使用C，使用FFmpeg保存部分视频

I am creating a simple video editing application using Java, JNI, C, and FFmpeg. The UI is being designed in Java and I am talking with FFmpeg from a C file using JNI. Does anyone have any suggestions as to the best way I should go about saving part of a video file using FFmpeg? I am allowing the users to choose parts of the video to save and what I am thinking as of right now is to basically loop through all of the packets and decode each frame (if need to encode to a different format) then save the frame to a file. All the while seeking to different parts of the video based on the users start and stop sections of their crops. If this doesn't make sense I would be glad to clear it up. Any ideas are much appreciated as I am just looking to create the most efficient and correct way to go about doing this. Thanks!I am creating a simple video editing applicatio