This document describes technical aspects of coding tools included in the associated codec. This document is not a specification of the associated codec. Instead, it summarizes the highlighted features of coding tools for new developers. This document should be updated when significant new normative changes have been integrated into the associated codec.
CDEF: Constrained directional enhancement
CfL: Chroma from Luma
IntraBC: Intra block copy
LCU: Largest coding unit
OBMC: Overlapped Block Motion Compensation
The largest coding block unit (LCU) applied in this codec is 128×128. In addition to no split mode PARTITION_NONE, the partition tree supports 9 different partitioning patterns, as shown in below figure.
According to the number of sub-partitions, the 9 partition modes are summarized as follows: 1. Four partitions: PARTITION_SPLIT, PARTITION_VERT_4, PARTITION_HORZ_4 2. Three partitions (T-Shape): PARTITION_HORZ_A, PARTITION_HORZ_B, PARTITION_VERT_A, PARTITION_HORZ_B 3. Two partitions: PARTITION_HORZ, PARTITION_VERT
Among all the 9 partitioning patterns, only PARTITION_SPLIT mode supports recursive partitioning, i.e., sub-partitions can be further split, other partitioning modes cannot further split. Particularly, for 8x8 and 128x128, PARTITION_VERT_4, PARTITION_HORZ_4 are not used, and for 8x8, T-Shape partitions are not used either.
For both intra and inter coded blocks, the coding block can be further partitioned into multiple transform units with the partitioning depth up to 2 levels. The mapping from the transform size of the current depth to the transform size of the next depth is shown in the following Table 1.
Furthermore, for intra coded blocks, the transform partition is done in a way that all the transform blocks have the same size, and the transform blocks are coded in a raster scan order. An example of the transform block partitioning for intra coded block is shown in the Figure 2.
For inter coded blocks, the transform unit partitioning can be done in a recursive manner with the partitioning depth up to 2 levels. The transform partitioning supports 1:1 (square), 1:2/2:1, and 1:4/4:1 transform unit sizes ranging from 4×4 to 64×64. If the coding block is smaller than or equal to 64x64, the transform block partitioning can only apply to luma component, for chroma blocks, the transform block size is identical to the coding block size. Otherwise, if the coding block width or height is greater than 64, then both the luma and chroma coding blocks will implicitly split into multiples of min(W, 64)x min(H, 64) and min(W, 32)x min(H, 32) transform blocks, respectively.
Directional intra prediction modes are applied in intra prediction, which models local textures using a given direction pattern. Directional intra prediction modes are represented by nominal modes and angle delta. The nominal modes are similar set of intra prediction angles used in VP9, which includes 8 angles. The index value of angle delta is ranging from -3 ~ +3, and zero delta angle indicates a nominal mode. The prediction angle is represented by a nominal intra angle plus an angle delta. In total, there are 56 directional intra prediction modes, as shown in the following figure. In the below figure, solid arrows indicate directional intra prediction modes and dotted arrows represent non-zero angle delta.
The nominal mode index and angle delta index is signalled separately, and nominal mode index is signalled before the associated angle delta index. It is noted that for small block sizes, where the coding gain from extending intra prediction angles may saturate, only the nominal modes are used and angle delta index is not signalled.
In addition to directional intra prediction modes, four non-directional intra modes which simulate smooth textures are also included. The four non-directional intra modes include SMOOTH_V, SMOOTH_H, SMOOTH and PAETH predictor.
In SMOOTH V, SMOOTH H and SMOOTH modes, the prediction values are generated using quadratic interpolation along vertical, horizontal directions, or the average thereof. The samples used in the quadratic interpolation include reconstructed samples from the top and left neighboring blocks and samples from the right and bottom boundaries which are approximated by top reconstructed samples and the left reconstructed samples.
In PAETH predictor mode, the prediction for each sample is assigned as one from the top (T), left (L) and top-left (TL) reference samples, which has the value closest to the Paeth predictor value, i.e., T + L -TL. The samples used in PAETH predictor are illustrated in below figure.
