f you do not care that much
about file size then the best mode will be in constant quality
(fixed-quant). However if you do care about a file size but still want
to obtain good to excellent quality, then 2-pass or 3-pass is the option
for you.
2-pass or 3-pass的存在是为了降低文件大小。
Two pass encoding used to retain the best quality during conversion.
In the first pass of two-pass encoding, the input data from the source clip is analyzed and stored in a log file.
In the second pass, the collected data from the first pass is used to achieve the best encoding quality.
Two pass encoding 是为了转换的时候 保持最好的质量。
two-pass encoding的第1个阶段, 即first pass阶段，分析源文件并且生成一个log文件。
two-pass encoding的第2个阶段, 即second pass阶段，利用first pass生成的log文件来达到最好的编码质量。
2pass是2次编码的意思，stats是统计文档的名称，记录了1pass中的信息.
2次编码就相当于2次转换这么做虽然转换时间会漫长，但压出的片子会有更好的画质，画面细节更好，而且体积会更小。
2-pass主要是针对非实时的视频编码（如文件）来说的，通俗的说就是需要二次编码，
第一次编码是先对整个文件进行扫描，记录一些统计信息，
第二次编码时根据前面记录的统计信息再进行编码，这样的好处是可以提高编码的质量。
References:
https://www.jacksonlin.net/20170304-vbr-cbr/
https://www.quora.com/What-is-the-difference-between-1-pass-and-2-pass-video-encoding
https://blog.csdn.net/china_video_expert/article/details/5936391
https://www.jacksonlin.net/20170304-vbr-cbr/
