Video coding using adaptive resolution

Abstract

Reconstructing a frame or picture using adaptive resolution is described. Coding units of the picture coded at a reduced resolution are reconstructed. The picture comprising the reconstructed coding units is stored in a picture buffer at the reduced resolution. The picture is resampled to a full resolution, and a filter with block-level signaled parameters is applied to the picture at the full resolution.

Description

VIDEO CODING USING ADAPTIVE RESOLUTIONCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to and the benefit of U.S. Provisional Patent Application Serial No. 63 / 601,680, filed November 21, 2023, the entire disclosure of which is incorporated herein by reference.BACKGROUND

[0002] Digital video streams may represent video using a sequence of frames or still images. Digital video can be used for various applications including, for example, video conferencing, high-definition video entertainment, video advertisements, or sharing of usergenerated videos. A digital video stream can contain a large amount of data and consume a significant amount of computing or communication resources of a computing device for processing, transmission, or storage of the video data. Various approaches have been proposed to reduce the amount of data in video streams, including encoding or decoding techniques.SUMMARY

[0003] A first aspect of the teachings herein is a method for reconstructing a picture. Coding units of the picture coded at a reduced resolution are reconstructed. The picture comprising the reconstructed coding units is stored in a picture buffer at the reduced resolution. The picture is resampled to a full resolution, and a filter with block-level parameters is applied to the picture at the full resolution.

[0004] In some implementations, applying the filter includes sequentially applying more than one filter with respective block-level parameters to the picture at the full resolution.

[0005] In some implementations, the filter with block-level parameters includes a sample adaptive offset (SAG) filter.

[0006] In some implementations, the filter with block-level parameters includes an adaptive loop filter (ALF).

[0007] In some implementations, the filter with block-level parameters is used to filter the picture at the reduced resolution and to filter the picture at the full resolution.

[0008] In some implementations, a maximum number of classes of the ALF depends on a ratio of the reduced resolution compared to the full resolution.

[0009] In some implementations, the first aspect includes applying a filter that has no parameters signaled at a block level to the picture at the reduced resolution before storing the picture.

[0010] In some implementations, the filter that has no parameters signaled at the block level includes a deblocking filter.

[0011] In some implementations, the block-level parameters are derived and signaled at the full resolution.

[0012] In some implementations, a block size of filter local adaptation at the reduced resolution and at the full resolution are a same block size.

[0013] In some implementations, a block size of filter local adaptation at the reduced resolution and at the full resolution are a different block size and a number of filter blocks at the reduced resolution is equal to a number of filter blocks at the full resolution.

[0014] In some implementations, the first aspect includes outputting the filtered picture at the full resolution.

[0015] In some implementations, the first aspect includes using the picture stored in the picture buffer for inter prediction of a second picture coded after the picture.

[0016] A second aspect of the teachings herein is an apparatus comprising a processor configured to perform the method of any of the implementations described herein.

[0017] A third aspect of the teachings herein is a non-transitory, computer-readable medium storing a compressed bitstream comprising instructions to perform the method of any of the implementations described herein.

[0018] A fourth aspect of the teachings herein is a non-transitory computer-readable medium having stored thereon an encoded or compressed bitstream. The encoded bitstream can include frame residual data coded at a reduced resolution as compared to a higher, full resolution and block-level parameters for filtering the frame after reconstruction at a full resolution. The block-level parameters are derived at the full resolution.

[0019] In some implementations, the encoded bitstream includes frame-level parameters for filtering the frame after reconstruction at the reduced resolution.

[0020] These and other aspects of the present disclosure are disclosed in the following detailed description of the embodiments, the appended claims, and the accompanying figures.BRIEF DESCRIPTION OF THE DRAWINGS

[0021] The description herein refers to the accompanying drawings described below, wherein like reference numerals refer to like parts throughout the several views.

[0022] FIG. 1 is a schematic of a video encoding and decoding system.

[0023] FIG. 2 is a block diagram of an example of a computing device that can implement a transmitting station or a receiving station.

[0024] FIG. 3 is a diagram of a typical video stream to be encoded and subsequently decoded.

[0025] FIG. 4 is a block diagram of an encoder according to implementations of this disclosure.

[0026] FIG. 5 is a block diagram of a decoder according to implementations of this disclosure.

[0027] FIG. 6 is a block diagram of a representation of a portion of a frame.

[0028] FIG. 7 is a flowchart of a technique for reconstructing a frame or picture using adaptive resolution.

[0029] FIG. 8 is a block diagram of a structure that can implement the technique of FIG.7.

[0030] FIG. 9 is a block diagram of another structure that can implement the technique of FIG. 7.DETAIEED DESCRIPTION

[0031] Video compression schemes may include breaking respective images, or frames, of a video stream into smaller portions, such as coding tree blocks (CTBs) or coding tree units (CTUs) (sometimes referred to as superblocks) and generating an encoded bitstream using techniques to limit the information included for respective CTUs thereof. The bitstream can be decoded to re-create the source frames from the limited information. Encoding CTUs to or decoding CTUs from a bitstream can include predicting the values of pixels or CTUs based on similarities with other pixels or CTUs in the same frame or in one or more other frames that have already been coded.

[0032] Those similarities can be determined using intra prediction, which attempts to predict the pixel values of a coding unit (CU) (coding block, etc.) of a CTU using pixels peripheral to the CU (e.g., pixels that are in the same frame as the CU, but that are outside the CU). During encoding, the result of an intra-prediction mode performed against a CU is a prediction unit (PU) (prediction block, etc.). A prediction residual can be determined basedon a difference between the pixel values of the CU and the pixel values of the PU. The prediction residual and the intra-prediction mode used to ultimately obtain that prediction residual can then be encoded to a bitstream. During decoding, the prediction residual is reconstructed into a CU using a PU produced based on the intra-prediction mode and is thereafter included in an output video stream.

[0033] Similarly, inter prediction attempts to predict the pixel values of a CU of a CTU using pixels from one or more reference frames. During encoding, the result of an interprediction mode performed against a CU is also a PU. A prediction residual can be determined based on a difference between the pixel values of the CU and the pixel values of the PU. The prediction residual and the inter-prediction mode used to ultimately obtain that prediction residual can then be encoded to a bitstream. During decoding, the prediction residual is reconstructed into a CU using a PU produced based on the inter-prediction mode and is thereafter included in an output video stream.

