stored in a buffer. If the current frame is a P or B frame, then one of the stored set of filters may be used to filter the current frame if it leads to better RD cost. A flag is signaled to indicate usage of temporal prediction. If temporal prediction is used, then an index indicating which set of stored filters is used is signaled. No additional signaling of ALF coefficients may be needed. Block level ALF on/off control flags may be also signaled for a frame using temporal prediction.

Details of some aspects of ALF are summarized in this and the following paragraphs. Some aspects of ALF are related to pixel classification and geometry transformation. In one example, video encoder20and video decoder30may be configured to compute sums of absolute values of vertical, horizontal, and diagonal Laplacians at all pixels within a 6×6 window that covers each pixel in a reconstructed frame (before ALF). Video encoder20and video decoder30divide the reconstructed frame into non-overlapped 2×2 blocks. Video encoder20and video decoder30classify the four pixels in these blocks into one of twenty five (25) categories, denoted as Ck(k=0, 1, . . . , 24), based on the total Laplacian activity and directionality of that block. Additionally, video encoder20and video decoder30apply one of four geometry transformations (no transformation, diagonal flip, vertical flip, or rotation) to the filters based on the gradient directionality of that block.

Some aspects of adaptive loop filtering are related to filter derivation and prediction from fixed filters. For each class Ck, video encoder20and video decoder30first determine a best prediction filter from the pool for Ck, denoted as hpred,k, based on the SSE given by the filters. The SSE of Ck, which is to be minimized, can be written as below,
SSEk=Σx,y(Σi,j(hpred,k(i,j)+hΔ,k(i,j))p(x−i,y−j)−S(x,y))2,k=0, . . . ,24,(x,y)∈Ck,
where hΔ,kis the difference between the optimal filter for Ckand hpred,k. p′(x, y)=Σi,jhpred,k(i, j)p(x−i, y−j) is the result of filtering pixel p(x, y) by hpred,k. Then the expression for SSEkcan be re-expressed as

By making the partial derivative of SSEkwith respect to hΔ,k(i,j) equal to 0, the modified Wiener-Hopf equation can be obtained as follows:

In one example of adaptive loop filtering, video encoder20calculates and transmits only the difference between the optimal filter and the fixed prediction filter. If none of the candidate filters available in the pool is a good predictor, then video encoder20and video decoder30uses the identity filter (i.e., the filter with only one non-zero coefficient equal to 1 at the center that makes the input and output identical) as the predictor.

Some aspects of adaptive loop filtering relate to the merging of pixel classes. Classes are merged to reduce the overhead of signaling filter coefficients. The cost of merging two classes is increased with respect to SSE. Two classes Cmand Cnwith SSEs given by SSEmand SSEn, respectively can be determined. Cm+ncan denote the class obtained by merging Cmand Cnwith SSE, SSEm+n. SSEm+ncan always be greater than or equal to SSEm+SSEn. ΔSSEm+ncan denote the increase in SSE caused by merging Cmand Cn, which is equal to SSEm+n−(SSEm+SSEn). To calculate SSEm+n, video encoder20may derive hΔ,m+n, the filter prediction error for Cm+n, using the following expression similar to (1):
Σi,jhΔ,m+n(i,j)(Rpp,m(i−u,j−v)+Rpp,n(i−u,j−v))=R′ps,m(u,v)+R′ps,n(u,v)  (2)
Video encoder20may calculate the SSE for the merged category Cm+nas:
SSEm+n=−Σu,vhΔ,m+n(u,v)(R′ps,m(u,v)+R′ps,n(u,v))+(Rss,m+Rss,n)

To reduce the number of classes from N to N−1, two classes, Cmand Cn, may be determined, such that merging them leads to the smallest ΔSSEm+ncompared to any other combinations. In some ALF designs, video encoder20is configured to check every pair of available classes for merging to find the pair with the smallest merge cost.

If Cmand Cn(with m<n) are merged, then video encoder20and video decoder30may mark Cnas unavailable for further merging and the auto- and cross-correlations for Cmare changed to the combined auto- and cross-correlations as follows:
Rpp,m=Rpp,m+Rpp,n
R′ps,m=R′ps,m+R′ps,n
Rss,m=Rss,m+Rss,n.

Video encoder20may determine an optimal number of ALF classes after merging for each frame based on the RD cost. In one example, this is done by starting with twenty-five (25) classes and merging a pair of classes (from the set of available classes) successively until there is only one class left. For each possible number of classes (1, 2, . . . , 25) left after merging, video encoder20may store a map indicating which classes are merged together. Video encoder20can then select the optimal number of classes such that the RD cost is minimized as follows:

Nopt=arg⁢⁢min⁢N⁢(J⁢N⁢=DN+λ⁢⁢R⁢N),
where D|Nis the total SSE of using N classes (D|N=Σk=0N-1SSEk), R|Nis the total number of bits used to code the N filters, and λ is the weighting factor determined by the quantization parameter (QP). Video encoder20may transmit the merge map for Noptnumber of classes, indicating which classes are merged together, to video decoder30.

