Transform split restriction and skip texture context derivation optimization for temporally interpolated picture frame prediction

Abstract

Hardware-friendly approaches to generated reference frame prediction include transform split restriction and skip texture context derivation optimization. Transform split restriction generally refers to the restriction of transform sizes available for a current block to a single transform size corresponding to a size of the current block based on the current block having been predicted using a generated reference frame. Skip texture context derivation optimization generally refers to the splitting of contexts for entropy coding distributions of a transform skip symbol for blocks predicted using generated reference frame prediction and those predicted without using same. The transform split restriction and skip texture context derivation optimization approaches disclosed herein enable effective generated reference frame prediction in a hardware coder use case by, in relevant part, preventing the unnecessary consumption of valuable bitstream space as well as the suboptimal adaptation of distribution values for entropy coding.

Description

TRANSFORM SPLIT RESTRICTION AND SKIP TEXTURE CONTEXTDERIVATION OPTIMIZATION FOR TEMPORALLY INTERPOLATED PICTUREFRAME PREDICTIONBACKGROUND

[0001] 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

[0002] Disclosed herein are, inter alia, systems and techniques for transform split restriction and skip texture context derivation optimization for generated reference frame (e.g., temporally interpolated picture (TIP) frame) prediction.

[0003] A method for transform split restriction for generated reference frame prediction according to an implementation of this disclosure comprises: generating a prediction block for a current block of a video frame using a generated reference frame that is generated using forward and backward reference frames of the video frame; determining, for a prediction residual generated using the prediction block, a transform size limited for use with the prediction residual based on the use of the generated reference frame to generate the prediction block; generating transform coefficients based on the prediction residual and the transform size; and encoding the transform coefficients to a bitstream without an indication of the transform size.

[0004] In some implementations of the method, determining the transform size limited for use with the prediction residual based on the use of the generated reference frame to generate the prediction block comprises: limiting a selection of multiple transform sizes available for use with prediction blocks to only the transform size based on the use of the generated reference frame to generate the prediction block.

[0005] In some implementations of the method, determining the transform size limited for use with the prediction residual based on the use of the generated reference frame to generate the prediction block comprises: determining that the current block is predicted using generated reference frame prediction.

[0006] In some implementations of the method, the transform size is identical to a size of the prediction block.

[0007] In some implementations of the method, generating the transform coefficients based on the prediction residual and the transform size comprises: generating the transform coefficients without splitting the prediction residual into multiple smaller blocks.

[0008] In some implementations of the method, encoding the transform coefficients to the bitstream without the indication of the transform size causes a decoder to infer a size of a transform block corresponding to the transform coefficients.

[0009] In some implementations of the method, the generated reference frame is a temporally interpolated picture reference frame.

[0010] A non-transitory computer readable medium according to an implementation of this disclosure has stored thereon an encoded bitstream configured for decoding by operations for transform split restriction for generated reference frame prediction, the operations comprising: determining, for a prediction residual generated using a prediction block for a current block of a video frame, a transform size limited prediction residual based on a use of a generated reference frame to generate the prediction block; generating transform coefficients based on the prediction residual and the transform size; and encoding the transform coefficients to a bitstream without an indication of the transform size.

[0011] In some implementations of the non-transitory computer readable medium, the transform size is identical to a size of the prediction block.

[0012] In some implementations of the non-transitory computer readable medium, determining the transform size limited for use with the prediction residual based on the use of the generated reference frame to generate the prediction block comprises: limiting available transform sizes to a single transform size corresponding to the size of the prediction block.

[0013] In some implementations of the non-transitory computer readable medium, generating the transform coefficients based on the prediction residual and the transform size comprises: generating the transform coefficients without splitting the prediction residual into multiple smaller blocks.

[0014] In some implementations of the non-transitory computer readable medium, the operations comprise: generating the prediction block using the generated reference frame,wherein the generated reference frame is generated using forward and backward reference frames of the video frame.

[0015] In some implementations of the non-transitory computer readable medium, the generated reference frame is a temporally interpolated picture reference frame.

[0016] An apparatus for transform split restriction for generated reference frame prediction according to an implementation of this disclosure comprises: a memory; and a processor configured to execute instructions stored in the memory to: generate transform coefficients based on a prediction residual generated using a prediction block and a transform size determined for the prediction residual, wherein the transform size is limited based on a use of a generated reference frame to generate the prediction block; and encode the transform coefficients to a bitstream without an indication of the transform size.

[0017] In some implementations of the apparatus, the processor is configured to execute the instructions to: determine the transform size for the prediction residual, wherein the transform size is limited for use with the prediction residual based on the use of the generated reference frame to generate the prediction block.

[0018] In some implementations of the apparatus, a selection of multiple transform sizes available for use with prediction blocks is limited to only the transform size based on the use of the generated reference frame to generate the prediction block.

[0019] In some implementations of the apparatus, the processor is configured to execute the instructions to: generate the prediction block for a current block of a video frame using the generated reference frame, wherein the generated reference frame is generated using forward and backward reference frames of the video frame.

[0020] In some implementations of the apparatus, the transform size is identical to a size of the prediction block.

[0021] In some implementations of the apparatus, the transform coefficients are generated without splitting the prediction residual into multiple smaller blocks.

