Classifications

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  • H04N19/50—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using predictive coding
  • H04N19/503—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using predictive coding involving temporal prediction
  • H04N19/51—Motion estimation or motion compensation
  • H04N19/577—Motion compensation with bidirectional frame interpolation, i.e. using B-pictures
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
  • H04N19/00—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals
  • H04N19/10—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using adaptive coding
  • H04N19/102—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using adaptive coding characterised by the element, parameter or selection affected or controlled by the adaptive coding
  • H04N19/103—Selection of coding mode or of prediction mode
  • H04N19/105—Selection of the reference unit for prediction within a chosen coding or prediction mode, e.g. adaptive choice of position and number of pixels used for prediction
• • H—ELECTRICITY
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  • H04N19/00—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals
  • H04N19/10—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using adaptive coding
  • H04N19/134—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using adaptive coding characterised by the element, parameter or criterion affecting or controlling the adaptive coding
  • H04N19/157—Assigned coding mode, i.e. the coding mode being predefined or preselected to be further used for selection of another element or parameter
  • H04N19/159—Prediction type, e.g. intra-frame, inter-frame or bidirectional frame prediction
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
  • H04N19/00—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals
  • H04N19/10—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using adaptive coding
  • H04N19/189—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using adaptive coding characterised by the adaptation method, adaptation tool or adaptation type used for the adaptive coding
  • H04N19/19—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using adaptive coding characterised by the adaptation method, adaptation tool or adaptation type used for the adaptive coding using optimisation based on Lagrange multipliers
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
  • H04N19/00—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals
  • H04N19/50—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using predictive coding
  • H04N19/503—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using predictive coding involving temporal prediction
  • H04N19/51—Motion estimation or motion compensation
  • H04N19/513—Processing of motion vectors
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
  • H04N19/00—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals
  • H04N19/50—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using predictive coding
  • H04N19/503—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using predictive coding involving temporal prediction
  • H04N19/51—Motion estimation or motion compensation
  • H04N19/513—Processing of motion vectors
  • H04N19/517—Processing of motion vectors by encoding
  • H04N19/52—Processing of motion vectors by encoding by predictive encoding
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
  • H04N19/00—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals
  • H04N19/50—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using predictive coding
  • H04N19/503—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using predictive coding involving temporal prediction
  • H04N19/51—Motion estimation or motion compensation
  • H04N19/537—Motion estimation other than block-based
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
  • H04N19/00—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals
  • H04N19/50—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using predictive coding
  • H04N19/503—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using predictive coding involving temporal prediction
  • H04N19/51—Motion estimation or motion compensation
  • H04N19/573—Motion compensation with multiple frame prediction using two or more reference frames in a given prediction direction

Definitions

• 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 user-generated 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 compression and other encoding techniques.
• One technique for compression uses a reference frame to generate a prediction block corresponding to a current block to be encoded. Differences between the prediction block and the current block can be encoded, instead of the values of the current block themselves, to reduce the amount of data encoded.
• This disclosure relates generally to encoding and decoding video data and more particularly relates to using block-based optical flow estimation for motion compensated prediction in video compression.
• Frame-level based optical flow estimation is also described that can interpolate a co-located reference frame for motion compensated prediction in video compression.
• This disclosure describes encoding and decoding methods and apparatuses.
• a method determining a first frame portion of a first frame to be predicted, the first frame being in a video sequence, determining a first reference frame from the video sequence for forward inter prediction of the first frame, determining a second reference frame from the video sequence for backward inter prediction of the first frame, generating an optical flow reference frame portion for inter prediction of the first frame portion by performing an optical flow estimation using the first reference frame and the second reference frame, and performing a prediction process for the first frame portion using the optical flow reference frame.
• the first frame portion and optical flow reference frame portion can be a block, or an entire frame, for example.
• An apparatus includes a non- transitory storage medium or memory and a processor.
• the medium includes instructions executable by the processor to carry out a method including determining a first frame to be predicted in a video sequence, and determining an availability of a first reference frame for forward inter prediction of the first frame and a second reference frame for backward inter prediction of the first frame.
• the method also includes, responsive to determining the availability of both the first reference frame and the second reference frame, generating a respective motion field for pixels of a first frame portion using the first reference frame and the second reference frame as input into an optical flow estimation process, warping a first reference frame portion to the first frame portion using the motion fields to form a first warped reference frame portion, the first reference frame portion comprising pixels of the first reference frame co-located with the pixels of the first frame portion, warping a second reference frame portion to the first frame portion using the motion fields to form a second warped reference frame portion, the second reference frame portion comprising pixels of the second reference frame co-located with the pixels of the first frame portion, and blending the first warped reference frame portion and the second warped reference frame portion to form an optical flow reference frame portion for inter prediction of at least one block of the first frame.
• Another apparatus also includes a non-transitory storage medium or memory and a processor.
• the medium includes instructions executable by the processor to carry out a method including generating an optical flow reference frame portion for inter prediction of a block of a first frame of a video sequence using a first reference frame from the video sequence and a second reference frame of the video sequence by initializing motion fields for pixels of a first frame portion in a first processing level for an optical flow estimation, the first processing level representing downscaled motion within the first frame portion and comprising one level of multiple levels, and, for each level of the multiple levels warping the first reference frame portion to the first frame portion using the motion fields to form a first warped reference frame portion, warping the second reference frame portion to the first frame portion using the motion fields to form a second warped reference frame portion, estimating motion fields between the first warped reference frame portion and the second warped reference frame portion using the optical flow estimation, and updating the motion fields for pixels of the first frame portion using the motion fields between the first warped reference frame portion and the
• the method also includes, for a final level of the multiple levels, warping the first reference frame portion to the first frame portion using the updated motion fields to form a final first warped reference frame portion, warping the second reference frame portion to the first frame portion using the updated motion fields to form a final second warped reference frame portion, and blending the final first warped reference frame portion and the second warped reference frame portion to form the optical flow reference frame portion.
• Another apparatus includes a non- transitory storage medium or memory and a processor.
• the medium includes instructions executable by the processor to carry out a method including determining a first frame portion of a first frame to be predicted, the first frame being in a video sequence, determining a first reference frame from the video sequence for forward inter prediction of the first frame, determining a second reference frame from the video sequence for backward inter prediction of the first frame, generating an optical flow reference frame portion for inter prediction of the first frame portion by performing an optical flow estimation using the first reference frame and the second reference frame, and performing a prediction process for the first frame portion using the optical flow reference frame.
• FIG. 1 is a schematic of a video encoding and decoding system.
• FIG. 2 is a block diagram of an example of a computing device that can implement a transmitting station or a receiving station.
• FIG. 3 is a diagram of a typical video stream to be encoded and subsequently decoded.
• FIG. 4 is a block diagram of an encoder according to implementations of this disclosure.
• FIG. 5 is a block diagram of a decoder according to implementations of this disclosure.
• FIG. 6 is a block diagram of an example of a reference frame buffer.
• FIG. 7 is a diagram of a group of frames in a display order of a video sequence.
• FIG. 8 is a diagram of an example of a coding order for the group of frames of FIG. 7.