[0034] A frame, and hence its CTUs and CUs may include a luminance, also referred to as luma, component and two chrominance, also referred to as chroma, components. These luma and chroma components may in some case be referred to as a luma block and chroma blocks. The luma component may, for example, be expressed within a Y plane and the chroma components may be expressed either within U and V planes or Cr and Cb planes. The luma component is understood to include some number of luma samples and each chroma component is understood to include some number of chroma samples. Generally, the luma samples provide measures of brightness throughout a frame and thus represent the structural qualities of the video content of the frame, whereas the chroma samples provide measures of color throughout the frame. The number of luma samples can indicate the spatial resolution (or simply the resolution) of the frame.

[0035] Conventionally, the spatial resolution of a video bitstream can change by encoding an intra-coded frame into the output video stream, where an intra-coded frame is a frame with CUs encoded only using intra prediction. It is desirable, however, to allow spatial resolution to change at inter-coded pictures. One way this may be achieved is by using reference picture resampling. Specifically, when such a resolution change occurs, the decoding process of a picture (used interchangeably with a frame herein) may refer to one or more previous reference pictures that have a different spatial resolution for inter prediction, and consequently a resampling of the reference pictures for operation of the inter-prediction process may be applied.

[0036] Reference frame resampling may either down-sample or up-sample a reference picture to predict a current picture having a different resolution. To minimize calculations, the resampling occurs as part of the motion compensation process and is performed at the block (e.g., CTU) level. Namely, a scaling ratio is used together with motion information (e.g., reference frame and motion vector) to locate the reference samples in the reference picture to be used in the interpolation process. Different interpolation filters with different frequency responses may be used with selection of the interpolation filter, for example, depending on the scaling ratio.

[0037] Generally, reference picture resampling provides new functionality without increasing the signaling required to represent the current frame. In fact, compared to forcing the insertion of an intra-coded picture for switching resolution, allowing inter prediction from reference pictures of different resolutions can improve coding efficiency and mitigate bit rate spikes that may result from the insertion of the inter-coded picture. Reference picture resampling can also improve coding efficiency. For example, reference picture resampling can be used to adaptively adjust picture resolution at the group of pictures (GOP) level, where a GOP is a sequence of pictures (such as 8 frames) that starts with an intra-coded picture. Using reference picture resampling, the whole GOP may be coded at a reduced resolution so that higher coding efficiency may be achieved after all reduced-resolution pictures are up- sampled to the original resolution. These techniques may be more generally referred to as adaptive resolution.

[0038] Implementations of this disclosure describe a new coding mode when adaptive resolution is used. With this mode, decoders output (such as to display or other applications) the reconstructed pictures at the full resolution even for pictures coded with reduced resolutions while putting the reconstructed pictures at actual coded resolution into the reference picture buffer. Further details of techniques for video coding using adaptive resolution are described herein with initial reference to a system in which they can be implemented.

[0039] FIG. 1 is a schematic of a video encoding and decoding system 100. A transmitting station 102 can be, for example, a computer having an internal configuration of hardware such as that described in FIG. 2. However, other implementations of the transmitting station 102 are possible. For example, the processing of the transmitting station 102 can be distributed among multiple devices.

[0040] A network 104 can connect the transmitting station 102 and a receiving station 106 for encoding and decoding of the video stream. Specifically, the video stream can beencoded in the transmitting station 102, and the encoded video stream can be decoded in the receiving station 106. The network 104 can be, for example, the Internet. The network 104 can also be a local area network (LAN), wide area network (WAN), virtual private network (VPN), cellular telephone network, or any other means of transferring the video stream from the transmitting station 102 to, in this example, the receiving station 106.

[0041] The receiving station 106, in one example, can be a computer having an internal configuration of hardware such as that described in FIG. 2. However, other suitable implementations of the receiving station 106 are possible. For example, the processing of the receiving station 106 can be distributed among multiple devices.

[0042] Other implementations of the video encoding and decoding system 100 are possible. For example, an implementation can omit the network 104. In another implementation, a video stream can be encoded and then stored for transmission at a later time to the receiving station 106 or any other device having memory. In one implementation, the receiving station 106 receives (e.g., via the network 104, a computer bus, and / or some communication pathway) the encoded video stream and stores the video stream for later decoding. In an example implementation, a real-time transport protocol (RTP) is used for transmission of the encoded video over the network 104. In another implementation, a transport protocol other than RTP may be used, e.g., a video streaming protocol based on the Hypertext Transfer Protocol (HTTP).

[0043] When used in a video conferencing system, for example, the transmitting station 102 and / or the receiving station 106 may include the ability to both encode and decode a video stream as described below. For example, the receiving station 106 could be a video conference participant who receives an encoded video bitstream from a video conference server (e.g., the transmitting station 102) to decode and view and further encodes and transmits his or her own video bitstream to the video conference server for decoding and viewing by other participants.

[0044] FIG. 2 is a block diagram of an example of a computing device 200 that can implement a transmitting station or a receiving station. For example, the computing device 200 can implement one or both of the transmitting station 102 and the receiving station 106 of FIG. 1. The computing device 200 can be in the form of a computing system including multiple computing devices, or in the form of one computing device, for example, a mobile phone, a tablet computer, a laptop computer, a notebook computer, a desktop computer, and the like.

[0045] A processor 202 in the computing device 200 can be a conventional central processing unit. Alternatively, the processor 202 can be another type of device, or multiple devices, capable of manipulating or processing information now existing or hereafter developed. For example, although the disclosed implementations can be practiced with one processor as shown (e.g., the processor 202), advantages in speed and efficiency can be achieved by using more than one processor.

[0046] A memory 204 in computing device 200 can be a read-only memory (ROM) device or a random-access memory (RAM) device in an implementation. However, other suitable types of storage device can be used as the memory 204. The memory 204 can include code and data 206 that is accessed by the processor 202 using a bus 212. The memory 204 can further include an operating system 208 and application programs 210, the application programs 210 including at least one program that permits the processor 202 to perform the techniques described herein. For example, the application programs 210 can include applications 1 through N, which further include a video coding application that performs the techniques described herein. The computing device 200 can also include a secondary storage 214, which can, for example, be a memory card used with a mobile computing device.Because the video communication sessions may contain a significant amount of information, they can be stored in whole or in part in the secondary storage 214 and loaded into the memory 204 as needed for processing.