[0022] In some implementations of the apparatus, the encoding of the transform coefficients to the bitstream without the indication of the transform size omits the transform size for the transform coefficients from the bitstream.

[0023] These and other aspects of this disclosure are disclosed in the following detailed description of the implementations, the appended claims and the accompanying figures.BRIEF DESCRIPTION OF THE DRAWINGS

[0024] The description herein makes reference to the accompanying drawings described below, wherein like reference numerals refer to like parts throughout the several views.

[0025] FIG. 1 is a schematic of an example of a video encoding and decoding system.

[0026] FIG. 2 is a block diagram of an example of a computing device that can implement a transmitting station or a receiving station.

[0027] FIG. 3 is a diagram of an example of a video stream to be encoded and decoded.

[0028] FIG. 4 is a block diagram of an example of an encoder.

[0029] FIG. 5 is a block diagram of an example of a decoder.

[0030] FIG. 6 is an illustration of examples of portions of a video frame.

[0031] FIG. 7 is an illustration of frames used in connection with TIP frame prediction.

[0032] FIG. 8 is a flowchart diagram of an example of a technique for transform split restriction for generated reference frame (e.g., TIP frame) prediction.

[0033] FIG. 9 is a flowchart diagram of an example of a technique for skip texture context derivation optimization for generated reference frame (e.g., TIP frame) prediction.DETAIEED DESCRIPTION

[0034] Video compression schemes may include breaking respective images, or frames, of a video stream into smaller portions, such as blocks, and generating an encoded bitstream by using encoding techniques to limit the information included for respective blocks thereof. The bitstream can be decoded to re-create the source frames from the limited information. A video stream can be compressed (i.e., encoded) by a variety of techniques to reduce the bandwidth required to transmit or store the video stream. Similarly, a variety of techniques can be used to decompress (i.e., decode) a compressed video stream from a bitstream, to prepare the video stream for viewing or further processing. Compression of the video stream often exploits spatial and temporal correlation of video signals through spatial and / or motion- compensated prediction. Motion-compensated prediction may also be referred to as interprediction. Inter-prediction uses one or more motion vectors to generate a block (also called a prediction block) that resembles a current block to be encoded using previously encoded and decoded pixels. By encoding the motion vector(s), and the difference between the two blocks (i.e., a residual), a decoder receiving the encoded signal can reconstruct the current block by generating the prediction block and adding pixels of the prediction block to the decoded residual block.

[0035] Each motion vector used to generate a prediction block in the inter-prediction process refers to a reference frame (i.e., a frame other than a current frame which includes the block that is under prediction). Reference frames can be located before or after the current frame in the sequence of the video stream and may be frames that are reconstructed before being used as a reference frame. In particular, a reference frame may be a forward reference frame (i.e., a frame used for forward prediction relative to the sequence) or a backward reference frame (i.e., a frame used for backward prediction relative to the sequence). One or more forward and / or backward reference frames can be used to encode or decode a block. In particular, because many conventional video compression and decompression schemes use a pyramid coding structure to achieve high compression efficiencies, many frames are encoded and decoded using bi-directional prediction, such as using a forward reference frame and a backward reference frame. Bi-directional prediction using forward and backward reference frames has been shown to substantially improve the quality of prediction and thus the overall compression performance for the subject video stream.

[0036] Some recent approaches for bi-directional prediction uses a TIP frame, which is a reference frame generated by interpolating reference blocks from a forward reference frame and a backward reference frame (e.g., as the nearest past and future reference frames relative to the current frame). In particular, the coded motion vectors available in the forward and backward reference frames are used to generate a motion field for the current frame, and the motion field is the used to fetch the reference blocks which are used to generate the TIP frame. TIP frame prediction thus refers to an inter-prediction mode whereby a TIP frame is used to predict the motion of a current frame. The TIP frame is independently generated at each of the encoder and the decoder. In particular, the encoder generates the TIP frame using data determined as part of an encoder search process, and the decoder generates the TIP frame using bitstream data indicative of that encoder search process. The TIP frame can be generated in full before it is used or generated piece by piece in a streaming manner by which individual portions are generated using the motion field. The use of this TIP mode for video coding has shown remarkable coding gain achievements relative to video coding schemes which do not use the TIP mode.

[0037] The use of TIP frame prediction may affect or otherwise have some relationship with other aspects of the video coding pipeline, as well. In one example, a video codec may specify certain transform sizes which are available for a given block to encode based on, for example, the size of the block. In one non-limiting example, the available transform sizes for a block of size NxM, in which N and M are integers of the same or a different value, areNxM, (N / 2)xM, Nx(M / 2), and (N / 2)x(M / 2), in which the block is maintained without a split when the transform size of NxM is used and the block is split into two or four smaller blocks when the other transform sizes are used. The transform size may thus be selected at an encoder and signaled within a bitstream to a decoder using an N-ary (e.g., 4-ary) symbol. However, observations made when experimenting with the use of TIP frame prediction have suggested that the transform size selected for a block predicted using a TIP frame is almost always NxM. Thus, when TIP frame prediction is used, unnecessary data and resources are consumed in regard to the signaling and eventual decoding of the N-ary symbol representing the transform size in the bitstream. Despite this, the N-ary symbol representing the transform size in the bitstream is important for blocks which are not predicted using TIP frame prediction, it being recognized that the N-ary symbol, having a non-negligent probability, uses a same context for entropy coding purposes with all blocks, regardless of whether they are predicted using a TIP frame.