• FIG. 9 is a diagram used to explain the linear projection of a motion field according to the teachings herein.
• FIG. 10 is a flowchart diagram of a process for motion compensated prediction of a video frame using at least a portion of a reference frame generated using optical flow estimation.
• FIG. 11 is a flowchart diagram of a process for generating an optical flow reference frame portion.
• FIG. 12 is a flowchart diagram of another process for generating an optical flow reference frame portion.
• FIG. 13 is a diagram illustrating the processes of FIGS. 11 and 12.
• FIG. 14 is a diagram illustrating object occlusion.
• FIG. 15 is a diagram illustrating a technique for optimizing a decoder.
• a video stream can be compressed by a variety of techniques to reduce bandwidth required transmit or store the video stream.
• a video stream can be encoded into a bitstream, which involves compression, which is then transmitted to a decoder that can decode or decompress the video stream to prepare it 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.
• 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, a decoder receiving the encoded signal can re-create the current block. Inter-prediction may also be referred to as motion compensated prediction.
• Each motion vector used to generate a prediction block in the inter-prediction process refers to a frame other than a current frame, i.e., a reference frame.
• 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 some cases, there may be three reference frames used to encode or decode blocks of the current frame of the video sequence. One is a frame that may be referred to as a golden frame. Another is a most recently encoded or decoded frame. The last is an alternative reference frame that is encoded or decoded before one or more frames in a sequence, but which is displayed after those frames in an output display order.
• the alternative reference frame is a reference frame usable for backwards prediction.
• One or more forward and/or backward reference frames can be used to encode or decode a bock.
• the efficacy of a reference frame when used to encode or decode a block within a current frame can be measured based on a resulting signal-to-noise ratio or other measures of rate-distortion.
• the pixels that form prediction blocks are obtained directly from one or more of the available reference frames.
• the reference pixel blocks or their linear combinations are used for prediction of the given coding block in the current frame.
• This direct, block-based prediction does not capture the true motion activity available from the reference frames. For this reason, motion compensated prediction accuracy can suffer.
• references frame portions collocated with the current coding frame portions that use a per-pixel motion field calculated by optical flow to estimate the true motion activities in the video signal.
• Reference frame portions are interpolated that allow tracking of complicated non-translational motion activity, which is beyond the capability of conventional block-based motion compensated prediction determined directly from reference frames. Use of such reference frame portions can improve prediction quality.
• a frame portion refers to some of all of a frame, such as a block, a slice, or an entire frame.
• a frame portion in one frame is collocated with a frame portion in another frame if they have the same dimensions and are at the same pixel locations within the dimensions of each frame.
• 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 suitable implementations of the transmitting station 102 are possible. For example, the processing of the transmitting station 102 can be distributed among multiple devices.
• a network 104 can connect the transmitting station 102 and a receiving station 106 for encoding and decoding of the video stream.
• 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.
• the receiving station 106 in one example, can be a computer having an internal configuration of hardware such as that described in FIG. 2.
• Other suitable components such as that described in FIG. 2.
• the processing of the receiving station 106 can be distributed among multiple devices.
• an implementation can omit the network 104.
• 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 a non-transitory storage medium or memory.
• 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.
• a real-time transport protocol RTP
• a transport protocol other than RTP may be used, e.g., a Hypertext Transfer Protocol (HTTP) based video streaming protocol.
• HTTP Hypertext Transfer Protocol
• 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.
• 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 its own video bitstream to the video conference server for decoding and viewing by other participants.
• FIG. 2 is a block diagram of an example of a computing device 200 that can implement a transmitting station or a receiving station.
• 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.
• a CPU 202 in the computing device 200 can be a central processing unit.
• the CPU 202 can be any other type of device, or multiple devices, capable of manipulating or processing information now-existing or hereafter developed.
• the disclosed implementations can be practiced with one processor as shown, e.g., the CPU 202, advantages in speed and efficiency can be achieved using more than one processor.
• 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. Any other suitable type of storage device or non-transitory storage medium can be used as the memory 204.
• the memory 204 can include code and data 206 that is accessed by the CPU 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 CPU 202 to perform the methods described here.
• the application programs 210 can include applications 1 through N, which further include a video coding application that performs the methods described here.
• 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.
• 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 CPU 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.
• 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 light emitting diode (LED) display, such as an organic LED (OLED) display.
• LCD liquid crystal display
• CRT cathode-ray tube
• LED light emitting diode
• OLED organic LED
• 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.
• 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.
• the computing device 200 can also include or be in communication with a sound- sensing 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.
• FIG. 2 depicts the CPU 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 CPU 202 can be distributed across multiple machines (wherein individual machines can have one or more of 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.
• the bus 212 of the computing device 200 can be composed of multiple buses.
• 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.
• 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.
• 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, e.g., a frame 306.
• 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.
• 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.
• 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 one or 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.
• 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 CPU 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.
• 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.
• 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.
• respective frames 304 can be processed in units of blocks.
• respective blocks can be encoded using intra-frame prediction (also called intra-prediction) or inter- frame prediction (also called inter-prediction).
• intra-frame prediction also called intra-prediction
• inter-prediction also called inter-prediction
• a prediction block can be formed.
• intra-prediction a prediction block may be formed from samples in the current frame that have been previously encoded and reconstructed.
• inter- prediction a prediction block may be formed from samples in one or more previously constructed reference frames. The designation of reference frames for groups of blocks is discussed in further detail below.
• 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
• 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, the transform coefficients may be divided by the quantizer value and truncated.
• 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 the type of prediction used, transform type, motion vectors and quantizer value, 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.
• VLC variable length 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.
• the reconstruction path in FIG. 4 can be used to ensure that the encoder 400 and a decoder 500 (described below) 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 that are discussed in more detail below, 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).
• 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.
• encoder 400 can be used to encode the compressed bitstream 420.
• a non-transform based encoder can quantize the residual signal directly without the transform stage 404 for certain blocks or frames.
• an encoder can have the quantization stage 406 and the dequantization stage 410 combined in a common stage.
• 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 CPU 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.
• 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 deblocking filtering stage 514.
• stages to perform various functions to produce an output video stream 516 from the compressed bitstream 420 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 deblocking filtering stage 514.
• Other structural variations of the decoder 500 can be used to decode the compressed bitstream 420.
• 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.
• 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.
• 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.
• the deblocking 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.
• the decoder 500 can produce the output video stream 516 without the deblocking filtering stage 514.
• FIG. 6 is a block diagram of an example of a reference frame buffer 600.
• the reference frame buffer 600 stores reference frames used to encode or decode blocks of frames of a video sequence.
• the reference frame buffer 600 includes reference frames identified as a last frame LAST_FRAME 602, a golden frame GOLDEN_FRAME 604, and an alternative reference frame ALTREF_FRAME 606.
• a frame header of a reference frame may include a virtual index to a location within the reference frame buffer at which the reference frame is stored.
• a reference frame mapping may map the virtual index of a reference frame to a physical index of memory at which the reference frame is stored. Where two reference frames are the same frame, those reference frames will have the same physical index even if they have different virtual indexes.
