Added support to export annotations

[cheind]

The backend command supports sparse / dense annotations (as requested in #132) as and writes results in standard BeaverDam JSON format. Dense annotations are created between first and last keyframe using interpolation. Since BeaverDam uses html5 currentTime as timestamping mechanism, the command will need to know the fps of the video files. If not specified via command line, video files will automatically be probed using ffmpeg. This command should support local, remote video files and image lists.

Type python manage.py export_annotations --help for usage help.

A simple annotation viewer using OpenCV can be found here
https://gist.github.com/cheind/9850e35bb08cfe12500942fb8b55531f

Dense annotations will be stored in a new JSON field ('frames' per default) that follows this structure

{
    "frameid": 854,                     # Frame number with respect to start of video
    "frame": 68.32000000000001,         # Time measured in seconds from start computed as frameid * 1/FPS
    "state": "Red",                     # State copied from previous keyframe, or matching keyframe if refid is set
    "x": 1647.905947318195,             # interpolated x coordinate of bounds
    "y": 1142.1628561977332,            # interpolated y coordinate of bounds
    "w": 440.75167657459997,            # interpolated width of bounds
    "h": 451.9810186529337,             # interpolated height of bounds
    "continueInterpolation": true,      # -
    "refid": 17                         # Matched keyframe ID if any
}

Note that BeaverDam uses HTML5 currentTime to store keyframe times. These times vary between browsers and some browsers might even reduce the resolution of this field on purpose. See

https://stackoverflow.com/questions/32318333/html-5-currenttime-accuracy
https://bugs.chromium.org/p/chromium/issues/detail?id=555376

Therefore, the video probed FPS and hence computed frame times might not exactly match up the timestamps stored during browser annotation (except for image-lists where FPS is set to exactly 1).

[antingshen]

Thanks!

[cheind]

You're welcome. What I also wanted to mention is that I'm not really doing anyhting with the field continueInterpolation. What relevance does it have?