[0038] In another example, a video codec may use one or more entropy coding schemes to improve compression efficiency for determining what data to signal within a bitstream and how to signal the data. For example, a cumulative density function (CDF) may be used to represent a context consisting of a set of probabilities for video data, in which one or more of the probabilities may be updated after each relevant syntax element to encode has been parsed. These updates ensure that the encoder has an accurate understanding of the video data it is encoding, and the resulting probabilities are used to determine how to efficiently express the parsed syntax elements within the bitstream (e.g., using the fewest number of bits). In relevant part, transform size and related information may be entropy coded using a CDF. One such type of syntax that may be entropy coded is an N-ary (e.g., 2-ary) symbol indicating that transform processing has been skipped for a given block. However, observations made when experimenting with the use of TIP frame prediction have suggested that, because a current block refers to a TIP frame and the TIP frame is generally supposed to be as close as possible to a current frame that includes the current block, the distributions in entropy coding symbols (e.g., those of a CDF) for blocks predicted using a TIP frame are often significantly different from the distributions in entropy coding symbols for blocks predicted without the use of a TIP frame. In one non-limiting example, the average motion vector for blocks predicted without use of a TIP frame may have a much larger magnitude than the average motion vector for blocks predicted using TIP frame prediction since TIP frame prediction generally involves the use of relatively small motion vectors. As such, the distributions of symbols for these average motion vectors will be different. Where there is such a difference and therelevant syntax nevertheless all remain entropy coded using the same CDF, it results in a suboptimal CDF adaptation and a corresponding decrease in compression performance.

[0039] Implementations of this disclosure address problems such as these using novel approaches to transform split restriction and skip texture context derivation optimization. In some implementations, the splitting of a current block of a video frame during transform processing is restricted based on the current block having been predicted using a TIP frame. Thus, during encoding, when a determination is made that the current block was predicted using a TIP frame, the transform sizes available for the transform processing of the current block are limited to one transform size corresponding (i.e., identical) to the size of the current block, and no symbol representing the transform size for the current block is signaled within a bitstream to which the current block is encoded. During decoding, because a symbol is not signaled within the bitstream to represent the transform size for the current block, the decoder infers that the current block was not split during transform processing and thus uses a transform size corresponding (i.e., identical) to the size of the current block for inverse transform processing. In some implementations, an additional set of contexts is introduced and used for blocks which are predicted using TIP frame prediction, while blocks which are predicted without the use of TIP frame prediction are instead processed using an original set of contexts. In particular, a new set of contexts is introduced for entropy coding blocks which are predicted using a TIP frame based on an understanding that transform processing has been skipped for those blocks. Splitting the contexts for this N-ary transform skip symbol between TIP-predicted blocks and non-TIP-predicted blocks thus enables material coding gains to be achieved without a corresponding increase in computational cost.

[0040] While reference is made herein by example to superblocks, macroblocks, blocks, and the like, as are commonly used in video codecs such as VP9, AVI, and the currently indevelopment AV2, the implementations of this disclosure may be used with other video coding structures. In one particular but non-limiting example, the implementations of this disclosure may be used with coding tree units (CTUs), coding units (CUs), prediction units (Pus), and the like, as are commonly used in video codecs such as H.265, referred to as High- Efficiency Video Coding, and H.266, referred to as Versatile Video Coding. Accordingly, references herein to particular video coding structures such as superblocks, macroblocks, blocks, and the like shall be regarded as expressions of non-limiting example video coding structures with which the implementations of this disclosure may be used. Moreover, while references are made throughout this disclosure to TIP frames and the prediction thereof, the implementations of this disclosure may alternatively use, operate, or otherwise be performedfor other types of generated reference frames. As such, references to TIP frames throughout this disclosure may be substituted by “generated reference frame” or other types of generated reference frames.

[0041] Further details of techniques for transform split restriction and skip texture context derivation optimization for TIP frame prediction are described herein with initial reference to a system in which such techniques can be implemented. FIG. 1 is a schematic of an example 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.

[0042] 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 be encoded 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.

[0043] 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.

[0044] 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).

[0045] 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 videoconference 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.

[0046] In some implementations, the video encoding and decoding system 100 may instead be used to encode and decode data other than video data. For example, the video encoding and decoding system 100 can be used to process image data. The image data may include a block of data from an image. In such an implementation, the transmitting station 102 may be used to encode the image data and the receiving station 106 may be used to decode the image data.

[0047] Alternatively, the receiving station 106 can represent a computing device that stores the encoded image data for later use, such as after receiving the encoded or preencoded image data from the transmitting station 102. As a further alternative, the transmitting station 102 can represent a computing device that decodes the image data, such as prior to transmitting the decoded image data to the receiving station 106 for display.

[0048] 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.

[0049] 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.

[0050] 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 thetechniques described herein. For example, the application programs 210 can include applications 1 through N, which further include encoding and / or decoding software that performs, amongst other things, transform split restriction and skip texture context derivation optimization for TIP frame prediction as described herein.

