1) Enhanced directional intra prediction

To exploit more varieties of spatial redundancy in directional textures, in AV1, directional intra modes are extended to an angle set with finer granularity for blocks larger than $8 \times 8$. The original eight angles are made nominal angles, based on which fine angle variations in a step size of 3 degrees are introduced, i.e. the prediction angle is presented by a nominal intra angle plus an angle delta, which is $-3 \times 3$ multiples of the step size. To implement directional prediction modes in AV1 via a generic way, the 48 extension modes are realized by a unified directional predictor that links each pixel to a reference sub-pixel location in the edge and interpolates the reference pixel by a 2-tap bilinear filter. In total, there are 56 directional intra modes supported in AV1.

Another enhancement for directional intra prediction in AV1 is that, a low-pass filter is applied to the reference pixel values before they are used to predict the target block. The filter strength is pre-defined based on the prediction angle and block size.

2) New non-directional smooth intra predictors

VP9 has two non-directional intra prediction modes: DC_PRED and TM_PRED. AV1 expands on this by adding three new prediction modes: SMOOTH_PRED, SMOOTH_V_PRED, and SMOOTH_H_PRED. Also a fourth new prediction mode PAETH_PRED [19] replaces the existing mode TM_PRED. The new modes work as follows:

• • SMOOTH_PRED: Useful for predicting blocks that have a smooth gradient. It works as follows: estimate the pixels on the rightmost column with the value of the last pixel in the top row, and estimate the pixels in the last row of the current block using the last pixel in the left column. Then calculate the rest of the pixels by an average of quadratic interpolation in vertical and horizontal directions, based on distance of the pixel from the predicted pixels.

• • SMOOTH_V_PRED: Similar to SMOOTH_PRED, but uses quadratic interpolation only in the vertical direction.

• • SMOOTH_H_PRED: Similar to SMOOTH_PRED, but uses quadratic interpolation only in the horizontal direction.

• • PAETH_PRED: Calculate $base=left+top-top\_left$. Then predict this pixel as left, top, or top-left pixel depending on which of them is closest to “base”. The idea is: (i) if the estimated gradient is larger in horizontal direction, then we predict the pixel from “top”; (ii) if it is larger in vertical direction, then we predict pixel from “left”; otherwise (iii) if the two are the same, we predict the pixel from “top-left”.

3) Recursive-filtering-based intra predictor

To capture decaying spatial correlation with references on the edges, FILTER_INTRA modes are designed for luma blocks by viewing blocks as 2-D non-separable Markov models. Five filter intra modes are pre-designed for AV1, each represented by a set of eight 7-tap filters reflecting correlation between pixels in a $4 \times 2$ patch and the seven neighbors adjacent to the patch (e.g. $p_0 - p_6$ for the blue patch in Fig. 2). An intra block can pick one filter intra mode, and be predicted in batches of $4 \times 2$ patches. Each patch is predicted via the selected set of 7-tap filters weighting the neighbors differently at the 8 pixel locations. For those $4 \times 2$ units not fully attached to references on block boundary, e.g. the green patch in Fig. 2, predicted values of the immediate neighbors are used as the reference, meaning prediction is computed recursively among the $4 \times 2$ patches so as to combine more edge pixels at remote locations.

Fig. 2. Recursive-filter-based intra predictor.

4) Chroma predicted from Luma

Chroma from Luma (CfL) is a chroma-only intra predictor that models chroma pixels as a linear function of coincident reconstructed luma pixels. The predicted chroma pixels are obtained by adding the DC prediction to the scaled AC contribution. The DC prediction is computed using DC intra prediction mode on neighboring reconstructed chroma pixels. This is sufficient for most chroma content and has fast and mature implementations.

The scaled AC contribution is the result of multiplying the zero-mean-subsampled coincident reconstructed luma pixels by a scaling factor signaled in the bitstream. The subsampling step and average subtraction are combined to reduce the number of operations and also reduces rounding error.A scaling factors is signaled for each chroma plane but they are jointly-coded. Signaling scaling factors reduces decoder complexity and yields more precise RD-optimal predictions. Refer to Fig. 3 and [Reference Trudeau, Egge and Barr20] for more information.

Fig. 3. Outline of the operations required to build the proposed CfL prediction [Reference Trudeau, Egge and Barr20].

