2020年5月4日
By 1月时特约编辑
捷径

如何选择视频质量指标

了解更多关于视频质量指标测试流媒体西部!

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The tools I use or recommend: I used the Moscow State University video quality comparison tool. 我经常使用SSIMplus视频点播监视器, 我经常使用Hybrik(现在是杜比)媒体分析仪, 非常频繁的. The thing about Dolby is they've got a wonderful media analyzer tool but it's cloud encoding tool, 你不能直接访问媒体分析功能. 你必须使用云编码器. And since they charge by the month they've got a different kind of charge scheme. 不会用的人不多. 除非你在用他们的系统编码, 你无法接触到媒体分析器, 我不会在这上面花太多时间. It's great for me because if I need to process a hundred files or a thous和 files overnight, 我可以把它们上传到云端.

我可以让我的, my AWS account spin up a hundred machines 和 I can process those files in a very short period of time. If I tried to do that on my workstation, I could probably do it in four or five days. But again, that's not something you can access unless you're a HybriK customer. So the tools we'll spend our time on are the VQMT 和 the SSIMplus VOD monitor.

使用VQMT工具, I load the source file--a drag 和 drop here--和 then I can load one or two encoded files. And what we're showing here is the difference between constrained VBR 和 constant 位rate encoding. 我把源文件拖到这里, 这里的第一个进程文件, 这就是受限VBR, 这里是第二个进程文件, 这是恒定比特率. 然后我选择度规. 在这种情况下，我使用VMAF. 然后 I press start, 和 then it produces a results visualization.

这真的很关键. 我们一会儿看看我是怎么分析的.

But what this tells me is the per-frame VMAF score over the duration of the file. 因为这里有两个文件, 受约束的VBR用红色表示, 恒定比特率用绿色表示. And what I notice here is that there's a lot of downward spikes in the green file. 所以即使分数比较相似, I can look at this chart 和 see that there are some problem areas in the CBR file that probably represent visual quality differences that customers might notice.

顶部的图表显示了整个文件. 底部的图表显示了一个高亮显示的区域. 如果我把鼠标拖到这上面, it would show this region down here so I can zero in on the frames that I want to. 然后 once I've zeroed in, I click this, 和 I can see the actual frames. So I go from a pure number to the ability to say, "Hey, there's a real difference here."

这是刚刚发布的版本12的测试版. What they added in this version--for people who have this product but haven't seen the newest version--is the ability to see the actual VMAF score for the frames when you're looking at it in this mode. So this tells me that the CBR file has a VMAF rating of 47 和 the constrained VBR file over there has a VMAF rating of 69. 这基本上告诉了我这个, 尽管平均值是一样的, there are serious issues in the CBR file that are going to degrade viewer satisfaction.

这是CSV文件. 您可以以CSV或JSON格式获得输出. 这是源文件. 这是我们分析过的两个文件. 这很有趣. VMAF平均得分, we're seeing for the CBR is higher--the harmonic mean which takes into account the lower variability of the CBR score. 我们看到VBR分数更高了, And this is new for the Moscow State University tools: They added the harmonic mean to reflect files that have a lot of problem areas. 然后你可以看到最小值. That's really nice because that's a mathematical representation of the ugly frames we just saw--maximum value 和 then the location of those two files, 然后是标准差, which again is very valuable because the higher the st和ard deviation, 质量的可变性越高. So you can see right here that the VBR file has a st和ard deviation of seven, CBR文件, 所以在那个文件中有更多的质量可变性. That's another useful gauge in terms of how many problem areas there may be in that file. 然后 the variance numbers is just the st和ard deviation squared.

我喜欢莫斯科国家工具的哪一点? 它是负担得起的. 它是GUI和命令行. 非常直观. 你可以很容易地看到结果. 它可以处理除SSIMplus之外的大多数主要算法. 你们可以读一下我对这项技术的评论位.ly / VQMT_review.

另一方面，它只能比较类似帧率的文件. So if I have a 60 frame per second (fps) source 和 a 30fps output file, 我没法给那份文件打质量评级. 你可以用SSIMplus做到这一点. 这是它的主要优势之一.

然后, from my perspective the other thing about Hybrik--in addition to processing a lot of files--is it lets me download one CSV file that I can just populate into a spreadsheet, 完成了. 用莫斯科国立大学的工具. 如果我分析一百个文件, I've got to open up 100 CSV files 和 then copy 和 paste the results in. I'm not smart enough to know how to use JSON to automate that kind of input, 但如果你用的是CSV, 把数据输入需要很长时间.

这是SSIMwave视频点播监视器. It's based on the SSIMplus algorithm, which is by the same people who invented the SSIM algorithm. 所以这是一个进步. We saw that it rates videos on a scale that corresponds with, with human perception. 它有多种设备评级. It can compare different resolutions 和 different frame rates 和 they're here at 流媒体西部 if you're interested in the product. The other thing that the SSIMplus VOD Monitor can do is it can compute the BD-Rate functions that we learned a few minutes ago. 用莫斯科国立大学的工具, 如果我想计算BD-Rate, 我必须创建这些CSV文件, 输入Excel, 将宏应用于SSIMplus VOD监视器.

他们有比较模式. 在这个例子中，我们要看HEVC和H之间的区别.264. 我们有两个编码阶梯. 这是H.264编码阶梯. 这是HEVC编码阶梯. We see that at every point across the data rate spectrum, the blue line for HEVC is higher. 这个数字告诉我们，如果你使用HEVC梯子, you're going to save on average about 45%--the same quality at 45% lower data rate.

All you do is select the files you want to analyze 和 you can produce these numbers very quickly. It's a really good feature if you're doing a lot of codec analysis or a lot of encoder analysis.

我已经谈过几次Hybrik了. 您可以通过开源工具获得其中一些数字. 您可以使用FFmpeg和VMAF主机来计算VMAF. 我的网站上有两篇文章(位.ly / VMaster 和位.ly / ff_vmaf_win) that give you the code you need to do it 和 tell you how to do it. The problem with using the open source tools is you get the numbers only. 你无法看到数字下面的文件. 我发现这一点非常非常关键.