Aspects of signaling ALF parameters are described below. A brief step-by-step description of an example ALF parameter encoding process performed by video encoder20is given below. Video decoder30may be configured to perform a reciprocal process (e.g., signal from the perspective of video decoder30is the reception of syntax elements).1. Signal the frame level ALF on/off flag.2. If ALF is on, then signal the temporal prediction flag indicating the usage of the filters from the previous pictures.3. If temporal prediction is used, then signal the index of the frame from which the corresponding ALF parameters are used for filtering the current frame.4. If temporal prediction is not used, then signal the auxiliary ALF information and filter coefficients as follows:a. The following auxiliary ALF information may be signaled before signaling the filter coefficients. The auxiliary ALF information may include:i. The number of unique filters used after class merging.ii. Number of filter taps.iii. Class merge information indicating which classes share the filter prediction errors.iv. Index of the fixed filter predictor for each class.b. After signaling the auxiliary ALF information, filter coefficient prediction errors may be signaled as follows:i. A flag is signaled to indicate if the filter prediction errors are forced to zero (0) for some of the remaining classes after merging.ii. A flag is signaled to indicate if differential coding is used for signaling filter prediction errors (if the number of classes left after merging is larger than one (1)).iii. Filter coefficient prediction errors are then signaled using k-th order Exp-Golomb code, where the k-value for different coefficient positions is selected empirically.c. Filter coefficients for chroma components, if available, are directly coded without any prediction methods.5. Finally, the block-level ALF on/off control flags are signaled.
ALF with Clipping

Described below are examples ALF techniques with clipping that can be performed, for example, by video encoder20and/or video decoder30.

In some embodiments, decoded filter coefficients f(k,l) and clipping values c(k,l) are applied to a reconstructed image R(i,j) as follows:

FIG.2shows an example of ALF filter supports that can be used with techniques described in this disclosure. In particular,FIG.2shows a 5×5 diamond filter support200and a 7×7 diamond filter support210that can be used with ALF techniques with clipping.

A 7×7 filter (e.g., filter210) can applied to the luma component and a 5×5 filter (e.g., filter200) can be applied to chroma components.

Clipping value c(k, l) is calculated as follows. For the luma component:
c(k,l)=Round(2(BitDepthY*(4−clipIdx(k,l))/4))
Where BitDepthY is the bit depth for the luma component and clipIdx(k,l) is the clipping values at position (k,l). clipIdx(k,l), which can be 0, 1, 2 or 3.

For the chroma components:
c(k,l)=Round(2(BitDepthC−8)*2(8*(3−clipIdx[k,lj])/3))
Where BitDepthC is the bit depth for the chroma component and clipIdx(k,l) is the clipping values at position (k,l). clipIdx(k,l), which can be 0, 1, 2 or 3.
Pixel Classification

For the luma component, 4×4 blocks in the whole picture can be classified based on a 1D Laplacian direction (up to 5 directions) and a 2D Laplacian activity (up to 5 activity values). The direction Dirband unquantized activity Actbcan be calculated. Actbcan be further quantized to the range of 0 to 4 inclusively.

Values of two diagonal gradients, in addition to the horizontal and vertical gradients used in the existing ALF, are calculated using the 1-D Laplacian direction. As shown in (3) to (6) below, the sum of gradients of all pixels within an 8×8 window that covers a target pixel is employed as the represented gradient of the target pixel, where R (k, l) is the reconstructed pixels at location (k, l) and indices i and j refer to the coordinates of the upper left pixel in the 4×4 block. Each pixel is associated with four gradient values, with the vertical gradient denoted by gv, the horizontal gradient denoted by gh, the 135 degree diagonal gradient denoted by gd1, and the 45 degree diagonal gradient denoted by gd2.

gv=∑k=i-2i+5⁢∑l=j-2j+5⁢Vk,l,(3)Vk,l=|2R(k, l)−R(k, l−1)−R(k, l+1)| when both k and l are even numbers or both of k and l are not even numbers, 0 otherwise.

gh=∑k=i-2i+5⁢∑l=j-2j+5⁢Hk,l,(4)Hk,l=|2R(k, l)−R(k−1,l)−R(k+1,l)| when both k and l are even numbers or both of k and l are not even numbers, 0 otherwise.

gd⁢1=∑k=i-2i+5⁢∑l=j-3j+5⁢D⁢1k,l,(5)D1k,l=|2R(k, l)−R(k−1, l−1)−R(k+1, l+1)| when both k and l are even numbers or both of k and l are not even numbers, 0 otherwise.

gd⁢2=∑k=i-2i+5⁢∑j=j-2j+5⁢D⁢2k,l,(6)D2k,l=|2R(k, l)−R(k−1, l+1)−R(k+1, l−1)| when both k and l are even numbers or both of k and l are not even numbers, 0 otherwise.