• the number of reference positions within the reference frame buffer 600, the types, and the names used are examples only.
• the reference frames stored in the reference frame buffer 600 can be used to identify motion vectors for predicting blocks of frames to be encoded or decoded. Different reference frames may be used depending on the type of prediction used to predict a current block of a current frame. For example, in bi-prediction, blocks of the current frame can be forward predicted using either frame stored as the LAST_FRAME 602 or the GOLDEN_FRAME 604, and backward predicted using a frame stored as the ALTREF_FRAME 606.
• the reference frame buffer 600 can store up to eight reference frames, wherein each stored reference frame may be associated with a different virtual index of the reference frame buffer. Although three of the eight spaces in the reference frame buffer 600 are used by frames designated as the LAST_FRAME 602, the GOLDEN_FRAME 604, and the ALTREF_FRAME 606, five spaces remain available to store other reference frames. For example, one or more available spaces in the reference frame buffer 600 may be used to store further reference frames, in particular some or all of the interpolated reference frame described herein. Although the reference frame buffer 600 is shown as being able to store up to eight reference frames, other implementations of the reference frame buffer 600 may be able to store additional or fewer reference frames.
• ALTREF_FRAME 606 may be a frame of a video sequence that is distant from a current frame in a display order, but is encoded or decoded earlier than it is displayed.
• the alternative reference frame may be ten, twelve, or more (or fewer) frames after the current frame in a display order. Further alternative reference frames can be frames located nearer to the current frame in the display order.
• An alternative reference frame may not correspond directly to a frame in the sequence. Instead, the alternative reference frame may be generated using one or more of the frames having filtering applied, being combined together, or being both combined together and filtered.
• An alternative reference frame may not be displayed. Instead, it can be a frame or portion of a frame generated and transmitted for use only in a prediction process (i.e., it is omitted when the decoded sequence is displayed).
• FIG. 7 is a diagram of a group of frames in a display order of the video sequence.
• the group of frames is preceded by a frame 700, which can be referred to as a key frame or an overlay frame in some cases, and comprises eight frames 702 - 716.
• No block within the frame 700 is inter predicted using reference frames of the group of frames.
• the frame 700 is a key (also referred to as intra-predicted frame) in this example, which refers to its status that predicted blocks within the frame are only predicted using intra prediction.
• the frame 700 can be an overlay frame, which is an inter-predicted frame that can be a reconstructed frame of a previous group of frames.
• inter prediction In an inter-predicted frame, at least some of the predicted blocks are predicted using inter prediction.
• the number of frames forming each group of frames can vary according to the video spatial / temporal characteristics and other encoded configurations, such as the key frame interval selected for random access or error resilience, for example.
• the coding order for each group of frames can differ from the display order. This allows a frame located after a current frame in the video sequence to be used as a reference frame for encoding the current frame.
• a decoder such as the decoder 500, may share a common group coding structure with an encoder, such as the encoder 400.
• the group coding structure assigns different roles that respective frames within the group may play in the reference buff (e.g., a last frame, an alternative reference frame, etc.) and defines or indicates the coding order for the frames within a group.
• FIG. 8 is a diagram of an example of a coding order for the group of frames of FIG. 7.
• the coding order of FIG. 8 is associated with a first group coding structure whereby a single backward reference frame is available for each frame of the group. Because the encoding and decoding order is the same, the order shown in FIG. 8 is generally referred to herein as a coding order.
• the key or overlay frame 700 is designated the golden frame in a reference frame buffer, such as the GOLDEN_FRAME 604 in the reference frame buffer 600.
• the frame 700 is intra- predicted in this example, so it does not require a reference frame, but an overlay frame as the frame 700, being a reconstructed frame from a previous group, also does not use a reference frame of the current group of frames.
• the final frame 716 in the group is designated an alternative reference frame in a reference frame buffer, such as the ALTREF_FRAME 606 in the reference frame buffer 600.
• the frame 716 is coded out of the display order after the frame 700 so as to provide a backward reference frame for each of the remaining frames 702 - 714.
• the frame 700 serves as an available reference frame for blocks of the frame 716.
• FIG. 8 is only one example of a coding order for a group of frames.
• Other group coding structures may designate one or more different or additional frames for forward and/or backward prediction.
• an available reference frame portion may be a reference frame portion that is interpolated using optical flow estimation.
• a reference frame portion may be a block, a slice, or an entire frame, for example.
• the resulting reference frame is referred to as a co-located reference frame herein because the dimensions are the same as the current frame.
• This interpolated reference frame may also be referred to herein as an optical flow reference frame.
• FIG. 9 is a diagram used to explain the linear projection of a motion field according to the teachings herein.
• the optical flow (also called a motion field) of the current frame may be estimated using the nearest available reconstructed (e.g., reference) frames before and after the current frame.
• the reference frame 1 is a reference frame that may be used for forward prediction of the current frame 900
• the reference frame 2 is a reference frame that may be used for backward prediction of the current frame 900.
• the immediately preceding, or last, frame 704 e.g., the reconstructed frame stored in the reference frame buffer 600 as the LAST_FRAME 602
• the frame 716 e.g., the reconstructed frame stored in the reference frame buffer 600 as the ALTREF_FRAME 606
• motion vectors may be projected between the pixels in the reference frames 1 and 2 to the pixels in the current frame 900 assuming that the motion field is linear in time.
• the index for the current frame 900 is 3
• the index for the reference frame 1 is 0,
• the index for the reference frame 2 is 716.
• a projected motion vector 904 for a pixel 902 of the current frame 900 is shown.
• the display indexes of the group of frames of FIG. 7 would show that the frame 704 is temporally closer to the frame 706 than the frame 716.
• the single motion vector 904 shown in FIG. 9 represents a different amount of motion between reference frame 1 and the current frame 900 than between the reference frame 2 and the current frame 900.
• the projected motion field 906 is linear between the reference frame 1, the current frame 900, and the reference frame 2.
• FIG. 10 is a flowchart diagram of a method or process 1000 for motion compensated prediction of a frame of a video sequence using at least a portion of a reference frame generated using optical flow estimation.
• the reference frame portion may be a block, a slice, or an entire reference frame, for example.
• An optical flow reference frame portion may also be referred to as a co-located reference frame portion herein.
• the process 1000 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 CPU 202, may cause the computing device to perform the process 1000.
• the process 1000 can be implemented using specialized hardware or firmware. Some computing devices may have multiple memories or processors, and the operations described in the process 1000 can be distributed using multiple processors, memories, or both.
• a current frame to be predicted is determined.
• Frames may be coded, and hence predicted, in any order, such as in the coding order shown in FIG. 8.
• the frames to be predicted may also be referred to as a first, second, third, etc. frame.
• the label of first, second, etc. does not necessarily indicate an order of the frames. Instead the label is used to distinguish one current frame from another herein unless otherwise stated.
• the frame may be processed in units of blocks in a block coding order, such as a raster scan order.
• the frame may also be processed in units of blocks according to receipt of their encoded residuals within an encoded bitstream.
• forward and backward reference frames are determined.