5) Color palette as a predictor

Sometimes, especially for artificial videos like screen capture and games, blocks can be approximated by a small number of unique colors. Therefore, AV1 introduces the palette mode to the intra coder as a general extra coding tool. The palette predictor for each plane of a block is specified by (i) a color palette, with 2–8 colors, and (ii) color indices for all pixels in the block. The number of base colors determines the trade-off between fidelity and compactness. The base colors of a block are transmitted in the bitstream by referencing to the base colors of neighboring blocks. The base colors that are not present in the neighboring blocks' palettes are then delta-encoded. The color indices are entropy coded pixel-by-pixel, using neighborhood-based contexts. The luma and chroma channels can decide whether to use the palette mode independently. For the luma channel, each entry in the palette is a scalar value; for the chroma channels, each entry in the palate is a two-dimensional tuple. After the prediction of a block is established with the palette mode, transform coding and quantization is applied to the residue block, just like the other intra prediction modes.

6) Intra block copy

AV1 allows its intra coder to refer back to previously reconstructed blocks in the same frame, in a manner similar to how inter coder refers to blocks from previous frames. It can be very beneficial for screen content videos which typically contain repeated textures, patterns, and characters in the same frame. Specifically, a new prediction mode named IntraBC is introduced, and will copy a reconstructed block in the current frame as prediction. The location of the reference block is specified by a displacement vector in a way similar to motion vector compression in motion compensation. Displacement vectors are in whole pixels for the luma plane, and may refer to half-pel positions on corresponding chrominance planes, where bilinear filtering is applied for sub-pel interpolation.

The IntraBC mode is only available for keyframes or intra-only frames. It can be turned on and off by a frame-level flag. The IntraBC mode cannot refer to pixels outsize of current tile. To facilitate hardware implementations, there are certain additional constraints on the reference areas. For example, there is a 256-horizontal-pixel-delay between current superblock and the most recent superblock that IntraBC may refer to. Another constraint is that when the IntraBC mode is enabled for the current frame, all the in-loop filters, including deblocking filters, loop-restoration filters, and the CDEF filters, must be turned off. Despite all these constraints, the IntraBC mode still brings significant compression improvement for screen content videos.

1) Extended reference frames

AV1 extends the number of references for each frame from 3 to 7. In addition to VP9's LAST(nearest past) frame, GOLDEN(distant past) frame and ALTREF(temporal filtered future) frame, we add two near past frames (LAST2 and LAST3) and two future frames (BWDREF and ALTREF2) [Reference Lin21]. Figure 4 demonstrates the multi-layer structure of a golden-frame group, in which an adaptive number of frames share the same GOLDEN and ALTREF frames. BWDREF is a look-ahead frame directly coded without applying temporal filtering, thus more applicable as a backward reference in a relatively shorter distance. ALTREF2 serves as an intermediate filtered future reference between GOLDEN and ALTREF. All the new references can be picked by a single prediction mode or be combined into a pair to form a compound mode. AV1 provides an abundant set of reference frame pairs, providing both bi-directional compound prediction and uni-directional compound prediction, thus can encode a variety of videos with dynamic temporal correlation characteristics in a more adaptive and optimal way.

Fig. 4. Example of multi-layer structure of a golden-frame group [Reference Chen32].

2) Dynamic spatial and temporal motion vector referencing

Efficient motion vector (MV) coding is crucial to a video codec because it takes a large portion of the rate cost for inter frames. To that end, AV1 incorporates a sophisticated MV reference selection scheme to obtain good MV references for a given block by searching both spatial and temporal candidates. AV1 not only searches a wider spatial neighborhood than VP9 to construct a spatial candidate pool, but also utilizes a motion field estimation mechanism to generate temporal candidates. The motion field estimation process works in three stages: motion vector buffering, motion trajectory creation, and motion vector projection. First, for each coded frame, we store its reference frame indices and the associated motion vectors. This information will be referenced by next coding frame to generate its motion field. The motion field estimation examines the motion trajectories, e.g. $MV_{Ref2}$ in Fig. 5 pointing a block in one reference frame $Ref2$ to another reference frame $Ref0_{Ref2}$. It searches through the collocated $128 \times 128$ area to find all motion trajectories in $8 \times 8$ block resolution that pass each $64 \times 64$ processing unit. Next, at the coding block level, once the reference frame(s) have been determined, motion vector candidates are derived by linearly project passing motion trajectories onto the desired reference frames, e.g. converting $MV_{Ref2}$ in Fig. 5 to $MV_0$ or $MV_1$. Once all spatial and temporal candidates have been aggregated in the pool, they are sorted, merged, and ranked to obtain up to four final candidates [Reference Han, Xu and Bankoski22, Reference Han, Feng, Teng, Xu and Bankoski23]. The scoring scheme relies on calculating a likelihood of the current block having a particular MV as a candidate. To code an MV, AV1 signals the index of a selected reference MV from the list, followed by encoding the motion vector difference if needed.