To assign the directionality Dirb, the ratio of the maximum and the minimum of the horizontal and vertical gradients, denoted by Rh,vin (7) and the ratio of the maximum and the minimum of two diagonal gradients, denoted by Rd1,d2in (8) are compared against each other with two thresholds t1and t2.
Rh,v=gh,vmax/gh,vmin
whereingh,vmax=max(gh,gv),gh,vmin=min(gh,gv),  (7)
Rd0,d1=gd0,d1max/gd0,d1min
whereingd0,d1max=max(gd0,gd1),gd0,d1min=min(gd0,gd1)  (8)

By comparing the detected ratios of the horizontal/vertical and diagonal gradients, five direction modes, i.e., Dirbwithin the range of [0, 4] inclusive, are defined in (9). The values and physical meaning of Dirbare described in Table 1.

TABLE 1Values of Direction and Its Physical MeaningDirection valuesphysical meaning0Texture1Strong horizontal/vertical2horizontal/vertical3strong diagonal4diagonal

The activity value Act can be calculated as:

Act is further quantized to the range of 0 to 4 inclusive, and the quantized value is denoted as Â.

Quantization Process from Activity Value Act to Activity Index Â

In total, each 4×4 luma block can be categorized into one out of 25 (5×5) classes and an index is assigned to each 4×4 block according the value of Dirband Actbof the block. The group index can be denoted by C and is set equal to 5Dirb+Â wherein Â is the quantized value of Actb.

Geometry Transformations

For each category, one set of filter coefficients and clipping values may be signalled. To better distinguish different directions of blocks marked with the same category index, four geometry transformations, including no transformation, diagonal, vertical flip and rotation, are introduced.

FIG.3shows an example of an ALF filter support that can be used with techniques described in this disclosure. In particular,FIG.3shows a 5×5 diamond-shaped filter support300.

FIG.4shows examples of geometry transformations that can be used with techniques described in this disclosure. In particular,FIG.4can show geometry transformations400,410, and420. In some embodiments, the geometry transformations400,410, and420can be geometry transformations of the filter support300shown inFIG.3. In further embodiments, the geometry transformation400can be a diagonal transform of the filter support300, the geometry transformation410can be a vertical flip transform of the filter support300, and the geometry transformation420can be a rotation transform of the filter support400.

The geometric transformations shown inFIG.3andFIG.4can be represented in formula forms as follows:
Diagonal:fD(k,l)=f(l,k),cD(k,l)=c(l,k),
Vertical flip:fV(k,l)=f(k,K−l−1),cV(k,l)=c(k,K−l−1)
Rotation:fR(k,l)=f(K−l−1,k),cR(k,l)=c(K−l−1,k).  (11)
where K is the size of the filter and 0≤k, l≤K−1 are coefficients coordinates, such that location (0,0) is at the upper left corner and location (K−1, K−1) is at the lower right corner.

In embodiments when the diamond filter support is used, the coefficients with coordinates out of the filter support will be set to 0. In some embodiments, the geometry transformation index can be indicated by deriving it implicitly to avoid additional overhead. In GALF, the transformations are applied to the filter coefficients f(k, l) depending on gradient values calculated for that block. The relationship between the transformation and the four gradients calculated using (3)-(6) is described in Table 2. To summarize, the transformations are based on which one of two gradients (horizontal and vertical, or 45 degree and 135 degree gradients) is larger. Based on the comparison, more accurate direction information can be extracted. Therefore, different filtering results could be obtained due to the transformation while the overhead of filter coefficients is not increased.

In some embodiments, one luma filter parameter set can contain filter information (including filter coefficients and clipping values) for all 25 classes.

Fixed filters can be used to predict the filters for each class. A flag can be signaled for each class to indicate whether this class uses a fixed filter as its filter predictor. If yes, the fixed filter information is signaled.

To reduce the number of bits required to represent the filter coefficients, different classes can be merged. The information indicating which classes are merged is provided by sending an index iCfor each of the 25 classes. Classes having the same index iCshare the same filter coefficients that are coded. The mapping between classes and filters is signaled for each luma filter set. The index iCis coded with a truncated binary binarization method.