[0047] The computing device 200 can also include one or more output devices, such as a display 218. The display 218 may be, in one example, a touch sensitive display that combines a display with a touch sensitive element that is operable to sense touch inputs. The display 218 can be coupled to the processor 202 via the bus 212. Other output devices that permit a user to program or otherwise use the computing device 200 can be provided in addition to or as an alternative to the display 218. When the output device is or includes a display, the display can be implemented in various ways, including by a liquid crystal display (LCD), a cathode-ray tube (CRT) display, or a light emitting diode (LED) display, such as an organic LED (OLED) display.

[0048] The computing device 200 can also include or be in communication with an image-sensing device 220, for example, a camera, or any other image-sensing device 220 now existing or hereafter developed that can sense an image such as the image of a user operating the computing device 200. The image-sensing device 220 can be positioned such that it is directed toward the user operating the computing device 200. In an example, the position and optical axis of the image-sensing device 220 can be configured such that thefield of vision includes an area that is directly adjacent to the display 218 and from which the display 218 is visible.

[0049] The computing device 200 can also include or be in communication with a soundsensing device 222, for example, a microphone, or any other sound-sensing device now existing or hereafter developed that can sense sounds near the computing device 200. The sound-sensing device 222 can be positioned such that it is directed toward the user operating the computing device 200 and can be configured to receive sounds, for example, speech or other utterances, made by the user while the user operates the computing device 200.

[0050] Although FIG. 2 depicts the processor 202 and the memory 204 of the computing device 200 as being integrated into one unit, other configurations can be utilized. The operations of the processor 202 can be distributed across multiple machines (wherein individual machines can have one or more processors) that can be coupled directly or across a local area or other network. The memory 204 can be distributed across multiple machines such as a network-based memory or memory in multiple machines performing the operations of the computing device 200. Although depicted here as one bus, the bus 212 of the computing device 200 can be composed of multiple buses. Further, the secondary storage 214 can be directly coupled to the other components of the computing device 200 or can be accessed via a network and can comprise an integrated unit such as a memory card or multiple units such as multiple memory cards. The computing device 200 can thus be implemented in a wide variety of configurations.

[0051] FIG. 3 is a diagram of an example of a video stream 300 to be encoded and subsequently decoded. The video stream 300 includes a video sequence 302. At the next level, the video sequence 302 includes a number of adjacent frames 304. While three frames are depicted as the adjacent frames 304, the video sequence 302 can include any number of adjacent frames 304. The adjacent frames 304 can then be further subdivided into individual frames, for example, a frame 306. At the next level, the frame 306 can be divided into a series of planes or segments 308. The segments 308 can be subsets of frames that permit parallel processing, for example. The segments 308 can also be subsets of frames that can separate the video data into separate colors. For example, a frame 306 of color video data can include a luminance plane and two chrominance planes. The segments 308 may be sampled at different resolutions.

[0052] Whether or not the frame 306 is divided into segments 308, the frame 306 may be further subdivided into blocks 310, which can contain data corresponding to, for example, 16x16 pixels in the frame 306. The blocks 310 can also be arranged to include data from oneor more segments 308 of pixel data. The blocks 310 can also be of any other suitable size such as 4x4 pixels, 8x8 pixels, 16x8 pixels, 8x16 pixels, 16x16 pixels, or larger. Unless otherwise noted, the terms block and macroblock are used interchangeably herein.

[0053] FIG. 4 is a block diagram of an encoder 400 according to implementations of this disclosure. The encoder 400 can be implemented, as described above, in the transmitting station 102, such as by providing a computer software program stored in memory, for example, the memory 204. The computer software program can include machine instructions that, when executed by a processor such as the processor 202, cause the transmitting station 102 to encode video data in the manner described in FIG. 4. The encoder 400 can also be implemented as specialized hardware included in, for example, the transmitting station 102. In one particularly desirable implementation, the encoder 400 is a hardware encoder.

[0054] The encoder 400 has the following stages to perform the various functions in a forward path (shown by the solid connection lines) to produce an encoded or compressed bitstream 420 using the video stream 300 as input: an intra / inter prediction stage 402, a transform stage 404, a quantization stage 406, and an entropy encoding stage 408. The encoder 400 may also include a reconstruction path (shown by the dotted connection lines) to reconstruct a frame for encoding of future blocks. In FIG. 4, the encoder 400 has the following stages to perform the various functions in the reconstruction path: a dequantization stage 410, an inverse transform stage 412, a reconstruction stage 414, and a loop filtering stage 416. Other structural variations of the encoder 400 can be used to encode the video stream 300.

[0055] When the video stream 300 is presented for encoding, respective adjacent frames 304, such as the frame 306, can be processed in units of blocks. At the intra / inter prediction stage 402, respective blocks can be encoded using intra-frame prediction (also called intraprediction) or inter- frame prediction (also called inter-prediction). In any case, a prediction block can be formed. In the case of intra-prediction, a prediction block may be formed from samples in the current frame that have been previously encoded and reconstructed. In the case of inter-prediction, a prediction block may be formed from samples in one or more previously constructed reference frames.

[0056] Next, the prediction block can be subtracted from the current block at the intra / inter prediction stage 402 to produce a residual block (also called a residual). The transform stage 404 transforms the residual into transform coefficients in, for example, the frequency domain using block-based transforms. The quantization stage 406 converts the transform coefficients into discrete quantum values, which are referred to as quantizedtransform coefficients, using a quantizer value or a quantization level. For example, the transform coefficients may be divided by the quantizer value and truncated.

[0057] The quantized transform coefficients are then entropy encoded by the entropy encoding stage 408. The entropy-encoded coefficients, together with other information used to decode the block (which may include, for example, syntax elements such as used to indicate the type of prediction used, transform type, motion vectors, a quantizer value, or the like), are then output to the compressed bitstream 420. The compressed bitstream 420 can be formatted using various techniques, such as variable length coding (VLC) or arithmetic coding. The compressed bitstream 420 can also be referred to as an encoded video stream or encoded video bitstream, and the terms will be used interchangeably herein.

[0058] The reconstruction path (shown by the dotted connection lines) can be used to ensure that the encoder 400 and a decoder 500 (described below with respect to FIG. 5) use the same reference frames to decode the compressed bitstream 420. The reconstruction path performs functions that are similar to functions that take place during the decoding process (described below with respect to FIG. 5), including dequantizing the quantized transform coefficients at the dequantization stage 410 and inverse transforming the dequantized transform coefficients at the inverse transform stage 412 to produce a derivative residual block (also called a derivative residual). At the reconstruction stage 414, the prediction block that was predicted at the intra / inter prediction stage 402 can be added to the derivative residual to create a reconstructed block. The loop filtering stage 416 can be applied to the reconstructed block to reduce distortion such as blocking artifacts.