• the forward and backward reference frames are the nearest reconstructed frames before and after (e.g., in display order) the current frame, such as the current frame 900.
• the process 1000 ends. The current frame is then processed without considering optical flow.
• an optical flow reference frame portion may be generated using the reference frames at 1006. Generating the optical flow reference frame portion is described in more detail with reference to FIGS. 11-14.
• the optical flow reference frame portion may be stored at a defined position within the reference frame buffer 600 in some implementations. Initially, optical flow estimation according to the teachings herein is described. [0070] Optical flow estimation may be performed for respective pixels of a current frame portion by minimizing the following Lagrangian function (1):
• Jd ata is the data penalty based on the brightness constancy assumption (i.e., the assumption that an intensity value of a small portion of an image remains unchanged over time despite a position change).
• spatial is the spatial penalty based on the smoothness of the motion field (i.e., the characteristic that neighboring pixels likely belong to the same object item in an image, resulting in substantial the same image motion).
• the Lagrangian parameter ⁇ controls the importance of the smoothness of the motion field. A large value for the parameter ⁇ results in a smoother motion field and can better account for motion at a larger scale. In contrast, a smaller value for the parameter ⁇ may more effectively adapt to object edges and the movement of small objects.
• the data penalty may be represented by the data penalty function:
• £ " t are derivatives of pixel values of reference frame portions with respect to the horizontal axis x, the vertical axis y, and time t (e.g., as represented by frame indexes).
• the horizontal axis and the vertical axis are defined relative to the array of the pixels forming the current frame, such as the current frame 900, and the reference frames, such as the reference frames 1 and 2.
• the derivatives E x , E y , and E t may be calculated according to the following functions (3), (4), and (5):
• E x (index r2 — index cur ) / ⁇ index r2 — index rl ) ⁇ E x +
• E t £ (r2) - £ (rl) (5)
• E ⁇ ' is a pixel value at a projected position in the reference frame 1 based on the motion field of the current pixel location in the current frame being encoded.
• variable E ⁇ 2 ⁇ is a pixel value at a projected position in the reference frame 2 based on the motion field of the current pixel location in the current frame being encoded.
• variable index rl is the display index of the reference frame 1, where the display index of a frame is its index in the display order of the video sequence.
• variable index r2 is the display index of the reference frame 2
• variable index cur is the display index of the current frame 900.
• variable i3 ⁇ 4 is the horizontal derivative calculated at the reference frame 1
• variable E % is the horizontal derivative calculated at the reference
• the variable Ey is the vertical derivative calculated at the
• the variable Ey is the vertical derivative calculated at the reference frame 2 using a linear filter.
• the linear filter used for calculating the horizontal derivative is a 7-tap filter with filter coefficients [- 1/60, 9/60, -45/60, 0, 45/60, -9/60, 1/60] .
• the filter can have a different frequency profile, a different number of taps, or both.
• the linear filter used for calculating the vertical derivatives may be the same as or different from the linear filter used for calculating the horizontal derivatives.
• the spatial penalty may be represented by the spatial penalty function:
• ⁇ is the Laplacian of the horizontal component u of the motion field
• ⁇ is the Laplacian of the vertical component v of the motion field.
• FIG. 11 is a flowchart diagram of a method or process 1100 for generating an optical flow reference frame portion.
• the optical flow reference frame portion is an entire reference frame.
• the process 1100 can implement step 1006 of the process 1000.
• the process 1100 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 CPU 202, may cause the computing device to perform the process 1 100.
• the process 1100 can be implemented using specialized hardware or firmware. As described above, multiple processors, memories, or both, may be used.
• the estimated motion vectors from the current frame to the reference frames can be used to initialize the optical flow estimation for the current frame.
• all pixels within the current frame may be assigned an initialized motion vector. They define initial motion fields that can be utilized to warp the reference frames to the current frame for a first processing level to shorten the motion lengths between reference frames.
• the motion field mv cur of a current pixel may be initialized using a motion vector that represents a difference between the estimated motion vector mv r2 pointing from the current pixel to the backward reference frame, in this example reference frame 2, and the estimated motion vector mv r2 pointing from the current pixel to the forward reference frame, in this example reference frame 1, according to:
• Ttiv cur —mv rl ⁇ (index r2 — index rl )/ (index cur — index rl ), or
• Ttiv cur mv r2 ⁇ (index r2 — index rl ) / (index r2 — index cur ).
• one or more spatial neighbors having an initialized motion vector may be used. For example, an average of the available neighboring initialized motion vectors may be used.
• reference frame 2 may be used to predict a pixel of reference frame 1, where reference frame 1 is the last frame before the current frame being coded. That motion vector, projected on to the current frame using linear projection in a similar manner as shown in FIG. 9, results in a motion field mv cur at the intersecting pixel location, such as the motion field 906 at the pixel location 902.
• FIG. 11 refers to initializing the motion field for a first processing level because there are desirably multiple processing levels to the process 1100.
• FIG. 13 is a diagram that illustrates the process 1100 of FIG. 11 (and the process 1200 of FIG. 12 discussed below).
• the following description uses the phrase motion field. This phrase is intended to collectively refer to the motion fields for respective pixels unless otherwise clear from the context. Accordingly, the phrases "motion fields” or “motion field” may be used interchangeably when referring to more than one motion field. Further, the phrase optical flow may be used interchangeably with the phrase motion field when referring to the movement of pixels.
• a pyramid, or multi- layered, structure may be used.
• the reference frames are scaled down to one or more different scales.
• the optical flow is first estimated to obtain a motion field at the highest level (the first processing level) of the pyramid, i.e., using the reference frames that are scaled the most.
• the motion field is up-scaled and used to initialize the optical flow estimation at the next level. This process of upscaling the motion field, using it to initialize the optical flow estimation of the next level, and obtaining the motion field continues until the lowest level of the pyramid is reached (i.e., until the optical flow estimation is completed for the reference frame portions at full scale).
• the Lagrangian parameter ⁇ is set for solving the Lagrangian function (1) at 1104.
• the process 1100 uses multiple values for the Lagrangian parameter ⁇ .
• the first value at which the Lagrangian parameter ⁇ is set at 1104 may be a relatively large value, such as 100. While it is desirable that the process 1100 uses multiple values for the Lagrangian parameter ⁇ within the Lagrangian function (1), it is possible that only one value is used as described in the process 1200 described below.
• the reference frames are warped to the current frame according to the motion field for the current processing level. Warping the reference frames to the current frame may be performed using subpixel location rounding. It is worth noting that the motion field mv cur that is used at the first processing level is downscaled from its full resolution value to the resolution of the level before performing the warping. Downscaling a motion field is discussed in more detail below.
• the motion field to warp reference frame 1 is inferred by the linear projection assumption (e.g., that the motion projects linearly over time) as follows:
• the horizontal component u rl and the vertical component u rl of the motion field mv rl may be rounded to 1/8 pixel precision for the Y component and 1/16 pixel precision for the U and V component. Other values for the subpixel location rounding may be used.
• each pixel in a warped image E ⁇ , ai , ped is calculated as the referenced pixel given by the motion vector mv rl .