A signaled filter can be predicted from a previously signaled filter.

Adaptive Parameter Set

Adaptive parameter sets (APSs) can be used to carry ALF filter coefficients in the bitstream. An APS can contain a set of luma filter parameters or a set(s) of chroma filter parameters, or a combination thereof. A tile group (i.e., a group of one or more tiles) may only signal indices of APSs that are used for the current tile group in its tile group header. APSs can be used in various video coding standards, such as VVC.

CTU/CTB-Based Filter Set Switch

Filters generated from previously coded tile groups can be used for the current tile group to save the overhead for filter signaling. A luma CTU/CTB can use a filter set among fixed filter sets and non-fixed filter sets from an APSs. The filter set index can be signaled. All chroma CTBs may use a filter from the same APS. In the tile group header, the APSs used for luma and chroma CTBs of the current tile group can be signaled.

This disclosure describes techniques to further improve the coding gains and visual quality obtained by using adaptive loop filtering. Video encoder20and/or video decoder30may apply any of the following itemized techniques individually. Alternatively, video encoder20and/or video decoder30may apply any combination of the techniques discussed below.

In some embodiments, a filter (e.g., an ALF) for a current block signaled in one APS may be predicted from other filters in the same APS, filters in different APSs, or pre-defined filters. For instance, a filter for one tile/slice of a picture can be predicted based on a filter for a previous tile/slice in the same picture, a filter for a previous tile/slice in a different picture, or a set of pre-defined filters (e.g., maintained by a decoder and/or signalled from the encoder to the decoder for one or more video sequences). The video encoder20can indicate that the filter is to be predicted in the APS, can indicate which filter to use for the prediction, can indicate which APS to use to obtain the filter parameters used for prediction, and/or indicate the difference between the filter parameters for the current block and the filter parameters used for the prediction in the APS (if any). Based on the indications and filter parameters in the APS received from the video encoder20, the video decoder can determine the filter for the current block.

In further embodiments, filter parameters generated for one picture/slice/tile-group can be included in a single APS or can be included in multiple APSs. In some embodiments, an index can be signaled to indicate which APS(s) contain the filter parameters for a picture/slice/tile-group. This can result in reduced signaling as the filter parameters do not need to be signaled for each picture/slice/tile-group and can be, for example, consolidated in one APS or a group of APSs.

In other embodiments, all or some of the filters signaled in the same APS can use the same signaling parameter(s) to apply exponential-Golomb codes to signal filter coefficients. Additionally or alternatively, all or some of the filters signaled in the same APS can use the same signaling parameters to apply exponential-Golomb codes for the coefficients in the same positions of these filters. This can result in coding efficiencies due to allowing use of different order exponential-Golomb codes. For example, the order can be signaled for all or some of the filters and/or a default value can be used (e.g., if no order is signaled, a default flag is enabled, etc.).

In some embodiments, each filter may have its own parameters to apply exponential-Golomb codes to signal filter coefficients. In one example, all coefficients in a filter use the same parameters of exponential-Golomb codes. In another example, the parameters of exponential-Golomb codes can depend on the position of the coefficients in the filter. For example, for each filter, for each coefficient, or for each position, the order for the exponential-Golomb codes can be signaled or can be a default value (e.g., if no order is signaled, a default flag is enabled, etc.). This can result in coding efficiencies due to allowing use of different order exponential-Golomb codes for each filter, coefficient, position, etc.

In further embodiments, 0th order exponential-Golomb codes may be used to signal filter coefficients.

In additional embodiments, filter information (such as an APS index, a filter set index, and/or a filter index) of previously coded CTBs/CTUs/blocks, such as the filter information of a neighbor CTB/CTU/block, may be used to predict the filter information for current CTB/CTU/block. In some embodiments, the prediction may be used only when a number of filter candidates is larger than a threshold. For example, the threshold can be 100 and prediction may be used when the number of filter candidates is greater than 100. In further embodiments, the prediction information and filter information may be signaled with or without contexts. Example contexts include whether a neighboring block uses the same filter, whether a neighboring block used prediction, whether ALF is enabled/disabled for a neighboring block, and the like.

In some embodiments, an APS index can be predicted (inferred) from top and/or left neighboring CTBs/CTUs/blocks. In some embodiments, predicting the APS index from the top and/or left neighboring CTBs/CTUs/blocks can be the defined default behavior. In other embodiments, a flag can be used to indicate that the APS index is to be predicted from the top and/or left neighboring CTBs/CTUs/blocks.

In other embodiments, contexts used for signaling APS indices may be derived based on the APS indices signaled for neighboring CTBs/CTUs/blocks (located at the top and left of the current CTB/CTU/block). In other words, the APS indices signaled for neighboring CTBs/CTUs/blocks can be used as prediction information.

