2014年12月5日
By 1月时特约编辑
专题文章

3 Essential Tools to Own When Comparing Video Encoding Quality

I just finished a consulting project that involved shooting and encoding 32 1080p and 4K files into about 350 different configurations in H.264 and HEVC format and evaluating the moving and still image quality of each file. To meet my deadlines and time estimates—to say nothing of maintaining my sanity—I used three key tools.

的 first one is an objective quality metric. I’ve encoded video for more than 20 years and have always relied upon subjective comparisons. 然而, when you’re evaluating the comparative quality of more than 350 files, 一个值得信赖的, objective metric is worth its weight in Rolaids and NoDoz, particularly if it also enables subjective comparisons. 我得到的工具是 Moscow State University 视频质量 Metric Tool (VQMT), 我回顾一下出现在2014年冬季号 流媒体欧洲版.

的 VQMT can produce more than 20 different quality metrics, including peak signal to noise ratio (PSNR), 结构化相似度指数, 及视频质素量度(VQM，见描述). 郑重声明, I found VQM to be most useful in accurately predicting meaningful quality differences between the encoded files. VQMT also presents an excellent interface for comparing the frame quality of two or three encoded files, making it a one-stop shop for identifying and documenting qualitative differences between files. Even at $999, it’s a steal for high-volume jobs.

Fortunately, the second critical tool, FFmpeg, 是免费的, 虽然像所有命令行程序一样, 这是一个学习曲线, particularly if you’re not command line literate. 在我的项目中, FFmpeg proved to be the Swiss Army knife of file conversion tools, converting H.264, HEVC, and VP9 files to the YUV format that the VQMT tool uses for file comparisons, and also converting YUV files into AVI files I could import into Adobe Premiere for supplementary subjective comparisons and frame grabs.

FFmpeg是跨平台的, command line encoding tool that many developers use to create their own enterprise encoding tools. You can download the software and access multiple learning resources at ffmpeg.org. While there were other GUI-based tools I could have used to perform these conversions, most lack the simple and massive automatability that command line tools enable via 批处理文件s.

的 批处理文件 is the third tool necessary for high volume producers.

举个例子, one great feature of the VQMT is the ability to batch process multiple files. 在我的项目中, 我会批量编码16个文件, 然后使用VQMT产生PSNR, SSIM, 以及每个文件的VQM评级. 在较大的4K文件上, 每个测试运行大约需要3分钟, 所以如果通过GUI执行, analyzing the files would have taken an exceptionally boring 2.5个小时. 通过使用命令行, 我可以创建批处理, 走开, and copy and paste the results into my scoring spreadsheet. 的 VQM scores identified major qualitative differences, so I could load those files into VQMT and continue my 分析.

Put simply, 批处理文件s are files that contain commands to run multiple command line operations. 举个例子, the command line to analyze a file under VQMT has three elements, 程序名(VQMT.. Exe)，输入文件名(encodedfile . Exe).mp4)和测试(PSNR). To analyze 16 input files by three tests, 您创建了一个48行批处理文件, including the input files and required tests, and run it under the DOS command line (VQMT only supports Windows). 的批处理文件 presents the command line parameters to the program, 一个接一个, 直到所有进程完成.

的re a plenty of free resources on 批处理文件s; one I liked was an article at WikiHow you can find at bit.ly / batch_file. 注意，不是所有的编码, 分析, and conversion tools are accessible by command line, so make sure whatever tool you use for high-volume encoding work is command line-accessible.

By their very nature, large, high-volume encoding projects are cumbersome and time-consuming. Whether you’re producing them for a client, 或者供内部使用, using the right tools can help ensure both productivity and efficiency.

This article appears in the November/December 2014 issue of 流媒体杂志 作为“适合工作的工具”.”