• Subpixel interpolation may be performed using a conventional subpixel interpolation filter.
• mv r2 (index r2 — index cur ) / (index r2 — index rl ) ⁇ mv cur
• two warped reference frames exist.
• the two warped reference frames are used to estimate the motion field between them at 1108.
• Estimating the motion field at 1108 can include multiple steps.
• the derivatives E x , E y , and E t are calculated using the functions (3), (4), and (5).
• the frame boundaries of a warped reference frame may be expanded by copying the nearest available pixel. In this way, pixel values (i.e., E ⁇ 1 ⁇ and/or E ⁇ r2 ⁇ ) may be obtained when projected positions are outside of the warped reference frame.
• the derivatives are downscaled to the current level. As shown in FIG. 13, the reference frames are used to calculate the derivatives at the original scale to capture details. Downscaling the derivatives at each level 1 may calculated by averaging within a 2 1 by 2 1 block. It is worth noting that, because calculating the derivatives as well as downscaling by averaging them are both linear operations, the two operations may be combined in a single linear filter to calculate the derivatives at each level 1. This can lower complexity of the calculations.
• the motion field for pixels of the current frame is updated or refined using the estimated motion field between the warped reference frames.
• the current motion field for a pixel may be updated by adding the estimated motion field for a pixel on a pixel-by-pixel basis.
• a query is made at 1110 to determine whether there are additional values for the Lagrangian parameter ⁇ available. Smaller values for the Lagrangian parameter ⁇ can address smaller scales of motion. If there are additional values, the process 1100 can return to 1104 to set the next value for the Lagrangian parameter ⁇ . For example, the process 1100 can repeat while reducing the Lagrangian parameter ⁇ by half in each iteration.
• the motion field updated at 1108 is the current motion field for warping the reference frames at 1106 in this next iteration. Then, the motion field is again estimated at 1108.
• the processing at 1104, 1106, and 1108 continues until all of the possible Lagrangian parameters at 1110 are processed.
• the smallest value for the Lagrangian parameter ⁇ is 25 in an example.
• This repeating processing while modifying the Lagrangian parameter may be referred to as annealing the Lagrangian parameter.
• the process 1100 advances to 1112 to determine whether there are more processing levels to process. If there are additional processing levels at 1112, the process 1100 advances to 1114, where the motion field is up-scaled before processing the next layer using each of the available values for the Lagrangian parameter ⁇ starting at 1104. Upscaling the motion field may be performed using any known technique, including but not limited to the reverse of the downscaling calculations described previously.
• the optical flow is first estimated to obtain a motion field at the highest level of the pyramid. Thereafter, the motion field is upscaled and used to initialize the optical flow estimation at the next level. This process of upscaling the motion field, using it to initialize the optical flow estimation of the next level, and obtaining the motion field continues until the lowest level of the pyramid is reached (i.e., until the optical flow estimation is completed for the derivatives calculated at full scale) at 1112.
• the process 1100 advances to 1116.
• the number of levels can be three, such as in the example of FIG. 13.
• the warped reference frames blended at 1116 may be the full-scale reference frames that are warped again according to the process described at 1106 using the motion field estimated at 1108.
• the full-scale reference frames may be warped twice - once using the initial up-scaled motion field from the previous layer of processing and again after the motion field is refined at the full-scale level.
• the blending may be performed using the time linearity assumption (e.g., that frames are spaced apart by equal time periods) as follows:
• E cur) E ⁇ rped ⁇ ⁇ index r2 - index cur )/(index r2 - index rl ) + E ⁇ rped ⁇ (index cur — index rl ) / (index r2 — index rl )
• the pixel in only one of the warped reference frames rather than the blended value. For example, if a reference pixel in the reference frame 1 (represented by mv rl ) is out of bounds (e.g., outside of the dimensions of the frame) while the reference pixel in the reference frame 2 is not, then only the pixel in the warped image resulting from the reference frame 2 is used according to:
• Optional occlusion detection may be performed as part of the blending. Occlusion of objects and background commonly occurs in a video sequence, where parts of the object appear in one reference frame but are hidden in the other. Generally, the optical flow estimation method described above cannot estimate the motion of the object in this situation because the brightness constancy assumption is violated. If the size of the occlusion is relatively small, the smoothness penalty function may estimate the motion quite accurately. That is, if the undefined motion field at the hidden part is smoothed by the neighboring motion vectors, the motion of the whole object can be accurate.
• FIG. 14 is a diagram that illustrates object occlusion.
• the occluded part of object A shows in reference frame 1 and is hidden by object B in reference frame 2.
• the referenced pixel from reference frame 2 is from object B.
• using only the warped pixel from the reference frame 1 is desirable. Accordingly, using a technique that detects occlusions, instead of or in addition to the above blending, may provide a better blending result, and hence a better reference frame.
• the motion vector of the occluded part of object A points to object B in reference frame 2. This may result in the following situations.
• the first situation is that the warped pixel values E wa . p ed and E wa . ped are very different because they are from two different objects.
• the second situation is that the pixels in object B are referenced by multiple motion vectors, which are for object B in the current frame and for the occluded part of object A in the current frame.
• ⁇ warped tor h warped is greater than a threshold T pixe i
• [00124] is greater than a threshold T ref .
• N ⁇ ef is the total number of times that the referenced pixel in the reference frame 1 is referenced by any pixel in the current co-located frame. Given the existence of subpixel interpolation described above, N,-e/ * s coun ted when the reference subpixel location is within one pixel length of the interested pixel location. Moreover, if mv r2 points to a subpixel location, the weighted average of N,-e/ °f me f° ur neighboring pixels as the total number of references for the current subpixel location. N,-e/ ma ⁇ e similarly defined.
• an occlusion can be detected in the first reference frame using the first warped reference frame and the second warped reference frame. Then, the blending of the warped reference frames can include populating pixel positions of the optical flow reference frame corresponding to the occlusion with pixel values from the second warped reference frame. Similarly, an occlusion can be detected in the second reference frame using the first warped reference frame and the second warped reference frame. Then, the blending of the warped reference frames can include populating pixel positions of the optical flow reference frame corresponding to the occlusion with pixel values from the first warped reference frame
• the process 1100 provides substantial compression performance gains. These performance gains include 2.5% gains in PSNR and 3.3% in SSIM for a low-resolution set of frames, and 3.1% in PSNR and 4.0% in SSIM for a mid-resolution set of frames).
• the optical flow estimation performed according to the Lagrangian function (1) uses 2 * N linear equations to solve the horizontal component u and the vertical component v of the motion field for all N pixels of a frame.
• the computational complexity of optical flow estimation is a polynomial function of the frame size, which impose a burden on the decoder complexity. Accordingly, a sub-frame based (e.g., a block-based) optical flow estimation is next described, which can reduce the decoder complexity over the frame-based optical flow estimation described with regard to FIG. 11.
• FIG. 12 is a flowchart diagram of a method or process 1200 for generating an optical flow reference frame portion.
• the optical flow reference frame portion is less than an entire reference frame.
• the co-located frame portions in this example are described with reference to a block, but other frame portions may be processed according to FIG. 12.
