[FFmpeg-user] dshow and gdigrab formats missing NOSEEK flags?

[Diederick C. Niehorster]

Dear Sir, Madam,

I am trying to figure out whether a format is seekable. It was suggested to
me to check the context->iformat (AVInputFormat) flags and whether
read_seek/read_seek2 are non-NULL. That however does not work correctly for
the dshow and gdigrab avdevice input formats, as these do not set any of
AVFMT_NOBINSEARCH, AVFMT_NOGENSEARCH, AVFMT_NO_BYTE_SEEK. Is that an
oversight, since these avdevices are realtime and do not support seeking?

Trying to seek these formats anyway leads to a Invalid argument error, not
an operation not permitted as one would expect, see, e.g.:
ffprobe -f dshow -i "video=Integrated Webcam" -show_frames -read_intervals
12%13
[...]
Input #0, dshow, from 'video=Integrated Webcam':
  Duration: N/A, start: 331739.487000, bitrate: N/A
  Stream #0:0: Video: mjpeg (Baseline) (MJPG / 0x47504A4D), yuvj422p(pc,
bt470bg/unknown/unknown), 1280x720, 30 fps, 30 tbr, 10000k tbn, 10000k tbc
Could not seek to position 12000000: Invalid argument
Could not read packets in interval id:0 start:12 end:13

That is with ffprobe version 4.4-full_build-www.gyan.dev (commit ref
f68ab9de4e).

I'd be happy to provide a patch. Looking at the source, this is also the
case for at least the v4l2, vfwcap, x11grab, fbdev, kmsgrab and
android_camera devices, but i would not be able to test most of these.

Thanks and all the best,
Dee