Fig. 5. Motion field estimation [Reference Chen32].

3) Overlapped block motion compensation (OBMC)

OBMC can largely decrease prediction errors near block edges by smoothly combining predictions created from adjacent motion vectors. In AV1, a two-sided causal overlapping algorithm is designed to make OBMC easily fit in the advanced partitioning framework [Reference Chen and Mukherjee24]. It progressively combines the block-based prediction with secondary inter predictors in the above edge and then in the left, by applying predefined 1-D filters in vertical and horizontal directions. The secondary predictors only operate in restricted overlapping regions in top/left halves of the current block, so that they do not tangle with each other on the same side (see Fig. 6). AV1 OBMC is only enabled for blocks using a single reference frame, and only works with the first predictor of any neighbor with two reference frames, therefore the worst-case memory bandwidth is the same as what is demanded by a traditional compound predictor.

Fig. 6. Overlapping regions defined for AV1 OBMC.

4) Warped motion compensation

Warped motion models are explored in AV1 through two affine prediction modes, global and local warped motion compensation [Reference Parker, Chen and Mukherjee25]. The global motion tool is meant to handle camera motion, and allows frame-level signaling of an affine model between a frame and each reference. The local warped motion tool aims to capture varying local motion implicitly, using minimal overhead. Here, the model parameters are derived at the block level from 2D motion vectors signaled within the causal neighborhood. Both coding tools compete with translational modes at the block level, and are selected only if there is an advantage in RD cost. Additionally, affine models are limited to small degrees so that they can be implemented efficiently in SIMD and hardware through a consecutive horizontal and vertical shear (Fig. 7), each using an 8-tap interpolation filter at 1/64 pixel precision.

Fig. 7. Affine warping in two shears [Reference Chen32].

5) Advanced compound prediction

A collection of new compound prediction tools is developed for AV1 to make its inter coder more versatile. In this section, any compound prediction operation can be generalized for a pixel $(i, j)$ as: $p_f(i, j) = m(i, j) p_1(i, j) + (1 - m(i, j)) p_2(i, j)$, where $p_1$ and $p_2$ are two predictors, and $p_f$ is the final joint prediction, with the weighting coefficients $m(i, j)$ in $[0, 1]$ that are designed for different use cases and can be easily generated from predefined tables [Reference Joshi26].

• • Compound wedge prediction: Boundaries of moving objects are often difficult to be approximated by on-grid block partitions. The solution in AV1 is to predefine a codebook of 16 possible wedge partitions and to signal the wedge index in the bitstream when a coding unit chooses to be further partitioned in such a way. The 16-ary shape codebooks containing partition orientations that are either horizontal, vertical, or oblique with slopes ±2 or ±0.5, are designed for both square and rectangular blocks as shown in Fig. 8. To mitigate spurious high-frequency components, which often are produced by directly juxtaposing two predictors, soft-cliff-shaped 2-D wedge masks are employed to smooth the edges around the intended partition, i.e. $m(i, j)$ is close to 0.5 around the edges, and gradually transforms into binary weights at either end.

• • Difference-modulated masked prediction: In many cases, regions in one predictor will contain useful content that is not present in the second predictor. For example, one predictor might include information that was previously occluded by a moving object. In these instances, it is useful to allow some regions of the final prediction to come more heavily from one predictor than the other. AV1 provides the option to use a non-uniform mask where the pixel difference between $p_1$ and $p_2$ is used to modulate mask weights over some base value. The mask is generated by: $m(i, j) = b + a\vert p_1(i, j) - p_2(i, j)\vert$ where b controls how strongly one predictor is weighted over the other within the differing regions and a scaling factor a ensures a smooth modulation.

