不知道是不是因為大部分的人需要的都是 video 的 frame，而不是 motion vector（其實在解壓縮時只差一步），我 google 了好久都找不到工具可以用。後來發現 ffmpeg 的 ffplay 可以直接把 motion vector 邊撥邊畫出來，但是還是沒辦法 dump 出來。
$ ffplay -vismv 3 xxx.mpg
vismv 那個參數其實是三個 bit，第一個 bit 代表 P-Frame，第二個代表 B-Frame 的 forward motion vector，第三個 bit 則是 B-Frame 的 backward vector。
不過，這只能看。所以我參考 Using libavformat and libavcodec 跟 ffplay 的 source code 寫了一個程式。這個程式可以把 video 每個 frame (除了沒有 motion vector 的 I-Frame）的 motion vector 取出來。如果要改成取第幾個 frame、或是某個 range 之類的，也很好改。
g++ compile 時需要加上 -lavcodec -lavformat 兩個參數，當然也要先裝上 libavcodec 跟 libavformat 兩個 library。有裝 ffmpeg 就會有。
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