In further embodiments, for a CTB, instead of re-using filters in the APSs, some coefficient differences may be signaled, then the coefficients of the final filters may be equal to the sum of coefficients of filters in the APSs and the coefficient difference.

In additional embodiments, filters from coded APSs may be used to form one or multiple new filters. In some implementations, only the information about where the filters are from is signaled and, thus, the signal filter coefficients may not be signaled. For example, a new APS may be signaled in the bitstream, with an indication to obtain one or more filter parameters from a first APS and one or more filter parameters from a second APS that were previous signaled. As a further example, the new APS may be signaled with one or more filter parameters and may also include an indication to obtain one or more filter parameters from previously signalled APSs. Additionally or alternatively, information about how new filter sets are formed may be signaled in picture/slice/tile group headers, such that the new filter sets can be used in that picture/slice/tile group. An example of such information can be an indicator (e.g., an index) of an APS and/or an indicator (e.g., an index) of a filter within the APS.

In the example ofFIG.5, video encoder20includes a video data memory33, partitioning unit35, prediction processing unit41, summer50, transform processing unit52, quantization unit54, entropy encoding unit56. Prediction processing unit41includes motion estimation unit (MEU)42, motion compensation unit (MCU)44, and intra prediction unit46. For video block reconstruction, video encoder20also includes inverse quantization unit58, inverse transform processing unit60, summer62, filter unit64, and decoded picture buffer (DPB)66.

As shown inFIG.5, video encoder20receives video data and stores the received video data in video data memory33. Video data memory33may store video data to be encoded by the components of video encoder20. The video data stored in video data memory33may be obtained, for example, from video source18. DPB66may be a reference picture memory that stores reference video data for use in encoding video data by video encoder20, e.g., in intra- or inter-coding modes. Video data memory33and DPB66may be formed by any of a variety of memory devices, such as dynamic random access memory (DRAM), including synchronous DRAM (SDRAM), magnetoresistive RAM (MRAM), resistive RAM (RRAM), or other types of memory devices. Video data memory33and DPB66may be provided by the same memory device or separate memory devices. In various examples, video data memory33may be on-chip with other components of video encoder20, or off-chip relative to those components.

Partitioning unit35retrieves the video data from video data memory33and partitions the video data into video blocks. This partitioning may also include partitioning into slices, tiles, or other larger units, as wells as video block partitioning, e.g., according to a quadtree structure of LCUs and CUs. Video encoder20generally illustrates the components that encode video blocks within a video slice to be encoded. The slice may be divided into multiple video blocks (and possibly into sets of video blocks referred to as tiles). Prediction processing unit41may select one of multiple possible coding modes, such as one of multiple intra coding modes or one of multiple inter coding modes, for the current video block based on error results (e.g., coding rate and the level of distortion). Prediction processing unit41may provide the resulting intra- or inter-coded block to summer50to generate residual block data and to summer62to reconstruct the encoded block for use as a reference picture.

Intra prediction unit46within prediction processing unit41may perform intra-predictive coding of the current video block relative to one or more neighboring blocks in the same frame or slice as the current block to be coded to provide spatial compression. Motion estimation unit42and motion compensation unit44within prediction processing unit41perform inter-predictive coding of the current video block relative to one or more predictive blocks in one or more reference pictures to provide temporal compression.

Motion estimation unit42may be configured to determine the inter-prediction mode for a video slice according to a predetermined pattern for a video sequence. The predetermined pattern may designate video slices in the sequence as P slices or B slices. Motion estimation unit42and motion compensation unit44may be highly integrated, but are illustrated separately for conceptual purposes. Motion estimation, performed by motion estimation unit42, is the process of generating motion vectors, which estimate motion for video blocks. A motion vector, for example, may indicate the displacement of a PU of a video block within a current video frame or picture relative to a predictive block within a reference picture.

Motion compensation, performed by motion compensation unit44, may involve fetching or generating the predictive block based on the motion vector determined by motion estimation, possibly performing interpolations to sub-pixel precision. Upon receiving the motion vector for the PU of the current video block, motion compensation unit44may locate the predictive block to which the motion vector points in one of the reference picture lists. Video encoder20forms a residual video block by subtracting pixel values of the predictive block from the pixel values of the current video block being coded, forming pixel difference values. The pixel difference values form residual data for the block, and may include both luma and chroma difference components. Summer50represents the component or components that perform this subtraction operation. Motion compensation unit44may also generate syntax elements associated with the video blocks and the video slice for use by video decoder30in decoding the video blocks of the video slice.

