Whither SVT-AV1
M莫斯科州立大学(MSU)每年都会发布多个编解码器报告. I find these reports credible because they use 非常多样的内容和现实的编码参数,由大学与编解码器开发人员协商得出. I wrote about the 2020 crop of reports at go2sm.com/moscow2020.
One very noteworthy observation from these reports is the consistently dismal performance of Scalable Video Technology (SVT)-based codecs. In 1-fps tests in the “MSU Video Codecs Comparison 2020 Part 1: FullHD, Objective” report, SVT-HEVC and SVT-VP9 placed well behind x265 and WebM’s VP9; they’re even behind the x264 codec they were designed to replace. SVT-AV1 落后于开放媒体联盟(AOMedia)的libbaom 4个百分点,落后于Visionular的Aurora1编解码器25个百分点. Granted, SVT is more about performance than quality, but SVT-HEVC 在30帧/秒的测试中,SVT-VP9再次落后于x264和x265. (VP9 and Aurora didn’t make the performance cut.)
这些结果非常有趣,因为它们与SVT-AV1有关, the AV1 codec that AOMedia has gone all-in on. Specifically, in August 2020, AOMedia成立了一个软件工作组,利用SVT-AV1“创建AV1编码器实现,在应用程序中提供出色的视频压缩,消除了不断增长的视频传输市场的计算复杂性权衡。.” It’s only been about a year, so it’s hard to criticize these efforts, 但是基于svt的VP9和HEVC编解码器的落后性能引发了人们的疑问 amedia为AV1的持续发展选择了合适的架构.
I raised these issues with David Ronca, 他是facebook的视频编码主管,也是Net-flix的前编码技术主管. He responded, “As I understand it, SVT-HEVC和SVT-VP9设计用于实时编码, and [are] not really useful for other use cases. 线程性能几乎是线性的,这意味着更多的内核=更快的编码. 其他编解码器在处理许多线程时效率不高. SVT-AV1表现非常有利的libbaom,并提供坚实的增益 the way into x264 computational complexity. We (SVT社区)对SVT- av1感觉非常好,并将继续投资于它. It’s among the top most well-designed encoders.”
Regarding codec comparisons in general, Ronca补充说:“英特尔在SPIE上分享了一个编解码器的比较,这是如何评估现代编解码器的. 基本指标是每次射击,单线程,复杂性vs. efficiency.” Ronca was referring to a white paper titled “SVT-AV1编码器:概述,功能和速度-质量权衡,将SVT-AV1与libbaom、x264、x265和libvpx (VP9)进行了比较。. 该论文于2020年8月发表,下载费用为21美元. Or, 你可以在YouTube上观看Ronca对John Porterfield的采访视频(go2sm.com/av1ronca; about 25 minutes in).
这篇论文的测试是广泛的,有充分的文档和良好的呈现 touch idiosyncratic in my view, which raises questions as to how the results will translate to real-world applications. For example, 研究人员使用恒定速率因子编码进行所有编码, 在学术编解码器比较中最常用的是哪一种, but very seldom used for actual production. When I tested AV1 encoders for my article “AV1已经到来:比较来自AO-Media, Visionular和Intel/Netflix的编解码器,” I tested SVT-AV1 using two-pass encoding. 英特尔对此表示反对,称双通率控制尚未完成. As I explain in the article, 我留下了SVT-AV1,因为我已经投入了大量的时间来使用编解码器,因为它在GitHub上是免费提供的,没有任何关于双通道性能的警告. 白皮书也没有测试在我的测试中表现最好的Aurora1编解码器.
I asked Ronca about these issues, and he responded, “如果我要部署AV1,我的用例是2-pass, fixed GOP, I might want to test libvpx, SVT-AV1, 也许是一些封闭源的AV1编码器在2-pass模式与固定GOP.”
The white paper concludes, “关于更快预设的初步实验数据显示,SVT-AV1最终将成为广泛(视频点播)应用的首选编码器.” That may yet be the case, 但我要提醒的是,所有编解码器的比较都是学术的,直到他们使用编码参数用编码器测试你的内容. 它们越接近您的编码模式,它们可能就越准确.
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