[0059] Other variations of the encoder 400 can be used to encode the compressed bitstream 420. In some implementations, a non-transform based encoder can quantize the residual signal directly without the transform stage 404 for certain blocks or frames. In some implementations, an encoder can have the quantization stage 406 and the dequantization stage 410 combined in a common stage.

[0060] FIG. 5 is a block diagram of a decoder 500 according to implementations of this disclosure. The decoder 500 can be implemented in the receiving station 106, for example, by providing a computer software program stored in the memory 204. The computer software program can include machine instructions that, when executed by a processor such as the processor 202, cause the receiving station 106 to decode video data in the manner described in FIG. 5. The decoder 500 can also be implemented in hardware included in, for example, the transmitting station 102 or the receiving station 106.

[0061] The decoder 500, similar to the reconstruction path of the encoder 400 discussed above, includes in one example the following stages to perform various functions to produce an output video stream 516 from the compressed bitstream 420: an entropy decoding stage 502, a dequantization stage 504, an inverse transform stage 506, an intra / inter prediction stage 508, a reconstruction stage 510, a loop filtering stage 512, and a post filtering stage 514. Other structural variations of the decoder 500 can be used to decode the compressed bitstream 420.

[0062] When the compressed bitstream 420 is presented for decoding, the data elements within the compressed bitstream 420 can be decoded by the entropy decoding stage 502 to produce a set of quantized transform coefficients. The dequantization stage 504 dequantizes the quantized transform coefficients (e.g., by multiplying the quantized transform coefficients by the quantizer value), and the inverse transform stage 506 inverse transforms the dequantized transform coefficients to produce a derivative residual that can be identical to that created by the inverse transform stage 412 in the encoder 400. Using header information decoded from the compressed bitstream 420, the decoder 500 can use the intra / inter prediction stage 508 to create the same prediction block as was created in the encoder 400 (e.g., at the intra / inter prediction stage 402).

[0063] At the reconstruction stage 510, the prediction block can be added to the derivative residual to create a reconstructed block. The loop filtering stage 512 can be applied to the reconstructed block to reduce blocking artifacts. Other filtering can be applied to the reconstructed block. In this example, the post filtering stage 514 is applied to the reconstructed block to reduce blocking distortion, and the result is output as the output video stream 516. The output video stream 516 can also be referred to as a decoded video stream, and the terms will be used interchangeably herein. Other variations of the decoder 500 can be used to decode the compressed bitstream 420. In some implementations, the decoder 500 can produce the output video stream 516 without the post filtering stage 514 or otherwise omit the post filtering stage 514.

[0064] FIG. 6 is a block diagram of a representation of a portion 600 of a frame, such as the frame 306 of FIG. 3, according to implementations of this disclosure. As shown, the portion 600 of the frame includes four 64x64 blocks 610, which may be referred to as superblocks, in two rows and two columns in a matrix or Cartesian plane. A superblock can have a larger or a smaller size. While FIG. 6 is explained with respect to a superblock of size 64x64, the description is easily extendable to larger (e.g., 128x128) or smaller superblock sizes.

[0065] In an example, and without loss of generality, a superblock can be a basic or maximum coding unit, such as a CTU. Each superblock can be partitioned into four 32x32 blocks 620. Each 32x32 block 620 can include four 16x16 blocks 630. Each 16x16 block 630 can include four 8x8 blocks 640. Each 8x8 block 640 can include four 4x4 blocks 650. Each 4x4 block 650 can include 16 pixels, which can be represented in four rows and four columns in each respective block in the Cartesian plane or matrix. The pixels can include information representing an image captured in the frame, such as luminance information, color information, and location information. In an example, a block, such as a 16xl6-pixel block as shown, can include a luminance block 660, which can include luminance pixels 662; and two chrominance blocks 670 / 680, such as a U or Cb chrominance block 670, and a V or Cr chrominance block 680. The chrominance blocks 670 / 680 can include chrominance pixels 690. For example, the luminance block 660 can include 16x16 luminance pixels 662, and each chrominance block 670 / 680 can include 8x8 chrominance pixels 690, as shown.Although one arrangement of blocks is shown, any arrangement can be used. Although FIG. 6 shows NxN blocks, in some implementations, NxM, where N^M, blocks can be used. For example, 32x64 blocks, 64x32 blocks, 16x32 blocks, 32x16 blocks, or any other size blocks can be used. In some implementations, Nx2N blocks, 2NxN blocks, or a combination thereof can be used.

[0066] In some implementations, video coding can include ordered block-level coding. Ordered block-level coding can include coding blocks of a frame in an order, such as rasterscan order, wherein blocks can be identified and processed starting with a block in the upper left comer of the frame, or a portion of the frame, and proceeding along rows from left to right and from the top row to the bottom row, identifying each block in turn for processing. For example, the superblock in the top row and left column of a frame can be the first block coded, and the superblock immediately to the right of the first block can be the second block coded. The second row from the top can be the second row coded, such that the superblock in the left column of the second row can be coded after the superblock in the rightmost column of the first row.

[0067] In an example, coding a block can include using quad-tree coding, which can include coding smaller block units with a block in raster-scan order. The 64x64 superblock shown in the bottom-left corner of the portion of the frame shown in FIG. 6, for example, can be coded using quad-tree coding in which the top-left 32x32 block can be coded, then the top-right 32x32 block can be coded, then the bottom-left 32x32 block can be coded, and then the bottom-right 32x32 block can be coded. Each 32x32 block can be coded using quad-treecoding in which the top-left 16x16 block can be coded, then the top-right 16x16 block can be coded, then the bottom-left 16x16 block can be coded, and then the bottom-right 16x16 block can be coded. Each 16x16 block can be coded using quad-tree coding in which the top-left 8x8 block can be coded, then the top-right 8x8 block can be coded, then the bottom-left 8x8 block can be coded, and then the bottom-right 8x8 block can be coded. Each 8x8 block can be coded using quad-tree coding in which the top-left 4x4 block can be coded, then the topright 4x4 block can be coded, then the bottom-left 4x4 block can be coded, and then the bottom-right 4x4 block can be coded. In some implementations, 8x8 blocks can be omitted for a 16x16 block, and the 16x16 block can be coded using quad-tree coding in which the top-left 4x4 block can be coded, and then the other 4x4 blocks in the 16x16 block can be coded in raster-scan order.