• • Frame distance-based compound prediction: Besides non-uniform weights, AV1 also utilizes a modified uniform weighting scheme by accounting for frame distances. Frame distance is defined as the absolute difference between timestamps of two frames. Intuitively if one reference frame is right next to a current frame and the other frame located more distant from current frame, it is expected that the reference block from the first frame has higher correlation with the current block and hence should be weighted more than the other. Let $d_1$ and $d_2$ ($d_1\geq d_2$) represent distances from current frame to reference frames, from which $p_1$ and $p_2$ are computed. $w_1$ and $w_2$ are weights derived from $d_1$ and $d_2$. The most natural scheme is that weights are proportional to the reciprocal of frame distances, i.e. $w_1 / w_2 = d_2 / d_1$. However, a close observation reveals that the compound prediction should carry two major functionalities: exploiting the temporal correlation in the video signal and canceling the quantization noise from the reconstructed reference frames. The linear scheme does not take quantization noise into consideration. In a hierarchical coding structure with multiple reference frames, the relative distances of two reference frames from a current frame could differ substantially. A linear model would make the weight assigned to the block from a more distant frame too small to neutralize the quantization noise. On the other hand, the traditional average weighting, while not always tracking the temporal correlation closely, in general performs well to reduce the quantization noise. To balance these two factors, AV1 employs a modified weighting scheme derived based on the frame distances to give slightly more weight toward the distant predictor. The codebook is experimentally obtained and fixed in AV1:

  (1)\begin{align} p & = {\rm round}(w_1 * p_1 + w_2 * p_2 + 8) \gg 4 \notag\\ (w_1,w_2) & = \begin{cases} (9, 7), & {\rm if}\ 2 d_2 < 3 d_1 \\ (11, 5), & {\rm if}\ 2 d_2 \lt 5 d_1 \\ (12, 4), & {\rm if}\ 2 d_2 \lt 7 d_1 \\ (13, 3), & {\rm if}\ 2 d_2 \geq 7 d_1 \end{cases} \end{align}
  The advantage of such quantized weighting coefficient scheme is that it effectively embeds certain attenuation of the quantization noise, especially when $d_1$ and $d_2$ are far apart. As a complementary mode to the traditional average mode, AV1 adopts a hybrid scheme, where the codec could switch between the frame distance-based mode and the averaging mode, at coding block level, based on the encoder's rate-distortion optimization decision.

• • Compound inter-intra prediction: Compound inter-intra prediction modes, which combine intra prediction $p_1$ and single-reference inter prediction $p_2$, are developed to handle areas with emerging new content and old objects mixed. For the intra part, fourfrequently-used intra modes are supported. The mask $m(i, j)$ incorporates two types of smoothing functions: (i) smooth wedge masks similar to what is designed for wedge inter-inter modes, (ii) mode-dependent masks that weight $p_1$ in a decaying pattern oriented by the primary direction of the intra mode.

Fig. 8. Wedge codebooks for square and rectangular blocks [Reference Chen32].

1) Transform block partition

Instead of enforcing fixed transform unit sizes as in VP9, AV1 allows luma inter coding blocks to be partitioned into transform units of multiple sizes that can be represented by a recursive partition going down by up to two levels. To incorporate AV1's extended coding block partitions, we support square, 2:1/1:2, and 4:1/1:4 transform sizes from 4 × 4 to 64 × 64. For chroma blocks, only the largest possible transform units are allowed.

2) Extended transform Kernels

A richer set of transform kernels is defined for intra and inter blocks in AV1. The full 2-D kernel set is generated from horizontal/vertical combinations of four 1-D transform types, yielding 16 total kernel options [Reference Parker27]. The 1-D transform types are DCT and ADST, which have been used in VP9, flipADST, which applies ADST in reverse order, and the identity transform, which skips transform coding in order to preserve sharp edges. In practice, several of these kernels give similar results at large block sizes, allowing the gradual reduction of possible kernel types as transform size increases.

