Video Frame Capture: AVFoundation vs ffmpeg

Generate images

This repo contains python and swift code to generate video frame grabs using ffmpeg and AVFoundation, respectively.

AVFoundation

Command

Help message:

  swift run frameGrab --help

Example:

  swift run frameGrab --snaps 4 -o ../images/10_secs ../vids/10_secs.mp4 0

ffmpeg

Usage:

  python3 ff-snap.py <inputFile> <outDir> <numSnaps> <startMillis>

Example:

  python3 ff-snap.py ../vids/10_secs.mp4 ../images/10_secs 4 0

Discussion

The file vids/10_secs.mp4 is the first 10 seconds of example MBARI video. A frameRate of 6000/1001 is obtained using:

  ffprobe -v error \
    -select_streams v:0 \
    -of default=noprint_wrappers=1:nokey=1
    -show_entries stream=avg_frame_rate \
    vids/10_secs.mp4

Images

Note: The following comparisons use images in image/10_secs/

Comparing the ffmpeg images, the first three frame advances are:

    ff_10_secs_0_.jpg -> ff_10_secs_1_.jpg
    ff_10_secs_16_.jpg -> ff_10_secs_17_.jpg
    ff_10_secs_33_.jpg -> ff_10_secs_34_.jpg

Comparing AVFoundation images, the first three frame advances are:

    av_10_secs_16_.png -> av_10_secs_17_.png
    av_10_secs_33_.png -> av_10_secs_34_.png
    av_10_secs_50_.png -> av_10_secs_51_.png

This one-off frame timing continues through the video:

    ff_10_secs_6139_.jpg -> ff_10_secs_6140_.jpg
    av_10_secs_6139_.png -> av_10_secs_6140_.png

with ff_10_secs_6139_.jpg being the same as av_10_secs_6140_.png

The ffmpeg frame advance going from 0 to 1 millisecond seems peculiar to me.