[0068] In an example, video coding can include compressing the information included in an original, or input, frame by omitting some of the information in the original frame from a corresponding encoded frame. For example, coding can include reducing spectral redundancy, reducing spatial redundancy, reducing temporal redundancy, or a combination thereof.

[0069] In an example, reducing spectral redundancy can include using a color model based on a luminance component (Y) and two chrominance components (U and V or Cb and Cr), which can be referred to as the YUV or YCbCr color model or color space. Using the YUV color model can include using a relatively large amount of information to represent the luminance component of a portion of a frame and using a relatively small amount of information to represent each corresponding chrominance component for the portion of the frame. For example, a portion of a frame can be represented by a high-resolution luminance component, which can include a 16x16 block of pixels, and by two lower resolution chrominance components, each of which representing the portion of the frame as an 8x8 block of pixels. A pixel can indicate a value (e.g., a value in the range from 0 to 255) and can be stored or transmitted using, for example, eight bits. Although this disclosure is described with reference to the YUV color model, any color model can be used.

[0070] Reducing spatial redundancy can include transforming a block into the frequency domain as described above. For example, a unit of an encoder, such as the entropy encoding stage 408 of FIG. 4, can perform a discrete cosine transform (DCT) using transform coefficient values based on spatial frequency.

[0071] Reducing temporal redundancy can include using similarities between frames to encode a frame using a relatively small amount of data based on one or more referenceframes, which can be previously encoded, decoded, and reconstructed frames of the video stream. For example, a block or a pixel of a current frame can be similar to a spatially corresponding block or pixel of a reference frame. A block or a pixel of a current frame can be similar to a block or a pixel of a reference frame at a different spatial location. As such, reducing temporal redundancy can include generating motion information indicating the spatial difference (e.g., a translation between the location of the block or the pixel in the current frame and the corresponding location of the block or the pixel in the reference frame).

[0072] Reducing temporal redundancy can include identifying a block or a pixel in a reference frame, or a portion of the reference frame, that corresponds with a current block or pixel of a current frame. For example, a reference frame, or a portion of a reference frame, which can be stored in memory, can be searched for the best block or pixel to use for encoding a current block or pixel of the current frame. For example, the search may identify the block of the reference frame for which the difference in pixel values between the reference block and the current block is minimized, and can be referred to as motion searching. The portion of the reference frame searched can be limited. For example, the portion of the reference frame searched, which can be referred to as the search area, can include a limited number of rows of the reference frame. In an example, identifying the reference block can include calculating a cost function, such as a sum of absolute differences (SAD), between the pixels of the blocks in the search area and the pixels of the current block.

[0073] As mentioned initially, adaptive resolution may be used when the resolution of a frame is different from the previous frame. In general, when the reference frame is stored at a different resolution from the current frame, a portion of the frame used for inter prediction of a current block (e.g., as indicated by the motion vector) is resampled to the resolution of the current frame. After reconstruction of the frame by the sequential reconstruction of its blocks, the reconstructed frame may be filtered. Some implementations of these filtering techniques are described with reference to FIGS. 4 through 6. An encoder and a decoder may include inloop filtering in their reconstruction process. In the encoder 400 of FIG. 4, the in-loop filtering is represented by the loop filtering stage 416. In the decoder 500 of FIG. 5, the inloop filtering is represented by the loop filtering stage 512. In-loop filtering may include filtering techniques such as deblocking, sample adaptive offset (SAG), adaptive loop filter (ALF), or some combination thereof. Broadly, these filtering techniques are used to reduce distortion introduced by encoding.

[0074] More specifically, a deblocking filter is designed to smooth sharp edges between blocks (CTUs and / or CUs) of the frame. This is sometimes referred to as removing blockingartifacts. The deblocking filter is generally applied to the entire reconstructed picture. For example, the rules, parameters, etc., for deciding whether to modify a pixel value and how to modify a pixel value at an edge are established on a frame-level basis. For example, one of multiple (e.g., three) filter strengths may be signaled. The deblocking filter may be applied to an 8x8 sample grid, a 4x4 sample grid, or some other size grid. The deblocking filter may first apply horizontal filtering for vertical edges of the picture and thereafter apply vertical filtering for horizontal edges of the picture, or vice versa. In some implementations of a codec (i.e., an encoder and decoder combination), the deblocking filter is applied before other in-loop filtering, if any, but this is not required.

[0075] SAO filtering also reduces distortion by compensating the pixel value offset between reconstructed pixels and original pixels. In general, SAO filtering classifies reconstructed pixels and adds a respective offset to each class or group of pixels. SAO may use different offsets pixel sample by pixel sample in a region depending on the sample classification, and SAO parameters may vary from region to region. The offsets may be determined at a decoder using, e.g., a look up table that is based on a histogram analysis made by the encoder. The region size may be fixed to one coding tree block (CTB). That is, the SAO parameters may be signaled at CTB level.

[0076] More than one SAO type may be used. For example, an edge offset (EO) type and a band offset (BO) type may be used. The EO type may have multiple sub-types corresponding to processing along different directions. For example, the EO type may have four sub-types corresponding to processing along the horizontal, vertical, 135-degree, and 45- degree directions. For an EO sub-type, the value of a pixel (also called a sample) is compared to two of its neighbors using one of four different gradient patterns to classify the pixel. An offset is applied to pixels in each of the four gradient patterns. No offset may be applied for pixel values that do not match one of the gradient patterns. The BO type may be based where the sample values fall within multiple bands (such as 32 bands for values 0-255 in 8-bit coding). An offset is applied to pixels in at least some of the bands, which offset is determined for respective bands. Each color component of the picture may have its own SAO parameters. The SAO filtering can increase edge sharpness and reduce ringing and impulse artifacts.

[0077] Filtering performed by an ALF may be selectively performed before or preferably after SAO filtering. An ALF can minimize the mean squared error between an original picture and the reconstructed picture, such as the picture output from SAO filtering, to improve the quality of the reconstruction. The ALF is generally referred to as adaptivebecause the coefficients may be signaled in the bitstream so they can be designed to reflect the content and distortion of the reconstructed picture. Filtering may be performed generally by partitioning sample locations into classes and applying (e.g., Wiener) filters on a class basis. The filter shapes may differ for luma and chroma components. For example, a 7 x 7 diamond shape may be used for luma components, and a 5 x 5 diamond shape may be used for chroma components.

[0078] With regards to classification within ALF filtering, respective sub-blocks of the luma plane, such a 4 x 4 luma block, can be classified based on its directionality and two- dimensional (2D) Laplacian activity. The 2D Laplacian activity uses calculated gradients in multiple directions for the reconstructed luma samples, such in the horizontal, vertical, 135- degree, and 45-degree directions. Up to 25 classes may be used. For each used class, a filter is signaled from encoder to decoder.