After prediction processing unit41generates the predictive block for the current video block, either via intra prediction or inter prediction, video encoder20forms a residual video block by subtracting the predictive block from the current video block. The residual video data in the residual block may be included in one or more TUs and applied to transform processing unit52. Transform processing unit52transforms the residual video data into residual transform coefficients using a transform, such as a discrete cosine transform (DCT) or a conceptually similar transform. Transform processing unit52may convert the residual video data from a pixel domain to a transform domain, such as a frequency domain.

Following quantization, entropy encoding unit56entropy encodes the quantized transform coefficients. For example, entropy encoding unit56may perform context adaptive variable length coding (CAVLC), context adaptive binary arithmetic coding (CABAC), syntax-based context-adaptive binary arithmetic coding (SBAC), probability interval partitioning entropy (PIPE) coding or another entropy encoding methodology or technique. Following the entropy encoding by entropy encoding unit56, the encoded bitstream may be transmitted to video decoder30or archived for later transmission or retrieval by video decoder30. Entropy encoding unit56may also entropy encode the motion vectors and the other syntax elements for the current video slice being coded provided by prediction processing unit41. Further, entropy encoding unit56may also entropy encode VPS, SPS, PPS, and/or APS information. For example, the entropy encoding unit56may encode APS information that indicates that a filter for a current block is to be predicted based on information in a previous APS, as described above.

Inverse quantization unit58and inverse transform processing unit60apply inverse quantization and inverse transformation, respectively, to reconstruct the residual block in the pixel domain for later use as a reference block of a reference picture. Motion compensation unit44may calculate a reference block by adding the residual block to a predictive block of one of the reference pictures within one of the reference picture lists. Motion compensation unit44may also apply one or more interpolation filters to the reconstructed residual block to calculate sub-integer pixel values for use in motion estimation. Summer62adds the reconstructed residual block to the motion compensated prediction block produced by motion compensation unit44to produce a reconstructed block.

Filter unit64filters the reconstructed block (e.g. the output of summer62) and stores the filtered reconstructed block in DPB66for uses as a reference block. The reference block may be used by motion estimation unit42and motion compensation unit44as a reference block to inter-predict a block in a subsequent video frame or picture. Filter unit64may perform any type of filtering such as deblock filtering, SAO filtering, peak SAO filtering, ALF, and/or GALF, and/or other types of loop filters. A deblock filter may, for example, apply deblocking filtering to filter block boundaries to remove blockiness artifacts from reconstructed video. A peak SAO filter may apply offsets to reconstructed pixel values in order to improve overall coding quality. Additional loop filters (in loop or post loop) may also be used.

In addition, filter unit64may be configured to perform any of the techniques in this disclosure related to adaptive loop filtering. For example, as described above, filter unit64may be configured to determine parameters for filtering a current block based on parameters for filtering a previous block that were included in the same APS as the current block, a different APS, or pre-defined filters.

FIG.6is a block diagram illustrating an example video decoder30that may implement the techniques described in this disclosure. Video decoder30ofFIG.6may, for example, be configured to receive the signaling described above with respect to video encoder20ofFIG.5. In the example ofFIG.6, video decoder30includes video data memory78, entropy decoding unit80, prediction processing unit81, inverse quantization unit86, inverse transform processing unit88, summer90, DPB94, and filter unit92. Prediction processing unit81includes motion compensation unit82and intra prediction unit84. Video decoder30may, in some examples, perform a decoding pass generally reciprocal to the encoding pass described with respect to video encoder20fromFIG.5.

During the decoding process, video decoder30receives an encoded video bitstream that represents video blocks of an encoded video slice and associated syntax elements from video encoder20. Video decoder30stores the received encoded video bitstream in video data memory78. Video data memory78may store video data, such as an encoded video bitstream, to be decoded by the components of video decoder30. The video data stored in video data memory78may be obtained, for example, via link16, from storage device26, or from a local video source, such as a camera, or by accessing physical data storage media. Video data memory78may form a coded picture buffer (CPB) that stores encoded video data from an encoded video bitstream. DPB94may be a reference picture memory that stores reference video data for use in decoding video data by video decoder30, e.g., in intra- or inter-coding modes. Video data memory78and DPB94may be formed by any of a variety of memory devices, such as DRAM, SDRAM, MRAM, RRAM, or other types of memory devices. Video data memory78and DPB94may be provided by the same memory device or separate memory devices. In various examples, video data memory78may be on-chip with other components of video decoder30, or off-chip relative to those components.