[0051] 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.

[0052] 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.

[0053] 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 the field of vision includes an area that is directly adjacent to the display 218 and from which the display 218 is visible.

[0054] 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.

[0055] 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. Theoperations 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.

[0056] 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.

[0057] FIG. 3 is a diagram of an example of a video stream 300 to be encoded and decoded. The video stream 300 includes a video sequence 302. At the next level, the video sequence 302 includes a number of adjacent video 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 video frames, for example, a frame 306.

[0058] 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.

[0059] 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, NxM pixels in the frame 306, in which N and M may refer to the same integer value or to different integer values. The blocks 310 can also be arranged to include data from one or more segments 308 of pixel data. The blocks 310 can be of any suitable size, such as 4x4 pixels, 8x8 pixels, 16x8 pixels, 8x16 pixels, 16x16 pixels, or larger up to a maximum block size, which may be 128x128 pixels or another NxM pixels size.

[0060] FIG. 4 is a block diagram of an example of an encoder 400. 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 mannerdescribed in FIG. 4. The encoder 400 can also be implemented as specialized hardware included in, for example, the transmitting station 102. In some implementations, the encoder 400 is a hardware encoder.

[0061] 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.

[0062] In some cases, the functions performed by the encoder 400 may occur after a filtering of the video stream 300. That is, the video stream 300 may undergo pre-processing according to one or more implementations of this disclosure prior to the encoder 400 receiving the video stream 300. Alternatively, the encoder 400 may itself perform such preprocessing against the video stream 300 prior to proceeding to perform the functions described with respect to FIG. 4, such as prior to the processing of the video stream 300 at the intra / inter prediction stage 402.

[0063] When the video stream 300 is presented for encoding after the pre-processing is performed, 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 intra-prediction) or inter-frame prediction (also called inter-prediction). In any case, a prediction block can be formed. In the case of intraprediction, 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.

[0064] 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 quantized transform coefficients, using a quantizer value or a quantization level. For example, thetransform coefficients may be divided by the quantizer value and truncated.

[0065] 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 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.

[0066] 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).

[0067] 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 apply an in-loop filter or other filter to the reconstructed block to reduce distortion such as blocking artifacts. Examples of filters which may be applied at the loop filtering stage 416 include, without limitation, a deblocking filter, a directional enhancement filter, and a loop restoration filter.

[0068] 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.

[0069] FIG. 5 is a block diagram of an example of a decoder 500. 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 thereceiving station 106. In some implementations, the decoder 500 is a hardware decoder.

[0070] 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 filter stage 514. Other structural variations of the decoder 500 can be used to decode the compressed bitstream 420.

[0071] 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).

[0072] 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. Examples of filters which may be applied at the loop filtering stage 512 include, without limitation, a deblocking filter, a directional enhancement filter, and a loop restoration filter. Other filtering can be applied to the reconstructed block. In this example, the post filter 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.

[0073] 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 filter stage 514 or otherwise omit the post filter stage 514.

[0074] FIG. 6 is an illustration of examples of portions of a video frame 600, which may, for example, be the frame 306 shown in FIG. 3. The video frame 600 includes a number of 64x64 blocks 610, such as four 64x64 blocks 610 in two rows and two columns in a matrix or Cartesian plane, as shown. Each 64x64 block 610 may include up to four 32x32 blocks 620.Each 32x32 block 620 may include up to four 16x16 blocks 630. Each 16x16 block 630 may include up to four 8x8 blocks 640. Each 8x8 block 640 may include up to four 4x4 blocks 950. Each 4x4 block 950 may include 16 pixels, which may be represented in four rows and four columns in each respective block in the Cartesian plane or matrix. In some implementations, the video frame 600 may include blocks larger than 64x64 and / or smaller than 4x4. Subject to features within the video frame 600 and / or other criteria, the video frame 600 may be partitioned into various block arrangements.

[0075] The pixels may include information representing an image captured in the video frame 600, such as luminance information, color information, and location information. In some implementations, a block, such as a 16x16 pixel block as shown, may include a luminance block 660, which may 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 may include chrominance pixels 690. For example, the luminance block 660 may include 16x16 luminance pixels 662 and each chrominance block 670, 680 may include 8x8 chrominance pixels 690 as shown. Although one arrangement of blocks is shown, any arrangement may be used. Although FIG. 6 shows NxN blocks, in some implementations, NxM blocks may be used, wherein N and M are different numbers. For example, 32x64 blocks, 64x32 blocks, 16x32 blocks, 32x16 blocks, or any other size blocks may be used. In some implementations, Nx2N blocks, 2NxN blocks, or a combination thereof, may be used.

[0076] In some implementations, coding the video frame 600 may include ordered blocklevel coding. Ordered block-level coding may include coding blocks of the video frame 600 in an order, such as raster-scan order, wherein blocks may be identified and processed starting with a block in the upper left comer of the video frame 600, or portion of the video frame 600, 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 64x64 block in the top row and left column of the video frame 600 may be the first block coded and the 64x64 block immediately to the right of the first block may be the second block coded. The second row from the top may be the second row coded, such that the 64x64 block in the left column of the second row may be coded after the 64x64 block in the rightmost column of the first row.