[0079] In addition to this luma sub-block-level filter adaptation, ALF may incorporate superblock (CTB or CTU)-level filter adaptation. A luma block, such as the portion 600 of the frame described with regards to FIG. 6, can use a filter set calculated for the current slice or one of the filter sets calculated for a previously coded slice. The luma block can also use one of multiple (e.g., 16) offline trained filter sets. Within each luma CTB, which filter from the chosen filter set should be applied to each 4 x 4 block is determined by the class calculated for that block. With regards to chroma blocks, ALF may only use CTB-level filter adaptation. Each CTB can select one of a number of filters available to it for its chroma components. For example, up to 8 filters can be used for chroma components in a slice such that each CTB can select one of these filters.

[0080] The filtering described above is by example only. More, fewer, or different filters may be used for filtering in an encoder and decoder.

[0081] Referring again to FIGS. 4 and 5, reconstructed frames after the in-loop filtering at the loop filtering stage 416 and the loop filtering stage 512, respectively, may be added to a reference or decoded picture buffer for inter prediction. A reconstructed frame or picture at a reduced resolution may be directly up-sampled to the original (e.g., the highest) resolution for storage in the picture buffer for inter prediction.

[0082] According to some implementations of adaptive resolution, directly up-sampling a picture coded at a reduced resolution to the original resolution may not be efficient. In an example, if the current picture to be decoded was encoded at the reduced resolution or another resolution below the highest resolution, the up- sampled frame from the reference buffer will down-sample the reference picture portions to the reduced resolution in themotion compensation process. If the current picture is coded at the same resolution at which the reference picture was coded, up- sampling and then down- sampling the reference frame are not needed. Further, storing an up- sampled reconstructed picture within the decoded (or reference) picture buffer for use in inter prediction may reduce coding efficiency (e.g., because the picture at the coded resolution is the best source of predictors).

[0083] The coding mode described herein may be used with adaptive resolution. The coding mode allows the output to the reference picture buffer and the output to the display to be at different resolutions. Moreover, the current frame or picture after resampling that is for display is filtered (e.g., using parameters signaled at the block-level), while the reconstructed picture at the coding resolution may be output to the reference buffer (with or without blocklevel filtering). In this way, the picture at the coded resolution is available for predictors without degrading the output picture.

[0084] FIG. 7 is a flowchart of a technique 700 for reconstructing a frame using adaptive resolution. The technique 700 can be implemented, for example, as a software program that may be executed by computing devices such as transmitting station 102 or receiving station 106. The software program can include machine-readable instructions that may be stored in a memory such as the memory 204 or the secondary storage 214, and that, when executed by a processor, such as processor 202, may cause the computing device to perform the technique 700. The technique 700 may be implemented at least in part in the reconstruction stage of an encoder and / or the reconstruction path of a decoder. The technique 700 can be implemented using specialized hardware or firmware. Multiple processors, memories, or both, may be used.

[0085] At operation 702, a frame is reconstructed at a first resolution. The frame is coded using adaptive sampling, which may be expressly signaled in the bitstream or inferred based on information within the bitstream. The first resolution may also be referred to as the coding resolution as the reconstruction is initially performed at the resolution at which the frame was coded. The reconstruction may be performed on coding units (superblocks, etc.) in a coding order, such as raster scan order. For example, an encoder may perform a reconstruction for respective blocks as described with regards to the dequantization stage 410, the inverse transform stage 412, and the reconstruction stage 414, while a decoder may perform a reconstruction as described with regards to the entropy decoding stage 502, the dequantization stage 504, the inverse transform stage 506, and the reconstruction stage 510. At least one block of the current frame is inter predicted using adaptive sampling, such as reference frame resampling. Hence, at least some blocks / coding units / regions of a storedreference frame may be resampled to generate a prediction block for reconstructing a block of the current frame.

[0086] At operation 704, filtering without block-level signaled parameters is applied, if any. For example, filtering having filter parameters (signaled or inferred) for larger coding units than at the block- level is applied. If a codec signals filter parameters at the frame level, for example, frame-level filtering is performed on the reconstructed frame. In an example, deblocking filtering as described above may be performed as it does not signal any parameters at the block level. If the codec includes no filtering without block-level signaled parameters, operation 704 may be omitted.

[0087] At operation 706, the reconstructed frame (whether filtered or not) is stored in the (reference, decoded, etc.) picture buffer for use in inter-prediction for frames coded after the current frame.

[0088] At operation 708, the reconstructed frame is resampled to a second resolution. For example, the reconstructed frame may be below the full resolution, and resampling at operation 708 includes up-sampling to the full resolution. If the reconstructed frame is already at full resolution, operation 708 may be omitted.

[0089] At operation 710, filtering with block-level signaled parameters is applied to the reconstructed frame at the second resolution. As mentioned above, for example, both SAO filtering and ALF filtering have block-level (e.g., CTB-level) signaled parameters.Accordingly, at operation 710, the block- level signaled parameters are determined and used in filtering the picture at the second, e.g., full, resolution. The resulting picture may be output for display.

[0090] Examples of different structures to implement the method or technique 700 according to FIG. 7 are next shown with reference to FIGS. 8 and 9. FIG. 8 is a block diagram of a structure 800 that can implement the technique 700 of FIG. 7. FIG. 9 is a block diagram of another structure 900 that can implement the technique 700 of FIG. 7. The processing of FIG. 7 can occur at both an encoder and a decoder, so each of FIGS. 8 and 9 represents a portion of either an encoder or decoder. The structures 800, 900 show specific examples of filters that can be used, but these examples are non-limiting.

[0091] The structure 800 includes a reconstruction stage 802 that can correspond to the reconstruction stage 414 of the encoder 400 or the reconstruction stage 510 of the decoder 500. This explanation of operation uses an example where the current frame to be reconstructed was (en)coded at a first resolution that is lower than full resolution. As codingunits are reconstructed as described above with regards to FIG. 4 or FIG. 5 and operation 702, they are combined to reconstruct the frame at the reduced resolution.

