ffmpeg -y -i v1.mp4 -i v2.mp4 -lavfi ssim="stats_file=ssim.log" -f null -
压缩后的视频放在前面v1.mp4的位置原视频放在后面v2.mp4的位置，还是反过来放?

差别其实不在于实际上的ssim计算过程，而在于ffmpeg会自动把ref的格式转换成符合main的，main是第一个，ref是第二个（源在后面）

ffmpeg -h filter=ssim可以看到相关的help信息。
Filter ssim
Calculate the SSIM between two video streams.
Inputs:
#0: main (video)
#1: reference (video)
Outputs:
#0: default (video)
ssim AVOptions:
stats_file <string> ..FV...... Set file where to store per-frame difference information
f <string> ..FV...... Set file where to store per-frame difference information
framesync AVOptions:
eof_action <int> ..FV...... Action to take when encountering EOF from secondary input (from 0 to 2) (default repeat)
repeat 0 ..FV...... Repeat the previous frame.
endall 1 ..FV...... End both streams.
pass 2 ..FV...... Pass through the main input.
shortest <boolean> ..FV...... force termination when the shortest input terminates (default false)
repeatlast <boolean> ..FV...... extend last frame of secondary streams beyond EOF (default true)

lz
自己看看公式，xy反过来有区别吗

ffmpeg -i videoToCompare.mp4 -i originalVideo.mp4 -lavfi ssim="stats_file=ssim.log" -f null - 大概是这样子。
不过SSIM的理论值和视频的顺序无关，所以哪个在前哪个在后都无所谓。
楼主如果有问题的话不妨把视频什么的都发出来，这样也方便知道问题在哪