Extracting all the group of pictures (GOPs) of a video generated by the codec

Question

I am doing some remasterisation techniques on a video, and instead of using these techniques on the whole video at once I want to do them on the group of pictures(GOPs) of the video separately, so what I want to know is there any way to extract the group of pictures of a video which are generated by the codec.

Answer 1

Group of pictures (GOP) applies all video frames starting from a key frame, and ends one frame before the next key frame.
(The above definition assumes "Close GOPs").

The following post have examples for splitting a video into GOPs, but there is no Python code sample and I am not sure if this script is actually working.

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For splitting a video file into multiple files when each file is one GOP, we may use segment muxer with -segment_times argument.
-segment_times expects a list of timestamps. We shell provide a list of timestamps of all the key frames in the input file.

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Let's start by building an input video file for testing (using FFmpeg CLI):

ffmpeg -y -f lavfi -i testsrc=size=192x108:rate=1:duration=100 -vcodec libx264 -g 10 in.mp4

The above command builds synthetic a video file with fixed GOP size of 10 frames (for testing).
The frames are numbered, so it's easy to follow...

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Using FFprobe CLI for getting the timestamps of all the key-frames (for demonstrating the concept):

ffprobe -skip_frame nokey -select_streams v:0 -show_frames -show_entries frame=pkt_pts_time -of json in.mp4 > tmp.txt

(A similar command is going to be executed from Python).

The above command creates a text file with timestamps of all key-frames in JSON format:

{
    "frames": [
    ...    
    {
        "pkt_pts_time": "10.000000"
    },
    {
        "pkt_pts_time": "20.000000"
    },
    ...
}

The segment_times list is going to be: "10.000000,20.000000,30.000000...".

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Using a Python script for programmatically splitting video file to GOPs:

• Use FFprobe for getting the PTS timestamps of all key frames (get it in JSON format):

   data = sp.run(['ffprobe', '-skip_frame', 'nokey', '-select_streams', 'v:0', '-show_frames', '-show_entries', 'frame=pkt_pts_time', '-of', 'json', in_file_name], stdout=sp.PIPE).stdout
  

• Convert from JSON (string) to dictionary, and Get 'frames' out of the dictionary:

   dict = json.loads(data) 
   frames_dict = dict['frames']
  

• Building a comma separated string of timestamps:

   pts_list = [item['pkt_pts_time'] for item in frames_dict]
   segment_times = ",".join(pts_list)
  

• Use FFmpeg for splitting the input video by timestamps (files: out0000.mp4, out0001.mp4, out0002.mp4):

   sp.run(['ffmpeg', '-i', in_file_name, '-codec', 'copy', '-f', 'segment', '-reset_timestamps', '1', '-segment_times', segment_times, 'out%04d.mp4'])
  

The above code uses subprocess module for executing FFmpeg and FFprobe within Python.
Make sure ffmpeg and ffprobe are in the exaction path.

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Python code:

import subprocess as sp
import json

# Preparation: build synthetic video file for testing
# ffmpeg -y -f lavfi -i testsrc=size=192x108:rate=1:duration=100 -vcodec libx264 -g 10 in.mp4

in_file_name = 'in.mp4'  # Input file name

# Use FFprobe for getting the PTS timestamps of all key frames (get it in JSON format).
data = sp.run(['ffprobe', '-skip_frame', 'nokey', '-select_streams', 'v:0', '-show_frames', '-show_entries', 'frame=pkt_pts_time', '-of', 'json', in_file_name], stdout=sp.PIPE).stdout
dict = json.loads(data)  # Convert from JSON (string) to dictionary 
frames_dict = dict['frames']  # Get 'frames' out of the dictionary

pts_list = [item['pkt_pts_time'] for item in frames_dict]  # Convert to list: ['0.000000', '10.000000', '20.000000', ...]
segment_times = ",".join(pts_list)  # Convert list to comma separated string: '0.000000,10.000000,20.000000,...'

# Use FFmpeg for splitting the input video by timestamps (files: out0000.mp4, out0001.mp4, out0002.mp4)
# Each segment file is going to be a GOP - start from key-frame, and end one frame before the next key-frame.
sp.run(['ffmpeg', '-i', in_file_name, '-codec', 'copy', '-f', 'segment', '-reset_timestamps', '1', '-segment_times', segment_times, 'out%04d.mp4'])

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Note:

• The above solution may not work for all codecs and all file formats.
  For example, H.265 codec has segmenting issues (due to FFmpeg limitations).