[0092] In this example, the optional in-loop filtering described at operation 702 is performed at a deblocking filter stage 804. The deblocking filter stage 804 may correspond to a loop filtering stage of an encoder or decoder, such as the loop filtering stage 416 of FIG. 4 or the loop filtering stage 512 of FIG. 5. Parameters used for the deblocking filtering at the deblocking filter stage 804 are not determined (and optionally) signaled at the block level. For example, parameters used for the deblocking filter at the deblocking filter stage 804 may be determined, signaled, and used at the frame level. The in-loop filtering can use a different filter or additional filters where block-level parameters are not determined and used.

[0093] Once the in-loop filtering is complete, the filtered picture is stored in the picture buffer 806 as described with regards to operation 706. The filtered picture is a reconstructed picture at the first, here reduced, resolution that can be used for inter prediction at an intra / inter prediction stage 808. In some implementations, the intra / inter prediction stage 808 can correspond to the intra / inter prediction stage 402 of the encoder 400 or the intra / inter prediction stage 508 of the decoder 500.

[0094] The filtered picture output from the deblocking filter stage 804 is resampled at the resampling stage 810 to a second resolution, corresponding to operation 708 of FIG. 7. The second resolution, in this example, is the full resolution. Thereafter, filtering with block-level parameters is applied as described with regards to operation 710. This can also be referred to as filtering with block-level signaled parameters (or signaled block-level parameters) because such parameters are often determined at an encoder and signaled and not inferred at a decoder. As can be seen from this example, the process of up-sampling the reduced- resolution picture to a full-resolution picture is before one or more filters with block-level (parameters are signaled at the block level). In this example, the full-resolution picture is filtered first at a SAG filtering stage 812 and then at an ALF filtering stage 914. The SAO filtering stage 812 may use any SAO filter, such as the SAO filter described above. The ALF filtering stage 814 may use any ALF, such as the ALF described above.

[0095] In some implementations, the parameters of the filters that are used at the blocklevel are derived (and e.g., signaled) at the full resolution even if a picture is coded at a reduced resolution, as in this example. When the numbers of filter blocks are different between the full-resolution frame and the reduced-resolution frame, or the sizes of filter blocks are different between the full-resolution frame and the reduced-resolution frame, filter information (e.g., parameter) signaling at the block level may be partially interleaved withCTB signaling. In this example, before the maximum number of CTB at reduced resolution is reached, block-level filter information is interleaved with CTB signaling. After the last CTB, all the remaining block-level filter information is signaled. Alternatively, all the block-level filter information is signaled at the beginning of a slice, before all CTB information in the slice, or at the end of a slice (after all the CTB information in the slice).

[0096] Where the parameters are derived and signaled at the full resolution, the block size of filter local adaptation may be the same at reduced resolutions and the full resolution so that more block level filter parameters may be signaled. In an example, the full resolution is 1080p (1920x1080), the reduced resolution is 720p (1280x720), and the block size of signaling filter parameters is 128x128 (a luma CTB size). Without the proposed method, parameters of (1280 / 128) x (720 / 128) = 10x6 filter blocks may be signaled for a picture coded at the reduced resolution 720p. With the proposed method, parameters of (1920 / 128) x (1080 / 128) = 15x9 filter blocks may be signaled for a picture whose full resolution is 1080p but is coded at the reduced resolution of 720p.

[0097] Alternatively, the block sizes of filter local adaptation are different at the reduced resolutions and the full resolution, but the number of filter blocks at the reduced resolutions and the full resolution are the same. In other words, at reduced resolution, the size of the filter block is also down-sampled from the full resolution accordingly. Consequentially, the size of the filter block may not be 4x4 grid aligned.

[0098] In some implementations, one or more filters that use block-level parameters may be used as part of both reduced-resolution and full-resolution processing. For example, in an ALF filtering stage, such as the ALF filtering stage 814 of FIG. 8, the maximum number of classes can depend on the ratio between the resolution used for the current picture and full resolution. In another example, the classification process can be performed at reduced resolution, while the filtering process is performed at full resolution.

[0099] In another example of these implementations, more ALF classes are allowed for pictures coded at reduced resolutions than pictures coded at full resolution. As a result, ALF filtering can be used to enhance high-frequency regions after up-sampling better than using the same number of classes for each. The classification process for reduced resolutions and full resolution may be different. For example, more classes that correspond to high frequencies may be used at reduced resolution.

[0100] The output of the ALF filtering stage 814 is an output frame 816 at full resolution.

[0101] The structure 900 of FIG. 9 is a variation of the structure 800 of FIG. 8. The structure 900 differs in that filtering with block-level parameters is added in thereconstruction path of the picture (here the reduced-resolution picture) after optional filtering with other than block-level parameters and before the current reconstructed picture at the coded resolution is added to the picture buffer, such as the picture buffer 806. In the example of FIG. 9, after deblocking the reduced-resolution picture at the deblocking filtering stage 804, the filtered picture is input to an SAG filtering stage 902 and to an ALF filtering stage 904 in sequence for block-level filtering.

[0102] FIG. 9 illustrates one of a number of variations wherein filters such as ALF and SAO, a neural network-based filter, or a fixed filter may be applied at both reduced resolution and full resolution. Each filter can have its own control — that is, an encoder may expressly signal that a filter is used, or the encoder and a decoder may implicitly determine that a filter is used. For example, ALF may be on only at reduced resolution for one picture, SAO is only on at full resolution for another picture, while one neural network-based (e.g., block-based) filter is used at reduced resolution for a picture and the same (or different) neural networkbased filter is used at full resolution for the same picture. Various combinations are possible to improve coding efficiency, image quality, or both.

[0103] The new coding mode described herein can improve the efficiency of adaptive resolution by outputting (such as to display or other applications) reconstructed pictures at full resolution even for pictures coded with reduced resolutions while putting the reconstructed pictures at actual coded resolution into the reference picture buffer.Reconstruction filters may be applied at either the block level or the frame level. If a filter needs to signal and / or use parameters at the block level (e.g., the CTB level), that filter is included after up- sampling as the number of block- level filter parameter sets may be different before and after up-sampling.

[0104] In a previous coding mode, when coding a picture with a reduced resolution, all filters may be applied at the reduced resolution. The new coding mode described herein allows some filters to be applied at a full resolution based on whether the filter needs to signal and / or use parameters at the block level. This may determine at which resolution to derive and signal the block- level filter parameters. Thus, all filters with no block- level signaled parameters may be applied directly at the reduced resolution, and all filters with block-level signaled parameters may be applied at the same resolution at which the parameters are derived.

[0105] For simplicity of explanation, the techniques herein are depicted and described as respective series of steps or operations. However, the steps or operations in accordance with this disclosure can occur in various orders and / or concurrently. Additionally, other steps oroperations not presented and described herein may be used. Furthermore, not all illustrated steps or operations may be required to implement a technique in accordance with the disclosed subject matter.

