Create I Frame out of P and B frames

Question

I've written a C++ converter based on FFMpeg which can receive a link to hls-stream and convert it into local .mp4 video. So far, so good, converter works like a charm, no questions about that.

PROBLEM: No matter what input source I'm providing to the converter, at the end of convertation I need to receive video with key-frames ONLY. I need such video due to perfect seeking forward and reverse.

It's a well-known fact that subsidiary video frames (P and B) dependent on their owner-frame (I frame), because this frame contains full pixel map. According to that, we can recreate a I frame for each P and B frame by merging their data with their I frame. That's why such ffmpeg command ffmpeg -i video.mp4 output%4d.jpg works.

QUESTION: How can I implement an algorithm of merging of frames in order to recreate Key-frames ONLY video at the end? What kind of quirks I need to know about merging datas of AVPacket s?

Thanks.

Answer 1

You cannot "merge" P and B-frames of a compressed stream (eg with H.264 codec), to obtain I-frames.

What ffmpeg does with

ffmpeg -i video.mp4 output%4d.jpg

is decoding each frame (thus it needs to start from an I-frame, then decode all subsequent P and B-frames in the stream), and compress them back to JPEG and output a JPEG image for each frame in the original input stream.

If you want to convert an input stream with P/B frames to an intra-only stream (with all I-frames), you need to transcode the stream. That means decode all frames from the original stream and encode them back to an intra-only stream.