• the process 1200 can implement step 1006 of the process 1000.
• the process 1200 can be
• 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 CPU 202, may cause the computing device to perform the process 1200.
• the process 1200 can be implemented using specialized hardware or firmware. As described above, multiple processors, memories, or both, may be used.
• all pixels within the current frame are assigned an initialized motion vector. They define initial motion fields that can be utilized to warp the reference frames to the current frame for a first processing level to shorten the motion lengths between reference frames.
• the initialization at 1202 can be performed using the same processing as described with regard to the initialization at 1102, so the description is not repeated here.
• the reference frames such as the reference frames 1 and 2
• the warping at 1204 can be performed using the same processing as described with regard to the warping at 1106 except that, desirably, the motion field mv cur initialized at 1202 is not downscaled from its full resolution value before warping the reference frames.
• the process 1200 can estimate the motion field between the two reference frames using a multi-level process similar to that described with regard to FIG. 13. Broadly stated, the process 1200 calculates the derivatives for a level, performs an optical flow estimation using the derivatives, and upscales the resulting motion field for the next level until all levels are considered.
• the motion field mv cur for a block at the current (or first) processing level is initialized at 1206.
• the block may be a block of the current frame selected in a scan order (e.g., a raster scan order) of the current frame.
• the motion field mv cur for the block comprises the motion field for respective pixels of the block.
• all pixels with a current block are assigned an initialized motion vector.
• the initialized motion vectors are used to warp reference blocks to the current block to shorten the lengths between the reference blocks in the reference frames.
• the motion field mv cur is downscaled from its full resolution value to the resolution of the level.
• the initialization at 1206 may comprise downscaling the motion field for respective pixels of the block from the full resolution values that were initialized at 1202. Downscaling may be performed using any technique, such as the downscaling described above.
• co-located reference blocks corresponding to the motion field in each of the warped reference frames are warped to the current block. Warping the reference blocks is performed similarly to the process 1100 at 1106. That is, knowing the optical flow mv cur of pixels of the reference block in reference frame 1, the motion field for warping is inferred by the linear projection assumption (e.g., that the motion projects linearly over time) as follows:
• the horizontal component u rl and the vertical component u rl of the motion field mv rl may be rounded to 1/8 pixel precision for the Y component and 1/16 pixel precision for the U and V component. Other values may be used.
• each pixel in a warped block e.g., E ⁇ , a . ped , is calculated as the referenced pixel given by the motion vector mv rl .
• Subpixel interpolation may be performed using a conventional subpixel interpolation filter.
• mv r2 (index r2 — index cur ) / (index r2 — index rl ) ⁇ mv cur
• the two warped reference blocks may be at full resolution.
• the derivatives E x , E y , and E t are calculated using the functions (3), (4), and (5).
• the frame boundaries may be expanded by copying the nearest available pixel to obtain out-of-bound pixel values as described with regard to the process 1100.
• neighboring pixels are often available in the reference frames warped at 1204.
• the pixels of neighboring blocks are available in the warped reference frames unless the block itself is at a frame boundary.
• pixels in neighboring portions of the warped reference frame may be used as the pixel values E ⁇ 1 ⁇ and E ⁇ r2 as applicable. If the projected pixels are outside of the frame boundaries, copying the nearest available (i.e., within bounds) pixel may still be used.
• the derivatives may be downscaled to the current level.
• the downscaled derivatives at each level 1 may calculated by averaging within a 2 1 by 2 1 block, as discussed previously. The complexity of the calculations may be reduced by combining the two linear operations of calculating and averaging the derivatives in a single linear filter, but this is not required.
• the downscaled derivatives can be used as inputs to the Lagrangian function (1) to perform optical flow estimation to estimate the motion field between the warped reference portions.
• One way is to assume zero correlation with neighboring blocks and assume a out-of-bound motion vector is the same as a motion vector at the nearest boundary location to the out-of-bound pixel location.
• Another way is to use the initialized motion vector for the current pixel (i.e., the motion field initialized at 1206) as the motion vector for an out-of-bound pixel location corresponding to the current pixel.
• the current motion field for the level is updated or refined using the estimation motion field between the warped reference blocks to complete the processing at 1210.
• the current motion field for a pixel may be updated by adding the estimated motion field for a pixel on a pixel-by-pixel basis.
• an additional loop is included to set decreasing values for the Lagrangian parameter ⁇ such that, at each level, the motion field is estimated and refined using increasingly smaller values for the Lagrangian parameter ⁇ .
• this loop is omitted. That is, in the process 1200 as shown, only one value for the Lagrangian parameter ⁇ is used to estimate the motion field at a current processing level. This can be a relatively small value, such as 25.
• Other values for the Lagrangian parameter ⁇ are possible, e.g., depending upon the smoothness of the motion, the image resolution, or other variables.
• the process 1200 can include the additional loop for varying the Lagrangian parameter ⁇ .
• the Lagrangian parameter ⁇ may be set before estimating the motion field at 1210 such that warping the reference blocks at 1208 and estimating and updating the motion field at 1210 are repeated until all values for the Lagrangian parameter ⁇ have been used as described with respect to the processing at 1104 and 1110 in the process 1100.
• the process 1200 advances to the query of 1212 after estimating and updating the motion field at 1210. This is done after the first and only motion field estimation and update at a level at 1210 when a single value for the Lagrangian parameter ⁇ is used. When multiple values for the Lagrangian parameter ⁇ are modified at a processing level, the process 1200 advances to the query of 1212 after estimating and updating the motion field at 1210 using the final value for the Lagrangian parameter ⁇ .
• the process 1200 advances to 1214, where the motion field is up-scaled before processing the next layer starting at 1206. Upscaling may be performed according to any known technique.
• the optical flow is first estimated to obtain a motion field at the highest level of the pyramid. Thereafter, the motion field is upscaled and used to initialize the optical flow estimation at the next level. This process of upscaling the motion field, using it to initialize the optical flow estimation of the next level, and obtaining the motion field continues until the lowest level of the pyramid is reached (i.e., until the optical flow estimation is completed for the derivatives calculated at full scale) at 1212.
• the process 1200 advances to 1216.
• the number of levels can be three, such as in the example of FIG. 13.
• the warped reference blocks are blended to form an optical flow reference block (e.g., E ⁇ cur ⁇ as described previously).
• the warped reference blocks blended at 1216 may be the full-scale reference blocks that are warped again according to the process described at 1208 using the motion field estimated at 1210.
• the full-scale reference blocks may be warped twice - once using the initial up-scaled motion field from the previous layer of processing and again after the motion field is refined at the full-scale level.
• the blending may be performed using the time linearity assumption similarly to the processing described at 1116.
• Optional occlusion detection as described at 1116 and shown by example in FIG. 14 may be incorporated as part of the blending at 1216.
• the process 1200 advances to 1218 to determine whether there are further frame portions (here, blocks) for prediction. If so, the process 1200 repeats starting at 1206 for the next block. The blocks may be processed in the scan order. Once there are no further blocks to consider in response to the query at 1218, the process 1200 ends.