Five filtering intra modes are defined, and each mode specify a set of eight 7-tap filters. Given the selected filtering mode index (0~4), the current block is divided into 4x2 sub-blocks. For one 4×2 sub-block, each sample is predicted by 7-tap interpolation using the 7 top and left neighboring samples as inputs. Different filters are applied for samples located at different coordinates within a 4×2 sub-block. The prediction process can be done recursively in unit 4x2 sub-block, which means that prediction samples generated for one 4x2 prediction block can be used to predict another 4x2 sub-block.
Chroma from Luma (CfL) is a chroma intra prediction mode, which models chroma samples as a linear function of co-located reconstructed luma samples. To align the resolution between luma and chroma samples for different chroma sampling format, e.g., 4:2:0 and 4:2:2, reconstructed luma pixels may need to be sub-sampled before being used in CfL mode. In addition, the DC component is removed to form the AC contribution. In CfL mode, the model parameters which specify the linear function between two color components are optimized by encoder signalled in the bitstream.
Motion vectors are predicted by neighboring blocks which can be either spatial neighboring blocks, or temporal neighboring blocks located in a reference frame. A set of MV predictors will be identified by checking all these blocks and utilized to encode the motion vector information.
Spatial motion vector prediction
There are two sets of spatial neighboring blocks that can be utilized for finding spatial MV predictors, including the adjacent spatial neighbors which are direct top and left neighbors of the current block, and second outer spatial neighbors which are close but not directly adjacent to the current block. The two sets of spatial neighboring blocks are illustrated in an example shown in Figure 8.
For each set of spatial neighbors, the top row will be checked from left to right and then the left column will be checked from top to down. For the adjacent spatial neighbors, an additional top-right block will be also checked after checking the left column neighboring blocks. For the non-adjacent spatial neighbors, the top-left block located at (-1, -1) position will be checked first, then the top row and left column in a similar manner as the adjacent neighbors. The adjacent neighbors will be checked first, then the temporal MV predictor that will be described in the next subsection will be checked second, after that, the non-adjacent spatial neighboring blocks will be checked.
For compound prediction which utilizes a pair of reference frames, the non-adjacent spatial neighbors are not used for deriving the MV predictor.
Temporal motion vector prediction
In addition to spatial neighboring blocks, MV predictor can be also derived using co-located blocks of reference pictures, namely temporal MV predictor. To generate temporal MV predictor, the MVs of reference frames are first stored together with reference indices associated with the reference frame. Then for each 8x8 block of the current frame, the MVs of a reference frame which pass the 8x8 block are identified and stored together with the reference frame index in a temporal MV buffer. In an example shown in Figure 5, the MV of reference frame 1 (R1) pointing from R1 to a reference frame of R1 is identified, i.e., MVref, which passes a 8x8 block (shaded in blue dots) of current frame. Then this MVref is stored in the temporal MV buffer associated with this 8x8 block. Figure 9: Motion field estimation by linear projection Finally, given a couple of pre-defined block coordinates, the associated MVs stored in the temporal MV buffer are identified and projected accordingly to derive a temporal MV predictor which points from the current block to its reference frame, e.g., MV0 in Figure 5. In Figure 6, the pre-defined block positions for deriving temporal MV predictors of a 16x16 block are shown and up to 7 blocks will be checked to find valid temporal MV predictors. Figure 10: Block positions for deriving temporal MV predictors The temporal MV predictors are checked after the nearest spatial MV predictors but before the non-adjacent spatial MV predictors.
All the spatial and temporal MV candidates will be put together in a pool, with each predictor associated with a weighting determined during the scanning of the spatial and temporal neighboring blocks. Based on the associated weightings, the candidates are sorted and ranked, and up to four candidates will be used as a list MV predictor list.
