cvideoffmpegmp4libavcodec

FFmpeg decoding .mp4 video file

I'm working on a project that needs to open .mp4 file format, read its frames 1 by 1, decode them and encode them with better type of lossless compression, and save them into a file.

From my understanding, it should go like this:

  1. Open input .mp4 file
  2. Find stream info -> find video stream index
  3. Copy codec pointer of found video stream index into AVCodecContext type pointer
  4. Find decoder -> allocate codec context -> open codec
  5. Read frame by frame -> decode the frame -> encode the frame -> save it into a file

So far, I encountered a couple of problems. For example, if I want to save a frame using av_interleaved_write_frame() function. I can't open input a .mp4 file using avformat_open_input() since it's going to populate filename part of the AVFormatContext structure with the input file name, and therefore, I can't "write" into that file.

I've tried a different solution using av_guess_format(), but when I dump format using dump_format(), I get nothing, So I can't find stream information about which codec it is using.

How should this be done?

Solution

  • See the "detailed description" in the muxing docs. You:

    1. set ctx->oformat using av_guess_format
    2. set ctx->pb using avio_open2
    3. call avformat_new_stream for each stream in the output file. If you're re-encoding, this is by adding each stream of the input file into the output file.
    4. call avformat_write_header
    5. call av_interleaved_write_frame in a loop
    6. call av_write_trailer
    7. close the file (avio_close) and clear up all allocated memory