Power efficient real time VVC decoder
Jeeva Raj A (Principal Engineer, Advanced Video)
Sagar Kotecha (Lead Engineer, Advanced Video)
Video is a powerful medium and its popularity as information, entertainment conduit is on the rise. As per market research, video is responsible for significant share of internet traffic. Additionally the current pandemic situation has significant impact on the exponential increase in online video traffic. Applications like virtual meetings at work, online education for the kids, virtual conferences and importantly OTT has seen massive increase in consumer demand.
Research and development in video compression and related technology ends up with new compression standards in market to address the demands. The recent VVC/H.266 video standard by JVET/MPEG provides significant advantage of compression gains and is versatile for use cases like adaptive bitrate streaming, 3D video, screen content encoding and HDR. Ittiam has been actively participating and contributing to the VVC standardization.
This blog serves as a primer to the various key tools of VVC and their applications. It also provides insights into Ittiam’s efficient implementation of VVC decoder on ARM based mobile client platform.
Versatile Video Coding (Rec. ITU-T H.266 | ISO/IEC 23090-3)  is the latest video coding standard finalized in July 2020 from JVET/MPEG. VVC achieves BD-rate gain of ~40% over the predecessor HEVC (Rec. ITU-T H.265 | ISO/IEC 23008-2) for the additional decoding complexity of ~60%.
What are the new tools that have been introduced or enhanced in VVC? How do these help in getting better compression efficiency?
|Category||Tool||High Level Details|
Max CTU 128x128
Maximum Coding Tree Unit size is 128 to get better compression efficiency in homogeneous regions especially in HD and UHD resolutions
Binary and Ternary Splits are allowed to get further compression gains by coding optimal block sizes
Separate Coding Structure for Chroma - Efficient coding of Chroma blocks independent of Luma coding structure
Directional Intra Modes
Wide Angle Prediction
Higher number of prediction modes(double as compared to HEVC)
Suitable for rectangular block intra prediction
Cross Component Linear Model Prediction of Cb from luma and Cr from Cb
Multiple Reference Lines used for Intra prediction
Intra Sub-Partitioning for better prediction and transform to preserve fine details and edges
Matrix Weighted Intra Prediction, tables are arrived through offline modeling/learning depending on block sizes
Extended Merge Prediction
Symmetric MVD, one direction MVD in bidirectional prediction blocks is signaled and other direction MVD is derived as inverse of signaled.
Merge list with additional entries from MV history, pair wise MVsMerge list with additional entries from MV history, pair wise MVsMerge list with additional entries from MV history, pair wise MVs
Merge MVD, Additional MVD to Merge MV with signaling of an index to discrete distance and direction for bi
History Based MVP, additional MV into Merge/MVP list based on the unique MVs from the recently coded units
Affine inter prediction for motion model like rotation, shear, zoom
Sub-block Temporal MVP to derive local motion field at 8x8 level within the CU
Adaptive Motion Vector Resolution- allows to signal the MVD precision to get better tradeoff between MV bits and Prediction accuracy.
Bi-Prediction with weights, allows weighted average of list 0 and list 1 Prediction signals instead of the only default 0.5 in HEVC
Bi-directional Optical Flow to get fine motion at 4x4 level based on Optical Flow for merge CUs.
Decoder Side Motion Vector Refinement for merge CUs. Additional MVD are obtained through decoder side refinement without any signaling overhead
Non Rectangular Partition for better prediction
Combined Intra and Inter Prediction
|Transform & Quantization|
For better Energy compaction
Different transform type selection for vertical and horizontal
Sub-Block Transform for Inter CU to save on partitioning where only sub-part of CU residuals are coded.
Low frequency non separable transform for Intra blocks to strive max decorrelation of residual signal
Alternating between 2 scalar quantizer based on state transition rule to select an optimum sequence of reconstruction values
Joint coding of Cb and Cr residuals
Core Cabac Engine
Multi-hypothesis Probability update model to improve probability estimation
Separate Residual Coding structure for transform coeff and transform skip coeff, Coeff group size depends on Transform block sizes
Selection of probability models for the syntax elements related to absolute values of transform coefficient levels depends on the values of the absolute levels or partially reconstructed absolute levels in a local neighborhood
In loop mapping of luma samples based on piecewise linear model
Luma dependent Chroma residual scaling. Scaling of signal across dynamic range
Better compression by removing blur artifacts across due to block processing
There are tools in VVC to improve the compression efficiency and flexibility for the real world application like ABR, 360 degree, Screen Content Coding, Video Conferencing.
|Special Tools||Use case|
|RPR||Reference Picture Resampling - improves compression efficiency for Adaptive Bitrate Streaming coding open GOP with resolution change by upscaling and downscaling reference Pictures|
|Sub-Pictures||Independently decodable sub-Pictures helps to improve parallelism and suitable for use cases like 360 degree videos and video conferencing|
|IBC, Transform Skip, Palette, BDPCM||Intra Block Copy, Transform Skip and palette mode for Screen Content Coding to improve the compression gains|
A real time decoder is the key enabler for widespread adoption of any video coding technology or Standard. One of Ittiam’s major focus areas is embedded software implementations, optimized for the widely-prevalent ARM based mobile platform.
In the year 2014, Google and Ittiam collaborated to bring HEVC Decoding capability to Android, making it available natively for a vast consumer ecosystem, and at scale . As a step forward for VVC, Ittiam demonstrated the real-time decoding capability of VVC on ARM based mobile devices in JVET and MPEG Plenaries during VVC development and standardization , . The main focus of this demonstration was to show the new compression tools in VVC are implementation friendly towards real-time decoding on ARM based platforms.