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Global warped motion
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Local warped motion
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Compound wedge prediction
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Difference-modulated masked prediction
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Frame distance-based compound prediction
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Compound inter-intra prediction
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The separable 2D transform process is applied on prediction residuals. For the forward transform, a 1-D vertical transform is performed first on each column of the input residual block, then a horizontal transform is performed on each row of the vertical transform output. For the backward transform, a 1-D horizontal transform is performed first on each row of the input de-quantized coefficient block, then a vertical transform is performed on each column of the horizontal transform output. The primary 1-D transforms include four different types of transform: a) 4-point, 8-point, 16-point, 32-point, 64-point DCT-2; b) 4-point, 8-point, 16-point asymmetric DST’s (DST-4, DST-7) and c) their flipped versions; d) 4-point, 8-point, 16-point, 32-point identity transforms. When transform size is 4-point, ADST refers to DST-7, otherwise, when transform size is greater than 4-point, ADST refers to DST-4.
For luma component, each transform block can select one pair of horizontal and vertical transform combination given a pre-defined set of transform type candidates, and the selection is explicitly signalled into the bitstream. However, the selection is not signalled when Max(width,height) is 64. When the maximum of transform block width and height is greater than or equal to 32, the set of transform type candidates depend on the prediction mode, as described in Table 3. Otherwise, when the maximum of transform block width and height is smaller than 32, the set of transform type candidates depend on the prediction mode, as described in Table 4.
The set of transform type candidates (namely transform set) is defined in Table 5.
For chroma component, the transform type selection is done in an implicit way. For intra prediction residuals, the transform type is selected according to the intra prediction mode, as specified in Table 4. For inter prediction residuals, the transform type is selected according to the transform type selection of the co-located luma block. Therefore, for chroma component, there is no transform type signalling in the bitstream.
The computational cost of large size (e.g., 64-point) transforms is further reduced by zeroing out all the coefficients except the following two cases:
Both the DCT-2 and ADST (DST-4, DST-7) are implemented using butterfly structure [1], which included multiple stages of butterfly operations. Each butterfly operations can be calculated in parallel and different stages are cascaded in a sequential order.
Quantization of transform coefficients may apply different quantization step size for DC and AC transform coefficients, and different quantization step size for luma and chroma transform coefficients. To specify the quantization step size, in the frame header, a base_q_idx syntax element is first signalled, which is a 8-bit fixed length code specifying the quantization step size for luma AC coefficients. The valid range of base_q_idx is [0, 255].
After that, the delta value relative to base_q_idx for Luma DC coefficients, indicated as DeltaQYDc is further signalled. Furthermore, if there are more than one color plane, then a flag diff_uv_delta is signaled to indicate whether Cb and Cr color components apply different quantization index values. If diff_uv_delta is signalled as 0, then only the delta values relative to base_q_idx for chroma DC coefficients (indicated as DeltaQUDc) and AC coefficients (indicated as DeltaQUAc) are signalled. Otherwise, the delta values relative to base_q_idx for both the Cb and Cr DC coefficients (indicated as DeltaQUDc and DeltaQVDc) and AC coefficients (indicated as DeltaQUAc and DeltaQVAc) are signalled.
The above decoded DeltaQYDc, DeltaQUAc, DeltaQUDc, DeltaQVAc and DeltaQVDc are added to base_q_idx to derive the quantization indices. Then these quantization indices are further mapped to quantization step size according to two tables. For DC coefficients, the mapping from quantization index to quantization step size for 8-bit, 10-bit and 12-bit internal bit depth is specified by a lookup table Dc_Qlookup[3][256], and the mapping from quantization index to quantization step size for 8-bit, 10-bit and 12-bit is specified by a lookup table Ac_Qlookup[3][256].
Given the quantization step size, indicated as Qstep, the input quantized coefficients is further de-quantized using the following formula:
F = sign * ( (f * Qstep) % 0xFFFFFF ) / deNorm
, where f is the input quantized coefficient, F is the output dequantized coefficient, deNorm is a constant value derived from the transform block area size, as indicated by the following table:
| deNorm | Tx block area size |
|---|---|
| 1 | Less than 512 samples |
| 2 | 512 or 1024 samples |
| 4 | Greater than 1024 samples |
When the quantization index is 0, the quantization is performed using a quantization step size equal to 1, which is lossless coding mode.