1) Multi-symbol entropy coding

VP9 used a tree-based boolean non-adaptive binary arithmetic encoder to encode all syntax elements. AV1 moves to using a symbol-to-symbol adaptive multi-symbol arithmetic coder. Each syntax element in AV1 is a member of a specific alphabet of N elements, and a context consists of a set of N probabilities together with a small count to facilitate fast early adaptation. The probabilities are stored as 15-bit cumulative distribution functions (CDFs). The higher precision than a binary arithmetic encoderenables tracking probabilities of less common elements of an alphabet accurately. Probabilities are adapted by simple recursive scaling, with an update factor based on the alphabet size. Since the symbol bitrate is dominated by encoding coefficients, motion vectors, and prediction modes, all of which use alphabets larger than 2, this design in effect achieves more than a factor of 2 reduction in throughput for typical coding scenarios over pure binary arithmetic coding.

In hardware, the complexity is dominated by throughput and size of the core multiplier that rescales the arithmetic coding state interval. The higher precision required for tracking probabilities is not actually required for coding. This allows reducing the multiplier size substantially by rounding from 16 × 15 bits to an 8 × 9 bit multiplier. This rounding is facilitated by enforcing a minimum interval size, which in turn allows a simplified probability update in which values may become zero. In software, the operation count is more important than complexity, and reducing throughput and simplifying updates correspondingly reduces fixed overheads of each coding/decoding operation.

2) Level map coefficient coding

In VP9, the coding engine processes each quantized transform coefficient sequentially following the scan order. The probability model used for each coefficient is contexted on the previously coded coefficient levels, its frequency band, transform block sizes, etc. To properly capture the coefficient distribution in the vast cardinality space, AV1 alters to a level map design for sizeable transform coefficient modeling and compression [Reference Han, Chiang and Xu28]. It builds on the observation that the lower coefficient levels typically account for the major rate cost.

For each transform unit, AV1 coefficient coder starts with coding a skip sign, which will be followed by the transform kernel type and the ending position of all non-zero coefficients when the transform coding is not skipped. Then coefficients are broken down into sign plane and three level planes, where the sign plane cover the signs of coefficients and the three level planes correspond to different ranges of coefficient magnitudes. After the ending position is coded, the lower level and middle level planes are coded together in reverse scan order. Then the sign plane and higher level plane are coded together in forward scan order. The lower level plane corresponds to the range of 0–2, the middle level plane takes care of the range of 3–14, and the higher level plane covers the range of 15 and above.

Such separation allows one to assign a rich context model to the lower level plane, which accounts the transform directions: bi-directional, horizontal, and vertical; transform size; and up to five neighbor coefficients for improved compression efficiency, at the modest context model size. The middle level plane uses a context model similar to the lower level plane with number of context neighbor coefficients being reduced from 5 to 2. The higher level plane is coded by Exp-Golomb code without using context model. In the sign plane, except that the DC sign is coded using its neighbor transform units's dc signs as context information, other sign bits are coded directly without using context model.

1) Constrained directional enhancement filter (CDEF)

CDEF is a detail-preserving deringing filter designed to be applied after deblocking. It works by estimating the direction of edges and patterns and then applying a non-separable, non-linear low-pass directional filter of size $5 \times 5$ with 12 non-zero weights. To avoid signaling the directions, the decoder estimates the directions using a normative fast search algorithm. A full description of the decoding process can be found in Ref. [Reference Midtskogen and Valin29].

• • Direction estimation: The image to be filtered is divided into blocks of $8\times 8$ pixels, which is large enough for reliable direction estimation. For each direction d, a line number k is assigned to each pixel as shown in Fig. 9 and the pixel average for line k is determined. The optimal direction is found by minimizing the square error calculated as the sum of squared differences between individual pixel values and the average for the corresponding lines.

