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.mpgvismv 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。
http://cmlab.csie.org/~victor/archive/motion_vector.cpp
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The author needs more context to write a review comment on motion vector extraction. They suggest referring to specific tools like OpenCV, PyTorch, and TensorFlow, and evaluating their accuracy, efficiency, ease of use, and documentation. When evaluating a tool, consider accuracy, efficiency, ease of use, features, and community and support. The tool should capture the movement of objects in video, be fast, easy to use, offer advanced features, and have a large user community.
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The technique of figuring out how objects or pixels move between two successive frames in a video or image series is known as motion vector extraction. It is a crucial method for motion tracking, computer vision, and video compression applications. Motion vectors are created by examining the variations in pixel locations between frames to explain how elements of an image move, including the motion's direction and magnitude.Prostitution for money
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