Performance Assessment of AV1, x265 and VVenC Open-Source Encoder Implementations Compared to VVC and HEVC Reference Software Models

Video applications continue to gain a lot of traction and have an enormous demand. As a result, there is a strong demand to decrease the video transmission bitrate without reducing visual quality [1],[2]. The efforts to achieve bitrate savings, especially for high-definition video content started in 2010, when the ITU-T Video Coding Experts Group (VCEG) and ISO/IEC Moving Pictures Expert Group (MPEG) established a Joint Collaborative Team on Video Coding (JCT-VC) to work on the High-Efficiency Video Coding (HEVC) standard. Then, in 2013, the 1st version of the HEVC specification was approved by ITU-T as Recommendation H.265 and by ISO/IEC as MPEG-H, Part 2 [3]. When developing the H.265/MPEG-HEVC standard, high-resolution and high frame-rate video coding was considered as one of its main potential application scenarios, while keeping it applicable to almost all existing use cases that were already targeted by H.264/MPEG-AVC.

However, more efficient video compression techniques were still desired, especially for streaming Ultra HD video content as well as Panorama video content (so called, 360° video content) from concerts, shows, sport events, etc. Therefore, in order to fulfill this demand, the exploration phase for future video coding technologies beyond HEVC (ITU-T H.265 | ISO/IEC 23008-2) started in October 2015 by establishing a Joint Video Exploration Team (JVET) on Future Video Coding of ITU-T VCEG and ISO/IEC MPEG [4]. The development process of the video coding standard beyond HEVC was driven by the most recent scientific and technological achievements in the video coding field, and under the JVET development it was titled "Versatile Video Coding" [5], or in short, VVC [6]. The 1st version of the VVC standard (i.e. VVC v1) was officially finalized during the 19th JVET meeting, which took place between June 22 and July 1, 2020, and it was approved by ITU-T as Recommendation H.266 and by ISO/IEC as MPEG-I, Part 3 [6].

While the joint video coding standardization activities of ITU-T and ISO/IEC organizations rely on an open and collaborative process driven by its active members, several companies individually developed their own video coding formats [7], [8]. Such in 2015, the Alliance for Open Media(AOM) was formed with the objective to work towards next-generation media formats in general and with a particular short-term focus on the development of a video-coding scheme [9],[10],[11]. In April 2016, AOM released a baseline version of the developed video-coding scheme, which got the name of AV1. In turn, the final version of the 1st edition of AV1 released in 2018, claiming to provide a significant coding-efficiency gain over the current state-of-the-art video codecs [10]. In May 2020, AV1 codec had a major update and its 2nd edition, i.e. AV1 2.0, was released [12],[13]. In turn, AV1 3.0 with better compression performance was released in 2021 [12],[13]. However, experimental results related to coding efficiency comparison of AV1 versus standardized codecs such as HEVC and VVC, which are reported in the literature [14], are not consistent and are sometimes even contradictory. As a result, there is a lot of confusion about the ability of future versions of AV1 to compete with HEVC/VVC-based encoder implementations.

In order to put things into perspective and to provide relevant information in a reproducible and reliable form, this work presents detailed experimental results of a coding-efficiency comparison of AV1 (aomenc) v3.6 [12] versus VVC-based codecs -the open-source VVenC codec v1.7 [15] and VVC reference software model (VTM) v17.0, and versus HEVC-based codecs - the open-source x265 codec v3.5 [16],[17] and HEVC reference software model (HM) v17.0 [18], along with a detailed discussion of the selected software implementations, the choice of coding parameters, and the corresponding evaluation setup. All tested encoders have been set to the best quality operation modes (i.e. the slowest encoding options) with a Group of Pictures (GOP) size equal to 16. Specifically, in this work, the authors focus on 4K and 1080p video content (selected from JVET CTC [19]), since it is considered to be the most popular nowadays, especially within the consumer multimedia applications, and encoding such content is typically the most challenging due to significantly larger computational complexity in terms of encoding times compared to lower video resolutions. For the rate-distortion (R-D) performance assessment, the authors used a Bjøntegaard-Delta bit-rate (BD-BR) measurement method for calculating average bit-rate differences between R-D curves for the same objective quality (e.g., for the same PSNR_YUV) [20].

According to the experimental results, the coding efficiency of both AV1 and x265 was found to be significantly inferior to VVenC, while having the overhead of 31.2% and 118%, respectively. In terms of the encoding speed, the typical encoding time of AV1 is ~5 times slower than HM, and ~2 times slower than VVenC. On the other hand, the encoding time of x265 for the given best quality mode configuration is similar to that of HM, while HM provides bitrate savings of more than 22%. When compared to VTM, the HM runtime is more than 12 times faster, but with an overhead of ~52%.