[0077] In some implementations, coding a block of the video frame 600 may include using quad-tree coding, which may include coding smaller block units within a block in raster- scan order. For example, the 64x64 block shown in the bottom left corner of the portion of the video frame 600 may be coded using quad-tree coding wherein the top left32x32 block may be coded, then the top right 32x32 block may be coded, then the bottom left 32x32 block may be coded, and then the bottom right 32x32 block may be coded. Each 32x32 block may be coded using quad- tree coding wherein the top left 16x16 block may be coded, then the top right 16x16 block may be coded, then the bottom left 16x16 block may be coded, and then the bottom right 16x16 block may be coded. Each 16x16 block may be coded using quad-tree coding wherein the top left 8x8 block may be coded, then the top right 8x8 block may be coded, then the bottom left 8x8 block may be coded, and then the bottom right 8x8 block may be coded. Each 8x8 block may be coded using quad-tree coding wherein the top left 4x4 block may be coded, then the top right 4x4 block may be coded, then the bottom left 4x4 block may be coded, and then the bottom right 4x4 block may be coded. In some implementations, 8x8 blocks may be omitted for a 16x16 block, and the 16x16 block may be coded using quad-tree coding wherein the top left 4x4 block may be coded, then the other 4x4 blocks in the 16x16 block may be coded in raster-scan order.

[0078] In some implementations, coding the video frame 600 may include encoding the information included in the original version of the image or video frame by, for example, omitting some of the information from that original version of the image or video frame from a corresponding encoded image or encoded video frame. For example, the coding may include reducing spectral redundancy, reducing spatial redundancy, or a combination thereof. Reducing spectral redundancy may include using a color model based on a luminance component (Y) and two chrominance components (U and V or Cb and Cr), which may be referred to as the YUV or YCbCr color model, or color space. Using the YUV color model may include using a relatively large amount of information to represent the luminance component of a portion of the video frame 600, and using a relatively small amount of information to represent each corresponding chrominance component for the portion of the video frame 600. For example, a portion of the video frame 600 may be represented by a high-resolution luminance component, which may include a 16x16 block of pixels, and by two lower resolution chrominance components, each of which represents the portion of the image as an 8x8 block of pixels. A pixel may indicate a value, for example, a value in the range from 0 to 255, and may be stored or transmitted using, for example, eight bits.Although this disclosure is described in reference to the YUV color model, another color model may be used. Reducing spatial redundancy may include transforming a block into the frequency domain using, for example, a discrete cosine transform. For example, a unit of an encoder may perform a discrete cosine transform using transform coefficient values based on spatial frequency.

[0079] Although described herein with reference to matrix or Cartesian representation of the video frame 600 for clarity, the video frame 600 may be stored, transmitted, processed, or a combination thereof, in a data structure such that pixel values may be efficiently represented for the video frame 600. For example, the video frame 600 may be stored, transmitted, processed, or any combination thereof, in a two-dimensional data structure such as a matrix as shown, or in a one-dimensional data structure, such as a vector array. Furthermore, although described herein as showing a chrominance subsampled image where U and V have half the resolution of Y, the video frame 600 may have different configurations for the color channels thereof. For example, referring still to the YUV color space, full resolution may be used for all color channels of the video frame 600. In another example, a color space other than the YUV color space may be used to represent the resolution of color channels of the video frame 600.

[0080] FIG. 7 is an illustration of frames used in connection with TIP frame prediction. A current frame 700 represents a frame under prediction using the TIP mode described herein, for example, during encoding (e.g., at the intra / inter prediction stage 402) or decoding (e.g., at the intra / inter prediction stage 510). A TIP frame 702 is generated on a streaming basis (i.e., portion by portion) using a motion field derived based on a backward reference frame 704 and a forward reference frame 706 of the current frame 700. For example, where the current frame 700 is denoted as Fi, the backward reference frame 704 can be denoted at Fi-i, and the forward reference frame 706 can be denoted as Fi+i .

[0081] Generally, the backward reference frame 704 and the forward reference frame 706 will be the same distance apart from the current frame 700 in a display order of the video sequence that includes them. However, in some implementations, the backward reference frame 704 and the forward reference frame 706 may be different distances apart from the current frame 700 in the display order. A temporal motion vector predictor 708 represents a motion vector predictor pointing from the backward reference frame 704 to the forward reference frame 706. A motion vector 710 pointing from the current frame 700 to the TIP frame 702 represents a motion vector which may be used with the TIP frame 702 to predict the motion within one or more blocks of the current frame 700.

[0082] In particular, the reference frames 704 and 706 have already been coded by the time they are identified for use as reference frames for the current frame 700. As such, motion vectors of the reference frames 704 and 706 are already known and available, from the earlier coding of the reference frames 704 and 706. Thus, once the reference frames 704 and 706 are identified, a motion field is determined for the current frame 700 using the motion vectors ofthe reference frames 704 and 706. In particular, the motion field includes motion field motion vectors each pointing to one of the forward reference frame 706 or the backward reference frame 704. The motion field effectively represents how the motion field motion vectors can be projected to determine motion vectors for the current frame 700, since the current frame 700 is in between the backward reference frame 704 and the forward reference frame 706. In some cases, a compound motion vector derivation approach may be used.