Entropy decoding unit80of video decoder30entropy decodes the video data stored in video data memory78to generate quantized coefficients, motion vectors, and other syntax elements. Entropy decoding unit80forwards the motion vectors and other syntax elements to prediction processing unit81. Video decoder30may receive the syntax elements at the video slice level and/or the video block level. Entropy decoding unit80may also decode VPS, SPS, PPS, and/or APS information. For example, the entropy decoding unit80may decode APS information that indicates that a filter for a current block is to be predicted based on information in a previous APS, as described above.

When the video slice is coded as an intra-coded (I) slice, intra prediction unit84of prediction processing unit81may generate prediction data for a video block of the current video slice based on a signaled intra prediction mode and data from previously decoded blocks of the current frame or picture. When the video frame is coded as an inter-coded slice (e.g., B slice or P slice), motion compensation unit82of prediction processing unit81produces predictive blocks for a video block of the current video slice based on the motion vectors and other syntax elements received from entropy decoding unit80. The predictive blocks may be produced from one of the reference pictures within one of the reference picture lists. Video decoder30may construct the reference frame lists, List0and List1, using default construction techniques based on reference pictures stored in DPB94.

Motion compensation unit82may also perform interpolation based on interpolation filters. Motion compensation unit82may use interpolation filters as used by video encoder20during encoding of the video blocks to calculate interpolated values for sub-integer pixels of reference blocks. In this case, motion compensation unit82may determine the interpolation filters used by video encoder20from the received syntax elements and use the interpolation filters to produce predictive blocks.

Inverse quantization unit86inverse quantizes, i.e., de-quantizes, the quantized transform coefficients provided in the bitstream and decoded by entropy decoding unit80. The inverse quantization process may include use of a quantization parameter calculated by video encoder20for each video block in the video slice to determine a degree of quantization and, likewise, a degree of inverse quantization that should be applied. Inverse transform processing unit88applies an inverse transform, e.g., an inverse DCT, an inverse integer transform, or a conceptually similar inverse transform process, to the transform coefficients in order to produce residual blocks in the pixel domain.

After prediction processing unit81generates the predictive block for the current video block using, for example, intra or inter prediction, video decoder30forms a reconstructed video block by summing the residual blocks from inverse transform processing unit88with the corresponding predictive blocks generated by motion compensation unit82. Summer90represents the component or components that perform this summation operation.

Filter unit92filters the reconstructed block (e.g. the output of summer90) and stores the filtered reconstructed block in DPB94for uses as a reference block and/or outputs the filtered reconstructed block (decoded video). The reference block may be used by motion compensation unit82as a reference block to inter-predict a block in a subsequent video frame or picture. Filter unit92may perform any type of filtering such as deblock filtering, SAO filtering, peak SAO filtering, ALF, and/or GALF, and/or other types of loop filters. A deblock filter may, for example, apply deblocking filtering to filter block boundaries to remove blockiness artifacts from reconstructed video. A peak SAO filter may apply offsets to reconstructed pixel values in order to improve overall coding quality. Additional loop filters (in loop or post loop) may also be used.

In addition, filter unit92may be configured to perform any of the techniques in this disclosure related to adaptive loop filtering. For example, as described above, filter unit92may be configured to determine parameters for filtering a current block based on parameters for filtering a previous block that were included in the same APS as the current block, a different APS, or pre-defined filters.

FIG.7shows an example implementation of filter unit92. Filter unit64may be implemented in the same manner. Filter units64and92may perform the techniques of this disclosure, possibly in conjunction with other components of video encoder20or video decoder30. In other embodiments, the filter unit64can be a post-processing unit that can perform the techniques of this disclosure outside of, for example, the video decoder30(e.g., after the decoded video is output from the video decoder30). In the example ofFIG.7, filter unit92includes deblock filter102, SAO filter104, and ALF/GALF filter106. SAO filter104may, for example, be configured to determine offset values for samples of a block in the manner described in this disclosure. ALF/GALF filter106may be configured to, for example, determine parameters for filtering a current block based on parameters for filtering a previous block that were included in the same APS as the current block, a different APS, or pre-defined filters.

Filter unit92may include fewer filters and/or may include additional filters. Additionally, the particular filters shown inFIG.7may be implemented in a different order. Other loop filters (either in the coding loop or after the coding loop) may also be used to smooth pixel transitions or otherwise improve the video quality. When in the coding loop, the decoded video blocks in a given frame or picture are then stored in DPB94, which stores reference pictures used for subsequent motion compensation. DPB94may be part of or separate from additional memory that stores decoded video for later presentation on a display device, such as display device32ofFIG.1.

FIG.8is a flowchart illustrating an example method of the disclosure. The techniques ofFIG.8may be performed by one or more structural units of video encoder20and video decoder30, including filter unit64and filter unit92. As discussed above, the term “coding” generically refers to both encoding and decoding. Likewise, the term “code” generically refers to both encode and decode.

