VP9

VP9 is an open and royalty-free video coding format developed by Google.
VP9 is the successor to VP8 and competes mainly with MPEG's High Efficiency Video Coding.
At first, VP9 was mainly used on Google's video platform YouTube. The emergence of the Alliance for Open Media, and its support for the ongoing development of the successor AV1, of which Google is a part, led to growing interest in the format.
In contrast to HEVC, VP9 support is common among modern web browsers. Android has supported VP9 since version 4.4 KitKat, while Safari 14 added support for VP9 in iOS / iPadOS / tvOS 14 and macOS Big Sur.
Parts of the format are covered by patents held by Google. The company grants free usage of its own related patents based on reciprocity, i.e. as long as the user does not engage in patent litigations.

History

VP9 is the last official iteration of the TrueMotion series of video formats that Google bought in 2010 for $134 million together with the company On2 Technologies that created it.
The development of VP9 started in the second half of 2011 under the development names of Next Gen Open Video and VP-Next. The design goals for VP9 included reducing the bit rate by 50% compared to VP8 while maintaining the same video quality, and aiming for better compression efficiency than the MPEG High Efficiency Video Coding standard. In June 2013 the "profile 0" of VP9 was finalized, and two months later Google's Chrome browser was released with support for VP9 video playback. In October of that year a native VP9 decoder was added to FFmpeg, and to Libav six weeks later. Mozilla added VP9 support to Firefox in March 2014. In 2014 Google added two high bit depth profiles: profile 2 and profile 3.
In 2013 an updated version of the WebM format was published, featuring support for VP9 together with Opus audio.
In March 2013, the MPEG Licensing Administration dropped an announced assertion of disputed patent claims against VP8 and its successors after the United States Department of Justice started to investigate whether it was acting to unfairly stifle competition.
Throughout, Google has worked with hardware vendors to get VP9 support into silicon. In January 2014, Ittiam, in collaboration with ARM and Google, demonstrated its VP9 decoder for ARM Cortex devices. Using GPGPU techniques, the decoder was capable of 1080p at 30fps on an Arndale Board. In early 2015 Nvidia announced VP9 support in its Tegra X1 SoC, and VeriSilicon announced VP9 Profile 2 support in its Hantro G2v2 decoder IP.
In April 2015 Google released a significant update to its libvpx library, with version 1.4.0 adding support for 10-bit and 12-bit bit depth, 4:2:2 and 4:4:4 chroma subsampling, and VP9 multithreaded decoding/encoding.
In December 2015, Netflix published a draft proposal for including VP9 video in an MP4 container with MPEG Common Encryption.
In January 2016, Ittiam demonstrated an OpenCL based VP9 encoder. The encoder is targeting ARM Mali mobile GPUs and was demonstrated on a Samsung Galaxy S6.
VP9 support was added to Microsoft's web browser Edge in 2016.
In March 2017, Ittiam announced the completion of a project to enhance the encoding speed of libvpx. The speed improvement was said to be 50-70%, and the code "publicly available as part of libvpx".

Features

VP9 is customized for video resolutions greater than 1080p and also enables lossless compression. It supports resolutions up to 65536×65536, whereas HEVC supports resolutions up to 8192×4320 pixels.
The VP9 format supports the following color spaces : Rec. 601, Rec. 709, Rec. 2020, SMPTE-170, SMPTE-240, and sRGB.
VP9 supports many transfer functions and supports HDR video with hybrid log–gamma or perceptual quantizer.

Efficiency

An early comparison that took varying encoding speed into account showed x265 to narrowly beat libvpx at the very highest quality whereas libvpx was superior at any other encoding speed, by SSIM.
In a subjective quality comparison conducted in 2014 featuring the reference encoders for HEVC, MPEG-4 AVC/H.264, and VP9, VP9, like H.264, required about two times the bitrate to reach video quality comparable to HEVC, while with synthetic imagery VP9 was close to HEVC.
By contrast, another subjective comparison from 2014 concluded that at higher quality settings HEVC and VP9 were tied at a 40 to 45% bitrate advantage over H.264.
Netflix, after a large test in August 2016, concluded that libvpx was 20% less efficient than x265, but by October the same year also found that tweaking encoding parameters could "reduce or even reverse the gap between VP9 and HEVC". At NAB 2017, Netflix shared that they had switched to the EVE encoder, which according to their studies offered better two-pass rate control and was 8% more efficient than libvpx.
An offline encoder comparison between libvpx, two HEVC encoders and x264 in May 2017 by Jan Ozer of Streaming Media Magazine, with encoding parameters supplied or reviewed by each encoder vendor, and using Netflix's VMAF objective metric, concluded that "VP9 and both HEVC codecs produce very similar performance" and "Particularly at lower bitrates, both HEVC codecs and VP9 deliver substantially better performance than H.264".