[0083] The motion field determined for the current frame 700 using the motion vectors of the backward reference frame 704 and the forward reference frame 706 has a same size as the current frame 700. The motion field for the current frame 700 may be separately determined at each of an encoder and a decoder to reduce bitstream size otherwise used for signaling the motion field. Once the motion field has been determined, the motion field motion vectors of the motion field can be stored for later use. For example, the motion field motion vectors may be stored in a memory buffer or cache.

[0084] Further details of techniques for transform split restriction and skip texture context derivation optimization are now described. FIG. 8 is a flowchart diagram of an example of a technique 800 for transform split restriction for TIP frame prediction. FIG. 9 is a flowchart diagram of an example of a technique 900 for skip texture context derivation optimization for TIP frame prediction during decoding.

[0085] The technique 800 and / or the technique 900 can be implemented, for example, as a software program that may be executed by computing devices such as the transmitting station 102 or the receiving station 106. For example, 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 the processor 202, may cause the computing device to perform the technique 800 and / or the technique 900. The technique 800 and / or the technique 900 can be implemented using specialized hardware or firmware. For example, a hardware component, such as a hardware coder, may be configured to perform the technique 800 and / or the technique 900. As explained above, some computing devices may have multiple memories or processors, and the operations described in the technique 800 and / or the technique 900 can be distributed using multiple processors, memories, or both. For simplicity of explanation, the technique 800 and the technique 900 are each depicted and described herein as a 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 or operations not presented and described herein may be used. Furthermore, not all illustrated steps or operations may be required to implement a techniquein accordance with the disclosed subject matter.

[0086] Referring first to FIG. 8, the technique 800 for transform split restriction for generated reference frame (e.g., TIP frame) prediction is shown. At 802, a prediction block is generated for a current block to encode using a TIP frame. The TIP frame is a frame generated using forward and backward reference frames of a current video frame that includes the current block. The forward and backward reference frames are previously coded frames that are each some distance apart from the current frame in a display order of a video sequence, in which the backward reference frame occurs prior to the current frame in that display order and the forward reference frame occurs after the current frame in that display order. For example, the forward and backward reference frames may each be a same distance (measured in terms of numbers of frames between them individually and the current frame) to the current frame in the display order. Generating the prediction block thus includes performing an inter-prediction process against the current block using the TIP frame.

[0087] At 804, a transform size is determined for a prediction residual generated using the prediction block. The transform size is limited based on the use of the TIP frame to generate the prediction block. In particular, there may be initially multiple transform sizes available for selection and use with prediction blocks generated during the encoding of a video sequence; however, the use of TIP frame prediction limits the availability of such transform sizes to a single transform size corresponding (i.e., identical) to a size of the prediction block and prediction residual for the current block. In this way, other transform sizes may remain available for blocks predicted without the use of TIP frame prediction, while bitstream space may be preserved for blocks predicted using TIP frame prediction by using a single transform size and, as will be described below, not signaling that transform size within a bitstream for those TIP-frame-predicted blocks. Determining the transform size may thus include limiting available transform sizes to a single transform size corresponding (i.e., identical) to the prediction block and / or prediction residual. In some cases, the transform size may be determined and limited based on a determination that the current block is predicted using TIP frame prediction.

[0088] At 806, transform coefficients are generated based on the prediction residual and the transform size, in which, per the above, the transform size was limited based on the use of the TIP frame to generate the prediction block from which the prediction residual resulted. In particular, because the limited transform size corresponds to the size of the prediction residual, the transform coefficients are generated for a transform block having the transform size, and thus without the prediction residual being split into multiple smaller blocks.

[0089] At 808, the transform coefficients are encoded to a bitstream. In particular, the transform coefficients are encoded to a bitstream without an indication of the transform size used for generating the transform coefficients and which was determined based on the use of the TIP frame to generate the prediction block for the current block. The transform size is accordingly omitted for the transform coefficients from the bitstream. Thus, during decoding, a decoder processing the encoded transform coefficients performs inverse transformation against the encoded transform coefficients by inferring that the size of the transform block corresponds to the size of the current block under decoding, and thus that multiple smaller blocks were not split and therefore do not have to be combined to decode the current block.

[0090] Referring next to FIG. 9, the technique 900 for skip texture context derivation optimization for generated reference frame (e.g., TIP frame) prediction is shown. At 902, a prediction block is generated for a current block to encode using a TIP frame. The TIP frame is a frame generated using forward and backward reference frames of a current video frame that includes the current block. The forward and backward reference frames are previously coded frames that are each some distance apart from the current frame in a display order of a video sequence, in which the backward reference frame occurs prior to the current frame in that display order and the forward reference frame occurs after the current frame in that display order. For example, the forward and backward reference frames may each be a same distance (measured in terms of numbers of frames between them individually and the current frame) to the current frame in the display order. Generating the prediction block thus includes performing an inter-prediction process against the current block using the TIP frame.

[0091] At 904, a determination is made to skip transform processing for a prediction residual generated using the prediction block. For example, the determination may be made based on content of the current block, content of the prediction residual, the prediction of the current block using the TIP frame, one or more other factors, or a combination thereof. In some implementations, determining to skip the transform processing for the prediction residual can include using a bitstream syntax (e.g., an N-ary flag) to indicate the skipped transform processing for the prediction residual.