In one example of the disclosure, video encoder20and/or video decoder30may, in element800, determine a set of ALF parameters for a block from an APS.

For example, as described above, ALF parameters for the current block may be predicted from other filters in the same APS, filters in different APSs, or pre-defined filters.

As an additional example, ALF parameters generated for one picture/slice/tile-group are included in a single APS or multiple APSs.

As an additional example, all or some of the filters signaled in the same APS can use the same parameters (orders) to apply exponential-Golomb codes to signal filter coefficients.

As an additional example, all or some of the filters signaled in the same APS can use the same parameters (orders) to apply exponential-Golomb codes to signal filter coefficients for the coefficients in the same filter positions.

As an additional example, in an APS, each filter may have its own parameters (orders) to apply exponential-Golomb codes to signal filter coefficients.

As an additional example, 0th order exponential-Golomb codes may be used to signal coefficients in a filter explicitly or implicitly.

As an additional example, the filter information (such as an APS index, an adaptive loop filter set index, and/or an adaptive filter index) of previously coded CTBs/CTUs/blocks, such as the filter information of a neighbor CTB/CTU/block, may be used to predict the filter information for current CTB/CTU/block. The prediction may be used only when a number of adaptive loop filter/APS candidates is larger than a threshold. The prediction information and filter information may be signaled with or without contexts.

As an additional example, the APS index of current CTB/CTU/block can be predicted (inferred) from top and/or left neighboring CTBs/CTUs/blocks.

As an additional example, the contexts used for signaling APS indices may be derived based on the APS indices signaled for neighboring CTBs/CTUs/blocks (located at the top and left of the current CTB/CTU/block).

As an additional example, for a CTB, instead of re-using filters in the APSs, some coefficient differences may be signaled, then the coefficients of the final filters may be equal to the sum of coefficients of filters in the APSs and the coefficient difference.

As an additional example, the filters from coded APSs may be used to form one or multiple new APSs. For example, only the information about where the filters are from (APS index and/or filter index) is signaled and, thus, the filter coefficients in the new APSs may not be signaled.

As an additional example, information about how new filter sets are formed (APS index and/or filter index) may be signaled in picture/slice/tile group headers, such that the new filter sets can be used in that picture/slice/tile group.

In element802, the video encoder and/or the video decoder30may adaptive loop filter the block in accordance with the respective set of adaptive loop filter parameters determined in the element800.

FIG.9is a flowchart illustrating another example method of the disclosure.FIG.9shows an example of the techniques ofFIG.8in more detail. For example, video encoder20and/or video decoder30may, in element900, code a block of video data in a current picture to create a reconstructed block of video data. That is, video encoder20and/or video decoder30may code the first block of video data in the current picture to create a first reconstructed block of video data.

Video encoder20and/or video decoder30may then, in element902, determine if a block level ALF On/Off flag is on. If no, video encoder20and/or video decoder30do not apply ALF, and instead proceed to code the next block of video data in element910. If yes, in element904, video encoder20and/or video decoder30code, for the block, a syntax element in an adaptive parameter set that indicates how to determine a set of ALF parameters.

In element906, video encoder20and/or video decoder30may then determine the ALF parameters based on the syntax element coded in element904(e.g., predicted based on parameters for previous ALFs in the APS or a different APS). In element908, video encoder20and/or video decoder30may then apply the adaptive loop filter (i.e., the ALF parameters determined in block906) to the reconstructed block. In element910, video encoder20and/or video decoder30may then proceed to code the next block of video data. For example, the next block of video data may be a second block of video data. The process ofFIG.9is then repeated.

FIG.10is a flowchart illustrating another example method of the disclosure.FIG.10shows an example of the techniques ofFIG.8in more detail. For example, video encoder20and/or video decoder30may, in element1000, obtain a block of video data.

In element1004, video encoder20and/or video decoder30may obtain an adaptive parameter set (APS) and, in element1006, determine a set of adaptive loop filter parameters for multiple filters for the block of video data based on the adaptive parameter set. Multiple adaptive loop parameters of the set of adaptive loop filter parameters may be signaled using the same signaling parameter for each of the multiple of filters of the adaptive parameter set.

In element1008, the video encoder20and/or video decoder30may code the block of video data using the set of adaptive loop filter parameters.

The previous description of the disclosed examples is provided to enable a person skilled in the art to make or use the disclosed examples. Various modifications to these examples will readily apparent to those skilled in the art, and the principles defined herein may be applied to other examples without departing from the scope of the disclosure. Thus, the present disclosure is not intended to be limited to the examples shown herein but is to be accorded the widest scope possible consistent with the principles and novel features as defined by the following claims.