开始测试的时间:FFmpeg第4轮.0和增加AV1支持
FFmpeg满4岁了.几个星期前, an event I planned to leave unheralded until I read Tim Siglin’s latest research report entitled “Real-World HEVC Insights: Adoption, Implications, and Workflows.” In addition to the much more important HEVC-related implementation details, 报告指出, 在406名合格的受访者中, 36%的人在编码工作流程中使用FFmpeg. Although FFmpeg has certainly weaved its way into my daily encoding chores, 我不知道总数会这么高, 使一个主要版本值得注意.
FFmpeg 4里有什么.0? 你可以 通读整个列表, but it’s the usual assortment of new codecs and features, 包括NVIDIA的NVDEC硬件加速编解码器, 改进的元数据编辑, 以及对海视SRT的支持. What really floated my boat, however, was the addition of AV1 support. Finally, after years of talking about the codec, I could finally run my own tests.
我发现了什么?? 结果喜忧参半. The quality was very good, but encoding times were long and decode requirements seemed very high. 编码时间有多长? 一个5秒的片段 钢铁之泪 1080p source footage took 23 hours and 46 minutes to encode (1,426 minutes) on my 48-core HP Z840 workstation using the default speed preset. This translates to 285 hours (and change) to encode one minute of 1080p video. 相比, encoding with x265 using the “veryslow” preset took about 3 minutes, VP9 using the “0 speed” setting (highest quality) took about 2 minutes, 和H.264使用“非常慢”预设花了10秒. 在这个简单的试验中, AV1的时间是x265的475倍, 比VP9长713倍, 大概是8,比H长500倍.264.
我会说在AV1编码期间, FFmpeg在Z840上只消耗一个或两个内核, and for later trials I was able to encode 10 files simultaneously while only extending encoding time by about 10 percent. But the same is true for VP9, and to a lesser extent, HEVC. Most commercial AV1 encoders will have to divide the video into chunks, 分别编码, and piece them back together to achieve any kind of throughput. 当然, 这减少了线性时间, 它不会减少机器时间, 这直接转化为成本. 不管你怎么看, AV1 streams will take an order of magnitude longer in machine time to process, 这直接转化为成本.
质量怎么样?? 这里我关注的是AV1 vs. HEVC. I targeted the AV1 encode at 3Mbps, and the file rendered at 2,854Kbps. 我以不同的数据速率运行了几个HEVC编码, 还有一个4,500Kbps HEVC file delivered almost identical video multimethod assessment fusion (VMAF) and peak signal-to-noise ratio (PSNR) quality. So, with my 5-second test clip, AV1 delivered equivalent quality at a 36.数据速率降低58%. 不管你怎么看，这都令人印象深刻.
解码证明并非如此. 在这里，我改用了我的惠普ZBook Studio G3笔记本电脑, powered by a single Xeon E3-1505M CPU and NVIDIA M1000M GPU, 哪个可能有两个H.264和HEVC解码硬件. Then I used FFmpeg to convert the encoded files to Y4M4 format on the notebook’s SSD drive. H.264转换为20.6倍实时，HEVC在12.1x, VP9在10点.2x和AV1 (1.实时. The AV1 results are close enough to real time to make you wonder if there’s some kind of speed gate in the decoder, 而我没有时间去探索.
It was right about then that I discovered that FFmpeg had just dropped a new build (4.01). 叹息. 随着最后期限的临近, 我不去, 并发誓要对新版本进行更广泛的测试, including asking MulticoreWare if I should tune for PSNR when measuring with VMAF, 我没有为上面提到的结果做什么. 即使质量结果站得住脚, nothing I saw in AV1 made me think that it would soon be relevant for producers who don’t periodically encode streams viewed by millions of people. If you’d like to see for yourself, check out the newly published 2018 edition of my book, Learn to Produce Video With FFmpeg in 30 Minutes or Less, which covers encoding AV1 and other new features in FFmpeg 4.0
[本文发表于2018年9月号。 流媒体杂志 当“FFmpeg”4岁时.0."]
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