• the process 1200 can implement 1006 in the process 1000. At the end of processing at 1006, whether performed according to the process 1100, the process 1200, or a variation of either as described herein, one or more warped reference frame portions exist.
• Performing a prediction process at an encoder can include generating a prediction block from an optical flow reference frame for a current block of the frame.
• the optical flow reference frame can be the optical flow reference frame output by the process 1100 and stored in a reference frame buffer, such as the reference frame buffer 600.
• the optical flow reference frame can be an optical flow reference frame generated by combining the optical flow reference portions output by the process 1200.
• Combining the optical flow reference portions may include arranging the optical flow reference portions (e.g., co-located reference blocks) according to the pixel positions of the respective current frame portions used in the generation of the each of the optical flow reference portions.
• the resulting optical flow reference frame can be stored for use in a reference frame buffer of the encoder, such as the reference frame buffer 600 of the encoder 400.
• Generating the prediction block at an encoder can include selecting the co-located block in the optical flow reference frame as the prediction block. Generating the prediction block at an encoder can instead include performing a motion search within the optical flow reference frame to select the best matching prediction block for the current block. However the prediction block is generated at the encoder, the resulting residual can be further processed, such as using the lossy encoding process described with regard to the encoder 400 of FIG. 4.
• the process 1000 may form part of a rate distortion loop for the current block that uses various prediction modes, including one or more intra prediction modes and both single and compound inter prediction modes using the available prediction frames for the current frame.
• a single inter prediction mode uses only a single forward or backward reference frame for inter prediction.
• a compound inter prediction mode uses both a forward and a backward reference frame for inter prediction.
• the rate e.g., the number of bits
• the distortion may be calculated as the differences between pixel values of the block before encoding and after decoding. The differences can be a sum of absolute differences or some other measure that captures the accumulated error for blocks of the frames.
• the optical flow reference frame may be excluded as a reference frame in any compound reference mode. This can simplify the rate distortion loop, and little additional impact on the encoding of a block is expected because the optical flow reference frame already considers both a forward and a backward reference frame.
• a flag may be encoded into the bitstream to indicate whether or not an optical flow reference frame is available for use in encoding the current frame. The flag may be encoded when any single block within the current frame is encoded using an optical flow reference frame block in an example. Where the optical flow reference frame is available for a current frame, it is possible to include an additional flag or other indicator (e.g., at the block level) indicating whether or not the current block was encoded by inter prediction using the optical flow reference frame.
• the prediction process at 1008 may be repeated for all blocks of the current frame until the current frame is encoded.
• performing a prediction process using the optical flow reference frame portion at 1008 may result from a determination that an optical flow reference frame is available for decoding the current frame.
• the determination is made by inspecting a flag that indicates that at least one block of the current frame was encoded using an optical flow reference frame portion.
• Performing the prediction process at 1008 in the decoder can include generating a prediction block. Generating the prediction block can include using an inter-prediction mode decoded from the encoded bitstream, such as in a block header. A flag or indicator can be decoded to determine the inter-prediction mode.
• the prediction block for the current block to be decoded is generated using pixels of an optical flow reference frame portion and a motion vector mode and/or a motion vector.
• the same processing to produce an optical flow reference frame for use in a prediction process as part of decoding may be performed at a decoder, such as the decoder 500, as was performed at the encoder.
• a decoder such as the decoder 500
• an entire optical flow reference frame can be generated and stored for use in the prediction process.
• additional savings in computational power at the decoder by modifying the process 1200 to limit performance of the process 1200 where coding blocks are identified as using the co- located/optical flow reference frame as an inter-prediction reference frame. This may be described by reference to FIG. 15, which is a diagram illustrating one technique for optimizing a decoder.
• pixels are shown along a grid 1500 with w representing a pixel location along a first axis of the grid 1500 and with y representing a pixel location along a second axis of the grid 1500.
• the grid 1500 represents pixel locations of a portion of the current frame.
• the prediction process at 1008 can include finding the reference block used to encode the current block (e.g., from header information, such as a motion vector).
• the motion vector for the current coding block 1502 points to a reference block represented by the inner dashed line 1504.
• the current coding block 1502 comprises 4x4 pixels.
• the reference block location is shown by the dashed line 1504 because the reference block is located in the reference frame, and not in the current frame.
• the reference block is located, all of the reference blocks that are spanned by (i.e., overlap) the reference block are identified. This may include extending the reference block size by half of the filter length at each boundary to consider sub-pixel interpolation filters.
• the sub-pixel interpolation filter length L is used to extend the reference block to the boundaries represented by the outer dashed line 1506.
• the motion vector results in a reference block that does not align perfectly with the full-pel locations.
• the darkened area in FIG. 15 represents the full-pel locations. All of the reference blocks that overlap the full-pel locations are identified.
• a first reference block that is co-located with the current block Assuming the block sizes are the same as the current coding block 1502, a first reference block that is co-located with the current block, a second reference block that is above the first reference block, two reference blocks that extend from the left of the first reference block, and two reference blocks that extend from the left of the second reference block are identified.
• the process 1200 may be performed at 1006 for only the blocks within the current frame that are co-located with the identified reference blocks so as to produce the co-located/optical flow estimated reference blocks. In the example of FIG. 15, this would result in six optical flow reference frame portions.
• reference block(s) for a subsequent block may overlap one or more reference blocks identified in the decoding process of the current block.
• optical flow estimation need be performed for any of the identified blocks only once to further reduce computing requirements at the decoder.
• a reference block generated at 1216 may be stored for use in decoding other blocks of the current frame.
• the prediction block is generated at the decoder, the decoded residual for the current block from the encoded bitstream can be combined with the prediction block to form a reconstructed block as described by example with regard to the decoder 500 of FIG. 5.
• the prediction process at 1008, whether performed after or in conjunction with the process 1200, may be repeated for all blocks of the current frame that were encoded using an optical flow reference frame portion until the current frame decoded.
• a block that is not encoded using an optical flow reference frame portion can be conventionally decoded according to the prediction mode decoded for the block from the encoded bitstream.
• the complexity of solving the optical flow formulation may be represented by 0(N*M), where M is the number of iterations to solve the linear equations. M is not related to the number of levels, or the number of values for the Lagrangian parameter ⁇ . Instead, M is related to the calculation precision in solving the linear equations. A larger value for M results in better precision. Given this complexity, moving from frame-level to sub-frame-level (e.g., block-based) estimation provides several options for reducing the decoder complexity. First, and because the constraint of motion field smoothness is relaxed at block boundaries, it is easier to converge to a solution when solving the linear equations for a block, resulting in a smaller M for a similar precision.
• each of the processes 1000, 1100, and 1200 is depicted and described 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 method in accordance with the disclosed subject matter.
• 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 preferred or advantageous over other aspects or designs. Rather, use of the word “example” is intended to present concepts in a concrete fashion.
• the term “or” is intended to mean an inclusive “or” rather than an exclusive “or”. That is, unless specified otherwise, or clear from context, "X includes A or B” is intended to mean any of the natural inclusive permutations. 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.
• Implementations of the transmitting station 102 and/or the receiving station 106 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.