[0092] At 906, the prediction residual is entropy coded. In particular, entropy coded data is generated by entropy coding the prediction residual using a context specific to blocks predicted using TIP frame prediction. The entropy coded data is thus generated, and the entropy coding is thus performed, based on the determination to skip the transform processing and based on the use of the TIP frame to generate the prediction block. In one example, the entropy coded data represents the bitstream syntax (i.e., the N-ary flag) used toindicate the skipped transform processing for the prediction residual. The context specific to blocks predicted using TIP frame prediction is different from a context used for blocks predicted without the use of TIP frame prediction. For example, a first block of a current frame may be predicted using a TIP frame (and thus using TIP frame prediction) and transform-related data associated therewith (i.e., a value of the above-described bitstream syntax) may be entropy coded using a first context or first set of contexts specific to blocks predicted using TIP frame prediction. A second block of the same current frame or of a different frame may be predicted without the use of a TIP frame (and thus without the use of TIP frame prediction) and transform-related data associated therewith (i.e., a value of the above-described bitstream syntax) may be entropy coded using a second context or second set of contexts for blocks predicted without use of TIP frame prediction. In this way, same types of data may be entropy coded for blocks predicted with or without the use of a TIP frame without using the same distributions, which would otherwise result in inaccurate probabilities being determined and used for the entropy coding.

[0093] At 908, the entropy coded data is encoded to a bitstream. A decoder later decoding the bitstream can thus similarly process the blocks to predict using TIP frame prediction and the blocks to predict without the use of TIP frame prediction without using the same distributions.

[0094] The implementations of this disclosure describe methods, systems, devices, apparatuses, and non-transitory computer readable media for transform split restriction for generated reference frame prediction, as well as methods, systems, devices, apparatuses, and non-transitory computer readable media for skip texture context derivation optimization for generated reference frame prediction.

[0095] In some implementations, a method for transform split restriction for generated reference frame prediction comprises: generating a prediction block for a current block of a video frame using a generated reference frame that is generated using forward and backward reference frames of the video frame; determining, for a prediction residual generated using the prediction block, a transform size limited for use with the prediction residual based on the use of the generated reference frame to generate the prediction block; generating transform coefficients based on the prediction residual and the transform size; and encoding the transform coefficients to a bitstream without an indication of the transform size. In some implementations, a non-transitory computer readable medium has stored thereon an encoded bitstream configured for decoding by operations for transform split restriction for generated reference frame prediction, the operations comprising: determining, for a prediction residualgenerated using a prediction block for a current block of a video frame, a transform size limited prediction residual based on a use of a generated reference frame to generate the prediction block; generating transform coefficients based on the prediction residual and the transform size; and encoding the transform coefficients to a bitstream without an indication of the transform size. In some implementations, an apparatus for transform split restriction for generated reference frame prediction comprises: a memory; and a processor configured to execute instructions stored in the memory to: generate transform coefficients based on a prediction residual generated using a prediction block and a transform size determined for the prediction residual, wherein the transform size is limited based on a use of a generated reference frame to generate the prediction block; and encode the transform coefficients to a bitstream without an indication of the transform size.

[0096] In some implementations of the method, the non-transitory computer readable medium, and / or the apparatus, determining the transform size limited for use with the prediction residual based on the use of the generated reference frame to generate the prediction block comprises: limiting a selection of multiple transform sizes available for use with prediction blocks to only the transform size based on the use of the generated reference frame to generate the prediction block.

[0097] In some implementations of the method, the non-transitory computer readable medium, and / or the apparatus, determining the transform size limited for use with the prediction residual based on the use of the generated reference frame to generate the prediction block comprises: determining that the current block is predicted using generated reference frame prediction.

[0098] In some implementations of the method, the non-transitory computer readable medium, and / or the apparatus, the transform size is identical to a size of the prediction block.

[0099] In some implementations of the method, the non-transitory computer readable medium, and / or the apparatus, generating the transform coefficients based on the prediction residual and the transform size comprises: generating the transform coefficients without splitting the prediction residual into multiple smaller blocks.

[0100] In some implementations of the method, the non-transitory computer readable medium, and / or the apparatus, encoding the transform coefficients to the bitstream without the indication of the transform size causes a decoder to infer a size of a transform block corresponding to the transform coefficients.

[0101] In some implementations of the method, the non-transitory computer readable medium, and / or the apparatus, the generated reference frame is a temporally interpolatedpicture reference frame.

[0102] In some implementations of the method, the non-transitory computer readable medium, and / or the apparatus, determining the transform size limited for use with the prediction residual based on the use of the generated reference frame to generate the prediction block comprises: limiting available transform sizes to a single transform size corresponding to the size of the prediction block.

[0103] In some implementations of the method, the non-transitory computer readable medium, and / or the apparatus, the method comprises, the operations comprise, and / or the processor is configured to execute the instructions for: generating the prediction block using the generated reference frame, wherein the generated reference frame is generated using forward and backward reference frames of the video frame.

[0104] In some implementations of the method, the non-transitory computer readable medium, and / or the apparatus, the method comprises, the operations comprise, and / or the processor is configured to execute the instructions for: determining the transform size for the prediction residual, wherein the transform size is limited for use with the prediction residual based on the use of the generated reference frame to generate the prediction block.

