Drop some parts of video and re-make key frames (c++ + libav) - c

Question

I'm trying to drop some parts of video in my app using libav, for example in a video that has 00:08:00 length, I try to drop frames 100-250 and 400-500 (Just for example).
I wrote this code that copy AVPacket and drop some packets, But there is a problem! In our videos every keyframe followed by 29 non-key frames. So when my code goes to drop frames 100-250 the frame 100 may be is a non-key frame, in this case the parts that are going to join (In this example frame 250 to frame 400) the frame 400 is positioned after a keyframe that is not belogs to.
In this section video frames shown garbled,
Video cutting speed is so important in my code, so I can't decode/re-encode all of video frames.
The question is that, How can I decode encode begin of each parts (from begin frame to first key frame) and make another frames copy without decode?
Or, Is there any another FAST solution for splitting/merging (Dropping some parts of video)?

Answer 1

The question is that, How can I decode encode begin of each parts (from begin frame to first key frame) and make another frames copy without decode?
You can't. It doesn't work that way.

Answer 2

Start to think about time, not in frames
you can get new videos fast in parts of base_video.mp4, for example,
ffmpeg -ss 00:00:00.000 -i base_video.mp4 -t 8.000 -c copy -strict -2 new_video_8seconds_fromstart.mp4
-ss 00:00:00.000 is the time to start the new video
-t is the duration in seconds and miliseconds, example, for 8 seconds of duration you have to use 8.000
-an if you dont want audio
-strict -2 if for copy some files codecs like DTS
but if you want with re-encoding remove -c copy but it never will be fast!
ffmpeg -ss 00:00:00.000 -i base_video.mp4 -t 8.000 new_video_8seconds_fromstart.mp4