• • Non-linear low-pass filter: The non-linear low-pass filter is designed to remove coding artifacts without blurring edges. This is achieved by selecting taps based on the identified direction and selecting filter strengths along and across the direction independently. The filter can be expressed as

  (2)\begin{align} y(i, j)& =\mathrm{round}\notag\\ & \quad \times \left(x(i, j) + g\left(\sum_{m,n \in R}w_{m,n}f(x\left(m, n) - x(i,j\right))\right)\right)\end{align}
  where R contains pixels in the neighborhood of $x(i, j)$ with the non-zero coefficients $w_{m,n}$, f and g are non-linear functions described below. Function $g(\cdot )$ ensures that the modification does not exceed the greatest difference between x and any $x(m,n)$. The function f constrains the pixel difference to be filtered by taking as arguments the difference d, a strength S, and a damping value D (see Fig. 10). The strength S controls the maximum difference allowed minus a ramp-down controlled by D. To allow control over the strength of the filtering along and across the identified direction, S is allowed to differ for different filter taps. Therefore for each direction, we define primary taps and secondary taps which have associated strengths, mapping to different sets of weights $w_{m,n}$. The strength values can be changed at $64\times 64$ resolution. Coding blocks with no prediction residual (“skip” blocks) are not filtered. The damping D is signaled at the frame level.

Fig. 9. Line number k following direction 0 to 7 in an $8\times 8$ block [Reference Midtskogen and Valin29].

Fig. 10. The CDEF constraint function [Reference Midtskogen and Valin29].

2) Loop restoration filters

AV1 adds a set of tools for application in-loop after CDEF, that are selected in a mutually exclusive manner in units of what is called the loop-restoration unit (LRU) of selectable size $64 \times 64$, $128 \times 128$, or $256 \times 256$. Specifically, for each LRU, AV1 allows selecting between one of two filters [Reference Mukherjee, Li, Chen, Anis, Parker and Bankoski30] as follows.

• • Separable symmetric normalized Wiener filter: Pixels are filtered with a 7 × 7 separable Wiener filter, the coefficients of which are signaled in the bit-stream. Because of the normalization and symmetry constraints, only three parameters need to be sent for each horizontal/vertical filter. The encoder makes a smart optimization to decide the right filter taps to use, but the decoder simply applies the filter taps as received from the bit-stream.

• • Dual self-guided filter: For each LRU, the decoder first applies two cheap integerized self-guided filters of support size 3 × 3 and 5 × 5 respectively with noise parameters signaled in the bitstream. (Note self-guided means the guide image is the same as the image to be filtered.) Next, the outputs from the two filters, $r_1$ and $r_2$, are combined with weights $(\alpha , \beta )$ also signaled in the bit-stream to obtain the final restored LRU as $x + \alpha (r_1 - x) + \beta (r_2 - x)$, where x is the original degraded LRU. Even though $r_1$ and $r_2$ may not necessarily be good by themselves, an appropriate choice of weights on the encoder side can make the final combined version much closer to the undegraded source.

3) Frame super-resolution

It is common practice among video streaming services to adaptively switch the frame resolution based on the current bandwidth. For example, when the available bandwidth is low, a service may send lower resolution frames and then upscale them to the display device resolution. However, such a scaling occurs outside of the video codec as of now.

The motivation behind the new Frame Super-resolution framework in AV1 is to make this scaling process more effective by making it a part of the codec itself. This coding mode allows the frame to be coded at lower spatial resolution and then super-resolved normatively in-loop to full resolution before updating the reference buffers. Later, these super-resolved reference buffers can be used to predict subsequent frames, even if they are at a different resolution, thanks to AV1's scaled prediction capability.

Super-resolution is almost always observed to be much better than upscaling lower resolution frames outside the codec in objective metrics. In addition, at very low bit-rates, it is sometimes observed to be better than full resolution too in terms of perceptual metrics. Furthermore, it provides an extra dimension to the encoder for rate and quality control.

While there's plenty of research in this area, most super-resolution methods in the image processing literature are far too complex for in-loop operation in a video codec. In AV1, to keep operations computationally tractable, the super-resolving process is decomposed into linear upscaling followed by applying the loop restoration tool at higher spatial resolution. Specifically, the Wiener filter is particularly good at super-resolving and recovering lost high frequencies. The only additional normative operation is then a linear upscaling prior to use of loop restoration. Further, in order to enable a cost-effective hardware implementation with no overheads in line-buffers, the upscaling/downscaling is constrained to operate only horizontally. Figure 11 depicts the overall architecture of the in-loop filtering pipeline when using frame super-resolution, where CDEF operates on the coded (lower) resolution, but loop restoration operates after the linear upscaler has expanded the image horizontally to resolve part of the higher frequencies lost. The downscaling factor is constrained to be in the range 15/16 to 8/16 (half).