[0105] In some implementations of the method, the non-transitory computer readable medium, and / or the apparatus, a selection of multiple transform sizes available for use with prediction blocks is limited to only the transform size based on the use of the generated reference frame to generate the prediction block.

[0106] In some implementations of the method, the non-transitory computer readable medium, and / or the apparatus, the encoding of the transform coefficients to the bitstream without the indication of the transform size omits the transform size for the transform coefficients from the bitstream.

[0107] 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.

[0108] 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 implementation unless described as such.

[0109] 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, or another encoder or decoder as disclosed herein) 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.

[0110] 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 purpose processor 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.

[0111] 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 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. Alternatively, the communications device can decode content stored locally on the communications device, forexample, 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.

[0112] Further, all or a portion of implementations of this 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.

[0113] The above-described implementations and other aspects have been described in order 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 so as to encompass all such modifications and equivalent arrangements.

Claims

What is claimed is:

1. A method for transform split restriction for generated reference frame prediction, the method comprising: generating a prediction block for a current block of a video frame using a generated reference frame that is generated using forward and backward reference frames of the video frame; determining, for a prediction residual generated using the prediction block, a transform size limited for use with the prediction residual based on the use of the generated reference frame to generate the prediction block; generating transform coefficients based on the prediction residual and the transform size; and encoding the transform coefficients to a bitstream without an indication of the transform size.

2. The method of claim 1, wherein determining the transform size limited for use with the prediction residual based on the use of the generated reference frame to generate the prediction block comprises: limiting a selection of multiple transform sizes available for use with prediction blocks to only the transform size based on the use of the generated reference frame to generate the prediction block.

3. The method of claim 1, wherein determining the transform size limited for use with the prediction residual based on the use of the generated reference frame to generate the prediction block comprises: determining that the current block is predicted using generated reference frame prediction.

4. The method of any one of claims 1 to 3, wherein the transform size is identical to a size of the prediction block.

5. The method of claim 4, wherein generating the transform coefficients based on the prediction residual and the transform size comprises: generating the transform coefficients without splitting the prediction residual intomultiple smaller blocks.

6. The method of any one of claims 1 to 3, wherein encoding the transform coefficients to the bitstream without the indication of the transform size causes a decoder to infer a size of a transform block corresponding to the transform coefficients.

7. The method of any one of claims 1 to 3, wherein the generated reference frame is a temporally interpolated picture reference frame.

8. A non-transitory computer readable medium having stored thereon an encoded bitstream, wherein an encoder is configured to generate the encoded bitstream by operations for transform split restriction for generated reference frame prediction, the operations comprising: determining, for a prediction residual generated using a prediction block for a current block of a video frame, a transform size limited prediction residual based on a use of a generated reference frame to generate the prediction block; generating transform coefficients based on the prediction residual and the transform size; and encoding the transform coefficients to a bitstream without an indication of the transform size.

9. The non-transitory computer readable medium of claim 8, wherein the transform size is identical to a size of the prediction block.

10. The non-transitory computer readable medium of claim 9, wherein determining the transform size limited for use with the prediction residual based on the use of the generated reference frame to generate the prediction block comprises: limiting available transform sizes to a single transform size corresponding to the size of the prediction block.

11. The non-transitory computer readable medium of claim 9, wherein generating the transform coefficients based on the prediction residual and the transform size comprises: generating the transform coefficients without splitting the prediction residual into multiple smaller blocks.

12. The non-transitory computer readable medium of any one of claims 8 to 11, the operations comprising: generating the prediction block using the generated reference frame, wherein the generated reference frame is generated using forward and backward reference frames of the video frame.

13. The non-transitory computer readable medium of any one of claims 8 to 11, wherein the generated reference frame is a temporally interpolated picture reference frame.

14. An apparatus for transform split restriction for generated reference frame prediction, the apparatus comprising: a memory; and a processor configured to execute instructions stored in the memory to: generate transform coefficients based on a prediction residual generated using a prediction block and a transform size determined for the prediction residual, wherein the transform size is limited based on a use of a generated reference frame to generate the prediction block; and encode the transform coefficients to a bitstream without an indication of the transform size.

15. The apparatus of claim 14, wherein the processor is configured to execute the instructions to: determine the transform size for the prediction residual, wherein the transform size is limited for use with the prediction residual based on the use of the generated reference frame to generate the prediction block.

16. The apparatus of claim 15, wherein a selection of multiple transform sizes available for use with prediction blocks is limited to only the transform size based on the use of the generated reference frame to generate the prediction block.

17. The apparatus of any one of claims 14 to 16, wherein the processor is configured to execute the instructions to: generate the prediction block for a current block of a video frame using the generatedreference frame, wherein the generated reference frame is generated using forward and backward reference frames of the video frame.

18. The apparatus of claim 17, wherein the transform size is identical to a size of the prediction block.

19. The apparatus of any one of claims 14 to 16, wherein the transform coefficients are generated without splitting the prediction residual into multiple smaller blocks.

20. The apparatus of any one of claims 14 to 16, wherein the encoding of the transform coefficients to the bitstream without the indication of the transform size omits the transform size for the transform coefficients from the bitstream.

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