FFMPEG nb_frames vs nb_read_frames

Question

I have videos captured at 200fps and I'm trying to label each frame with it's frame number. However, looking at the original videos with the following command:

ffprobe.exe -i YR7-020320-B1-2.avi -print_format json -loglevel fatal -show_streams -count_frames -select_streams v

and I get the following output:

{
    "streams": [
        {
            "index": 0,
            "codec_name": "dvvideo",
            "codec_long_name": "DV (Digital Video)",
            "codec_type": "video",
            "codec_time_base": "1001/30000",
            "codec_tag_string": "dvsd",
            "codec_tag": "0x64737664",
            "width": 720,
            "height": 480,
            "coded_width": 720,
            "coded_height": 480,
            "closed_captions": 0,
            "has_b_frames": 0,
            "sample_aspect_ratio": "8:9",
            "display_aspect_ratio": "4:3",
            "pix_fmt": "yuv411p",
            "level": -99,
            "chroma_location": "topleft",
            "refs": 1,
            "r_frame_rate": "30000/1001",
            "avg_frame_rate": "200/1",
            "time_base": "1/200",
            "start_pts": 0,
            "start_time": "0.000000",
            "duration_ts": 67735,
            "duration": "338.675000",
            "bit_rate": "192003405",
            "nb_frames": "67735",
            "nb_read_frames": "55652",
            "disposition": {
                "default": 0,
                "dub": 0,
                "original": 0,
                "comment": 0,
                "lyrics": 0,
                "karaoke": 0,
                "forced": 0,
                "hearing_impaired": 0,
                "visual_impaired": 0,
                "clean_effects": 0,
                "attached_pic": 0,
                "timed_thumbnails": 0
            }
        }
    ]
}

I was wondering what the difference between nb_frames and nb-read-frames is, to know which one denotes the real total number of frames.

Thanks!

score 3 · Answer 1 · answered Jun 29 '20 at 17:18

3

nb_frames is the number of frames as indicated in the file metadata - this may not always be recorded. nb_read_frames is the no. of frames returned by the decoder, so it requires full decoding of the stream.

answered Jun 29 '20 at 17:18

Gyan

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Thank you so much for your reply. Sorry for my slowness, what does full decoding of the stream mean? – Alex Willcox Jun 30 '20 at 08:32
All the frames are iuncompressed to produce a full picture. – Gyan Jun 30 '20 at 13:47

score 0 · Answer 2 · answered Aug 10 '22 at 23:01

nb_frames is written as metadata in the header of the video file; It might be in-accurate sometimes.

nb_read_frames are the real number of frames in the video file. ffmpeg/ffprobe actually read through the video file, decode every frame, count the frames, to achieve this number. It is slow to generate this number, but it is absolutely correct.

FFMPEG nb_frames vs nb_read_frames

2 Answers2