• IP intellectual property
• ASIC application-specific integrated circuits
• programmable logic arrays optical processors
• programmable logic controllers microcode, microcontrollers
• servers microprocessors, digital signal processors or any other suitable circuit.
• signal processors digital signal processors or any other suitable circuit.
• 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.
• a special purpose computer/processor can be utilized that contains other hardware for carrying out any of the methods, algorithms, or instructions described herein.
• the transmitting station 102 and the receiving station 106 can, for example, be implemented on computers in a video conferencing system.
• 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 hand-held communications device.
• the transmitting station 102 can encode content using an encoder 400 into an encoded video signal and transmit the encoded video signal to the communications device.
• the communications device can then decode the encoded video signal using a decoder 500.
• the communications device can decode content stored locally on the communications device.
• 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.
• 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 a semiconductor device. Other suitable mediums are also available.
• Example 1 A method, comprising: determining a first frame to be predicted in a video sequence; determining a first reference frame from the video sequence for forward inter prediction of the first frame; determining a second reference frame from the video sequence for backward inter prediction of the first frame; generating an optical flow reference frame for inter prediction of the first frame by performing an optical flow estimation using the first reference frame and the second reference frame; and performing a prediction process for the first frame using the optical flow reference frame.
• Example 2 The method of Example 1, wherein generating the optical flow reference frame comprises: performing the optical flow estimation by minimizing a Lagrangian function for respective pixels of the first frame.
• Example 3 The method of Example 1 or 2, wherein the optical flow estimation produces a respective motion field for pixels of the first frame, and generating the optical flow reference frame comprises: warping the first reference frame to the first frame using the motion fields to form a first warped reference frame; warping the second reference frame to the first frame using the motion fields to form a second warped reference frame; and blending the first warped reference frame and the second warped reference frame to form the optical flow reference frame.
• Example 4 The method of Example 3, wherein blending the first warped reference frame and the second warped reference frame comprises: combining co-located pixel values of the first warped reference frame and the second warped reference frame by scaling the co-located pixel values using distances between the first reference frame and the second reference frame and between the current frame and each of the first reference frame and the second reference frame.
• Example 5 The method of Example 3 or 4, wherein blending the first warped reference frame and the second warped reference frame comprises: populating pixel positions of the optical flow reference frame by one of combining co-located pixel values of the first warped reference frame and the second warped reference frame or using a single pixel value of one or the first warped reference frame or the second warped reference frame.
• Example 6 The method of any of Examples 3 to 5, further comprising: detecting an occlusion in the first reference frame using the first warped reference frame and the second warped reference frame, wherein blending the first warped reference frame and the second warped reference frame comprises: populating pixel positions of the optical flow reference frame corresponding to the occlusion with pixel values from the second warped reference frame.
• Example 7 The method of any of Examples 1 to 6, wherein performing the prediction process comprises: using the optical flow reference frame only for single reference inter prediction of blocks of the first frame.
• Example 8 The method of any of Examples 1 to 7, wherein the first reference frame is a nearest reconstructed frame in a display order of the video sequence to the first frame that is available for forward inter prediction of the first frame, and the second reference frame is a nearest reconstructed frame in the display order to the first frame that is available for backward inter prediction of the first frame.
• Example 9 The method of any of Examples 1 to 8, wherein performing the prediction process comprises: determining a reference block within the optical flow reference frame that is co-located with a first block of the first frame; and encoding a residual of the reference block and the first block.
• Example 10 An apparatus, comprising: a processor; and a non-transitory storage medium that includes instructions executable by the processor to carry out a method comprising: determining a first frame to be predicted in a video sequence; determining an availability of a first reference frame for forward inter prediction of the first frame and a second reference frame for backward inter prediction of the first frame; responsive to determining the availability of both the first reference frame and the second reference frame: generating a respective motion field for pixels of the first frame using the first reference frame and the second reference frame using optical flow estimation; warping the first reference frame to the first frame using the motion fields to form a first warped reference frame; warping the second reference frame to the first frame using the motion fields to form a second warped reference frame; and blending the first warped reference frame and the second warped reference frame to form the optical flow reference frame for inter prediction of blocks of the first frame.
• Example 11 The apparatus of Example 10, wherein the method further comprises: performing a prediction process for the first frame using the optical flow reference frame.
• Example 12 The apparatus of Example 10 or 11, wherein the method further comprises: using the optical flow reference frame only for single reference inter prediction of blocks of the first frame.
• Example 13 The apparatus of any of Examples 10 to 12, wherein generating a respective motion field comprises: calculating an output of a Lagrangian function for respective pixels of the first frame using the first reference frame and the second reference frame.
• Example 14 The apparatus of Example 13, wherein calculating the output of the Lagrangian function comprises: calculating a first set of motion fields for the pixels of the current frame using a first value for a Lagrangian parameter; and using the first set of motion fields as input to the Lagrangian function using a second value for the Lagrangian parameter to calculate a refined set of motion fields for the pixels of the current frame, wherein the second value for the Lagrangian parameter is smaller than the first value for the Lagrangian parameter, and the first warped reference frame and the second warped reference are warped using the refined set of motion fields.
• Example 15 An apparatus, comprising: a processor; and a non-transitory storage medium that includes instructions executable by the processor to carry out a method comprising: generating an optical flow reference frame for inter prediction of a first frame of a video sequence using a first reference frame from the video sequence and a second reference frame of the video sequence by: initializing motion fields for pixels of the first frame in a first processing level for an optical flow estimation, the first processing level representing downscaled motion within the first frame and comprising one level of multiple levels; for each level of the multiple levels: warping the first reference frame to the first frame using the motion fields to form a first warped reference frame; warping the second reference frame to the first frame using the motion fields to form a second warped reference frame; estimating motion fields between the first warped reference frame and the second warped reference frame using the optical flow
• Example 16 The apparatus of Example 15, wherein the optical flow estimation uses a Lagrangian function for respective pixels of a frame.
• Example 17 The apparatus of Example 16, wherein the method further comprises, for each level of the multiple levels: initializing a Lagrangian parameter in the Lagrangian function to a maximum value for a first iteration of warping the first reference frame, warping the second reference frame, estimating the motion fields, and updating the motion fields; and performing an additional iteration of warping the first reference frame, warping the second reference frame, estimating the motion fields, and estimating the motion fields using increasingly smaller values of a set of possible values for the Lagrangian parameter.
• Example 18 The apparatus of Example 16 or 17, wherein estimating the motion fields comprises: calculating derivatives of pixel values of the first warped reference frame and the second warped reference frame with respect to a horizontal axis, a vertical axis, and time; downscaling the derivatives responsive to the level being different from the final level; solving linear equations representing the Lagrangian function using the derivatives.
• Example 19 The apparatus of any of Examples 15 to 18, wherein the method further comprises: inter predicting the first frame using the optical flow reference frame.
• Example 20 The apparatus of any of Examples 15 to 19, wherein the processor and the non-transitory storage medium form a decoder.

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• 2018
  • 2018-05-10 EP EP18730489.4A patent/EP3673655A1/de active Pending
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