[0106] The aspects of encoding and decoding described above illustrate some examples of encoding and decoding techniques. However, it is to be understood that encoding and decoding, as those terms are used in the claims, could mean compression, decompression, transformation, or any other processing or change of data.

[0107] The word “example” is used herein to mean serving as an example, instance, or illustration. Any aspect or design described herein as “example” is not necessarily to be construed as being preferred or advantageous over other aspects or designs. Rather, use of the word “example” is intended to present concepts in a concrete fashion. As used in this application, the term “or” is intended to mean an inclusive “or” rather than an exclusive “or.” That is, unless specified otherwise or clearly indicated otherwise by the context, the statement “X includes A or B” is intended to mean any of the natural inclusive permutations thereof. That is, if X includes A; X includes B; or X includes both A and B, then “X includes A or B” is satisfied under any of the foregoing instances. In addition, the articles “a” and “an” as used in this application and the appended claims should generally be construed to mean “one or more,” unless specified otherwise or clearly indicated by the context to be directed to a singular form. Moreover, use of the term “an implementation” or the term “one implementation” throughout this disclosure is not intended to mean the same embodiment or implementation unless described as such.

[0108] Implementations of the transmitting station 102 and / or the receiving station 106 (and the algorithms, methods, instructions, etc., stored thereon and / or executed thereby, including by the encoder 400 and the decoder 500) can be realized in hardware, software, or any combination thereof. The hardware can include, for example, computers, intellectual property (IP) cores, application- specific integrated circuits (ASICs), programmable logic arrays, optical processors, programmable logic controllers, microcode, microcontrollers, servers, microprocessors, digital signal processors, or any other suitable circuit. In the claims, the term “processor” should be understood as encompassing any of the foregoing hardware, either singly or in combination. The terms “signal” and “data” are used interchangeably. Further, portions of the transmitting station 102 and the receiving station 106 do not necessarily have to be implemented in the same manner.

[0109] Further, in one aspect, for example, the transmitting station 102 or the receiving station 106 can be implemented using a general purpose computer or general purposeprocessor with a computer program that, when executed, carries out any of the respective methods, algorithms, and / or instructions described herein. In addition, or alternatively, for example, a special purpose computer / processor can be utilized which can contain other hardware for carrying out any of the methods, algorithms, or instructions described herein.

[0110] The transmitting station 102 and the receiving station 106 can, for example, be implemented on computers in a video conferencing system. Alternatively, the transmitting station 102 can be implemented on a server, and the receiving station 106 can be implemented on a device separate from the server, such as a handheld communications device. In this instance, the transmitting station 102, using an encoder 400, can encode content into an encoded video signal and transmit the encoded video signal to the communications device. In turn, the communications device can then decode the encoded video signal using a decoder 500. Alternatively, the communications device can decode content stored locally on the communications device, for example, content that was not transmitted by the transmitting station 102. Other suitable transmitting and receiving implementation schemes are available. For example, the receiving station 106 can be a generally stationary personal computer rather than a portable communications device, and / or a device including an encoder 400 may also include a decoder 500.

[0111] Further, all or a portion of implementations of the present disclosure can take the form of a computer program product accessible from, for example, a computer-usable or computer-readable medium. A computer-usable or computer-readable medium can be any device that can, for example, tangibly contain, store, communicate, or transport the program for use by or in connection with any processor. The medium can be, for example, an electronic, magnetic, optical, electromagnetic, or semiconductor device. Other suitable mediums are also available.

[0112] The above-described embodiments, implementations, and aspects have been described to facilitate easy understanding of this disclosure and do not limit this disclosure. On the contrary, this disclosure is intended to cover various modifications and equivalent arrangements included within the scope of the appended claims, which scope is to be accorded the broadest interpretation as is permitted under the law to encompass all such modifications and equivalent arrangements.

Claims

What is claimed is:

1. A method for reconstructing a picture, comprising: reconstructing coding units of the picture coded at a reduced resolution; storing the picture comprising the reconstructed coding units in a picture buffer at the reduced resolution; resampling the picture to a full resolution; and applying a filter with block-level signaled parameters to the picture at the full resolution.

2. The method of claim 1, wherein applying the filter comprises sequentially applying more than one filter with respective block-level signaled parameters to the picture at the full resolution.

3. The method of claim 1 or 2, wherein the filter with block-level signaled parameters comprises a sample adaptive offset (SAO) filter.

4. The method of any one of claims 1 to 3, wherein the filter with block-level signaled parameters comprises an adaptive loop filter (ALF).

5. The method of claim 3 or 4, wherein the filter with block-level signaled parameters is used to filter the picture at the reduced resolution and to filter the picture at the full resolution.

6. The method of claim 5, wherein a maximum number of classes of the ALF depends on a ratio of the reduced resolution compared to the full resolution.

7. The method of any one of claims 1 to 6, comprising: applying a filter that has no parameters signaled at a block level to the picture at the reduced resolution before storing the picture.

8. The method of claim 7, wherein the filter that has no parameters signaled at the block level comprises a deblocking filter.

9. The method of any one of claims 1 to 8, wherein the block- level signaled parameters are derived and signaled at the full resolution.

10. The method of claim 9, wherein a block size of filter local adaptation at the reduced resolution and at the full resolution are a same block size.

11. The method of claim 9, wherein a block size of filter local adaptation at the reduced resolution and at the full resolution are a different block size and a number of filter blocks at the reduced resolution is equal to a number of filter blocks at the full resolution.

12. The method of any one of claims 1 to 11, comprising: outputting the filtered picture at the full resolution.

13. The method of any one of claims 1 to 12, comprising: using the picture stored in the picture buffer for inter prediction of a second picture coded after the picture.

14. An apparatus comprising a processor configured to perform the method of any one of claims 1 to 13.

15. A non-transitory, computer-readable medium storing a compressed bitstream comprising instructions to perform the method of any one of claims 1 to 13.

16. A non-transitory computer-readable medium having stored thereon an encoded bitstream, wherein the encoded bitstream includes frame residual data corresponding to a frame coded at a reduced resolution as compared to a higher, full resolution and block-level parameters for filtering the frame after reconstruction at a full resolution, and the block- level parameters are derived at the full resolution.

17. The non-transitory computer-readable medium of claim 16, wherein the encoded bitstream includes frame-level parameters for filtering the frame after reconstruction at the reduced resolution.

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