Performance

An encoding speed versus efficiency comparison of the reference implementation in libvpx, x264 and x265 was made by an FFmpeg developer in September 2015: By SSIM index, libvpx was mostly superior to x264 across the range of comparable encoding speeds, but the main benefit was at the slower end of x264@veryslow, whereas x265 only became competitive with libvpx around 10 times as slow as x264@veryslow. It was concluded that libvpx and x265 were both capable of the claimed 50% bitrate improvement over H.264, but only at 10–20 times the encoding time of x264. Judged by the objective quality metric VQM in early 2015, the VP9 reference encoder delivered video quality on par with the best HEVC implementations.
A decoder comparison by the same developer showed 10% faster decoding for ffvp9 than ffh264 for same-quality video, or "identical" at same bitrate. It also showed that the implementation can make a difference, concluding that "ffvp9 beats libvpx consistently by 25–50%".
Another decoder comparison indicated 10–40 percent higher CPU load than H.264, and that on mobile, the Ittiam demo player was about 40 percent faster than the Chrome browser at playing VP9.

Profiles

There are several variants of the VP9 format, which successively allow more features; profile 0 is the basic variant, requiring the least from a hardware implementation:
; profile 0: color depth: 8 bit/sample, chroma subsampling: 4:2:0
; profile 1: color depth: 8 bit, chroma subsampling: 4:2:2, 4:2:0, 4:4:4
; profile 2: color depth: 10–12 bit, chroma subsampling: 4:2:0
; profile 3: color depth: 10–12 bit, chroma subsampling: 4:2:2, 4:2:0, 4:4:4

Levels

VP9 offers the following 14 levels:

Level	Luma Samples/s	Luma Picture Size	Max Bitrate	Max CPB Size for Visual Layer	Min Compression Ratio	Max Tiles	Min Alt-Ref Distance	Max Reference Frames	Examples for resolution @ frame rate
1			0.20	0.40	2	1	4	8	256×144@15
1.1			0.80	1.0	2	1	4	8	384×192@30
2			1.8	1.5	2	1	4	8	480×256@30
2.1			3.6	2.8	2	2	4	8	640×384@30
3			7.2	6.0	2	4	4	8	1080×512@30
3.1			12	10	2	4	4	8	1280×768@30
4			18	16	4	4	4	8	2048×1088@30
4.1			30	18	4	4	5	6	2048×1088@60
5			60	36	6	8	6	4	4096×2176@30
5.1			120	46	8	8	10	4	4096×2176@60
5.2			180	TBD	8	8	10	4	4096×2176@120
6			180	TBD	8	16	10	4	8192×4352@30
6.1			240	TBD	8	16	10	4	8192×4352@60
6.2			480	TBD	8	16	10	4	8192×4352@120

Technology

VP9 is a traditional block-based transform coding format. The bitstream format is relatively simple compared to formats that offer similar bitrate efficiency like HEVC.
VP9 has many design improvements compared to VP8. Its biggest improvement is support for the use of coding units of 64×64 pixels. This is especially useful with high-resolution video. Also, the prediction of motion vectors was improved. In addition to VP8's four modes, VP9 supports six oblique directions for linear extrapolation of pixels in intra-frame prediction.
New coding tools also include:

eighth-pixel precision for motion vectors,
three different switchable 8-tap subpixel interpolation filters,
improved selection of reference motion vectors,
improved coding of offsets of motion vectors to their reference,
improved entropy coding,
improved and adapted loop filtering,
the asymmetric discrete sine transform,
larger discrete cosine transforms, and
improved segmentation of frames into areas with specific similarities

In order to enable some parallel processing of frames, video frames can be split along coding unit boundaries into up to four rows of 256 to 4096 pixels wide evenly spaced tiles with each tile column coded independently. This is mandatory for video resolutions in excess of 4096 pixels. A tile header contains the tile size in bytes so decoders can skip ahead and decode each tile row in a separate thread. The image is then divided into coding units called superblocks of 64×64 pixels which are adaptively subpartitioned in a quadtree coding structure. They can be subdivided either horizontally or vertically or both; square units can be subdivided recursively down to 4×4 pixel blocks. Subunits are coded in raster scan order: left to right, top to bottom.
Starting from each key frame, decoders keep 8 frames buffered to be used as reference frames or to be shown later. Transmitted frames signal which buffer to overwrite and can optionally be decoded into one of the buffers without being shown. The encoder can send a minimal frame that just triggers one of the buffers to be displayed. Each inter frame can reference up to three of the buffered frames for temporal prediction. Up to two of those reference frames can be used in each coding block to calculate a sample data prediction, using spatially displaced content from a reference frame or an average of content from two reference frames. The remaining difference from the computed prediction to the actual image content is transformed using a DCT or ADST and quantized.
Something like a b-frame can be coded while preserving the original frame order in the bitstream using a structure named superframes. Hidden alternate reference frames can be packed together with an ordinary inter frame and a skip frame that triggers display of previous hidden altref content from its reference frame buffer right after the accompanying p-frame.
VP9 enables lossless encoding by transmitting at the lowest quantization level an additional 4×4-block encoded Walsh–Hadamard transformed residue signal.
In order to be seekable, raw VP9 bitstreams have to be encapsulated in a container format, for example Matroska, its derived WebM format or the Duck IVF format which is traditionally supported by libvpx, a library that originated at The Duck Corporation. VP9 is identified as V_VP9 in WebM and VP09 in MP4, adhering to respective naming conventions.