Fig. 11. In-loop filtering pipeline with optional super-resolution [Reference Chen32].

4) Film grain synthesis

Film grain synthesis in AV1 is normative post-processing applied outside of the encoding/decoding loop [Reference Norkin and Birkbeck31]. Film grain, abundant in TV and movie content, is often part of the creative intent and needs to be preserved while encoding. Its random nature makes it difficult to compress with traditional coding tools. Instead, the grain is removed from the content before compression, its parameters are estimated and sent in the AV1 bitstream. The decoder synthesizes the grain based on the received parameters and adds it to the reconstructed video (see Fig. 12 for details).

Fig. 12. Film grain estimation and synthesis framework [Reference Norkin and Birkbeck31].

The grain is modeled as an autoregressive (AR) process with up to 24 AR coefficients for luma and 25 for each chroma component (one more coefficient to capture possible correlation between the chroma and luma grain), which allows to support a wide range of different noise patterns. The AR process is used to generate $64 \times 64$ luma and $32 \times 32$ chroma grain templates (assuming 4:2:0 chroma subsampling). The $32 \times 32$ luma grain patches are then taken from pseudo-random positions in the template and applied to the video. To mitigate possible block artifacts from applying $32 \times 32$ patches separately to each block, an optional overlap operation can be applied to the noise samples before adding them to the reconstructed picture.

The tool supports flexible modeling of the relationship between the film grain strength and the signal intensity as follows:

\[ Y'= Y + f (Y) G_L,\]

where $Y'$ is the resulting luma re-noised with film grain, Y is the reconstructed value of luma before adding film grain, and $G_L$ is the luma film grain sample. Here, $f (Y)$ is a piecewise linear function that scales film grain depending on the luma component value. This piecewise linear function is signaled to the decoder and can be implemented as a precomputed look-up table (LUT) that is initialized before running the grain synthesis for the current frame. For a chroma component (e.g. Cb), the noise is modulated using the following formula to facilitate grain intensity modeling when the film grain strength in chroma depends on the luma component:

\begin{align*} Cb' & = Cb + f (u) G_{Cb},\\ u & = b_{Cb} Cb + d_{Cb} Y_{av} + h_{Cb}, \end{align*}

where u is an index in the LUT that corresponds to a Cb component scaling function, and parameters $b_{Cb}$, $d_{Cb}$, and $h_{Cb}$ are signaled to the decoder.

A set of film grain parameters can take up to approximately 145 bytes. Each frame can either receive a new set of grain parameters or re-use parameters from one of the previously decoded frames if those are available.

For grainy content, film grain synthesis significantly reduces the bitrate necessary to reconstruct the grain (up to $50\%$ bitrate savings can be found on sequences with heavy grain or noise). This tool is not used in the comparison in section 3 since it does not generally improve objective quality metrics because of the mismatch in positions of individual grains. More details on the film grain synthesis tool in AV1 can be found in Ref. [Reference Norkin and Birkbeck31].

1) AV1 tiles

AV1 supports independent tiles consisting of multiple superblocks, and tiles can be encoded and decoded in arbitrary orders. Defined by encoding parameters, the tiles can be uniform(i.e. tiles have the same size) or non-uniform(i.e. tiles can have different size). Independent tile support provides the coding flexibility, so that the encoder and decoder could process tiles in parallel, and thus get much faster.

In libaom codebase, multi-threading (MT) has been implemented in both encoder and decoder, which includes tile-based MT and row-based MT. While using of tiles is permitted, tile-based MT gives a significant speedup. While none or few tiles are used, row-based MT allows the thread to encode and decode one single superblock row, and gives a further boost to the speed. Using four tiles and four threads in a 720p video coding, the encoder speedup is about 3×, and the decoder speedup is about 2.5×.

2) Large-scale tiles

With the increasing popularity of virtual reality (VR) applications, for the first time, AV1 provides a solution to make real-time VR applications feasible. The large-scale tile tool allows the decoder to extract only an interesting section in a frame without the need to decompress the entire frame. This remarkably reduces the decoder complexity, and is extremely useful for real-time applications, such as light fields, which renders a single section of the frame following the viewer's head movement.