Entropy coding engine
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Coefficient coding
For each transform unit, the coefficient coding starts with coding a skip sign, which is followed by the signaling of primary transform kernel type and the end-of-block (EOB) position in case the transform coding is not skipped. After that, the coefficient values are coded in a multiple level map manner plus sign values. The level maps are coded as three level planes, namely lower-level, middle-level and higher-level planes, and the sign is coded as another separate plane. The lower-level, middle-level and higher-level planes correspond to correspond to different ranges of coefficient magnitudes. 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.
The three level planes are coded as follows. After the EOB position is coded, the lower-level and middle-level planes are coded together in backward scan order, and the scan order refers to zig-zag scan applied on the entire transform unit basis. Then the sign plane and higher-level plane are coded together in forward scan order. After that, the remainder (coefficient level minus 14) is entropy coded using Exp-Golomb code.
The context model applied to the lower level plane depends on the primary transform directions, including: bi-directional, horizontal, and vertical, as well as transform size, and up to five neighbor (in frequency domain) coefficients are used to derive the context. The middle level plane uses a similar context model, but the number of context neighbor coefficients is reduced from 5 to 2. The higher-level plane is coded by Exp-Golomb code without using context model. For the sign plane, except the DC sign that is coded using the DC signs from its neighboring transform units, sign values of other coefficients are coded directly without using context model.
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Direction Estimation
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Non-linear low-pass filter
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Separable symmetric normalized Wiener filter
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Dual self-guided filter
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To improve the coding performance of screen content coding, the associated video codec incorporates several coding tools，for example, intra block copy (IntraBC) is employed to handle the repeated patterns in a screen picture, and palette mode is used to handle the screen blocks with a limited number of different colors.
Intra Block Copy (IntraBC) [2] is a coding tool similar to inter-picture prediction. The main difference is that in IntraBC, a predictor block is formed from the reconstructed samples (before application of in-loop filtering) of the current picture. Therefore, IntraBC can be considered as “motion compensation” within current picture.
A block vector (BV) was coded to specify the location of the predictor block. The BV precision is integer. The BV will be signalled in the bitstream since the decoder needs it to locate the predictor. For current block, the flag use IntraBC indicating whether current block is IntraBC mode is first transmitted in bit stream. Then, if the current block is IntraBC mode, the BV difference diff is obtained by subtracting the reference BV from the current BV, and then diff is classified into four types according to the diff values of horizontal and vertical component. Type information needs to be transmitted into the bitstream, after that, diff values of two components may be signalled based on the type info.
IntraBC is very effective for screen content coding, but it also brings a lot of difficulties to hardware design. To facilitate the hardware design, the following modifications are adopted.
when IntraBC is allowed, the loop filters are disabled, which are de-blocking filter, the CDEF (Constrained Directional Enhancement Filter), and the Loop Restoration. By doing this, picture buffer of reconstructed samples can be shared between IntraBC and inter prediction.
To facilitate parallel decoding, the prediction cannot exceed the restricted areas. For one super block, if the coordinate of its top-left position is (x0, y0), the prediction at position (x, y) can be accessed by IntraBC, if y < y0 and x < x0 + 2 * (y0 - y)
To allow hardware writing back delay, immediate reconstructed areas cannot be accessed by IntraBC prediction. The restricted immediate reconstructed area can be 1 ∼ n super blocks. So on top of modification 2, if the coordinate of one super block's top-left position is (x0, y0), the prediction at position (x, y) can be accessed by IntraBC, if y < y0 and x < x0 + 2 * (y0 - y) - D, where D denotes the restricted immediate reconstructed area. When D is one super block, the prediction area is shown in below figure.
[1] J. Han, Y. Xu and D. Mukherjee, “A butterfly structured design of the hybrid transform coding scheme,” 2013 Picture Coding Symposium (PCS), San Jose, CA, 2013, pp. 17-20.
[2] J. Li, H. Su, A. Converse, B. Li, R. Zhou, B. Lin, J. Xu, Y. Lu, and R. Xiong, “Intra Block Copy for Screen Content in the Emerging AV1 Video Codec,” 2018 Data Compression Conference, Snowbird, Utah, USA.