Frame count for remuxed mkv

When creating a mkv file, the video stream does not contain several important bits of metadata, such as the number of frames.

I am not sure if this is a deficiency of ffmpeg, a feature(?) of the mkv container or an omission in the remuxing process. I believe this may be related to why mkv files do not seek either (probably missing other necessary metadata).

Of course, you can get a frame count by scanning the whole file, but this is very slow.

The example below, adapted from the pyav docs, shows the problem.

TIA for your help.

import av
import av.datasets


input_ = av.open(av.datasets.curated("pexels/time-lapse-video-of-night-sky-857195.mp4"))
output = av.open("remuxed.mkv", "w")

# Make an output stream using the input as a template. This copies the stream
# setup from one to the other.
in_stream = input_.streams.video[0]
out_stream = output.add_stream(template=in_stream)

for packet in input_.demux(in_stream):

    print(packet)

    # We need to skip the "flushing" packets that `demux` generates.
    if packet.dts is None:
        continue

    # We need to assign the packet to the new stream.
    packet.stream = out_stream

    output.mux(packet)

input_.close()
output.close()

remuxed = av.open("remuxed.mkv")
print(remuxed.frames)

ffprobe dump below confirms that the necessay metadata is not in the file:

Input #0, matroska,webm, from 'remuxed.mkv':
  Metadata:
    ENCODER         : Lavf58.76.100
  Duration: 00:00:06.72, start: 0.000000, bitrate: 2471 kb/s
  Stream #0:0: Video: h264 (High), yuv420p(tv, bt709, progressive), 1280x720 [SAR 1:1 DAR 16:9], 25 fps, 25 tbr, 1k tbn, 50 tbc (default)
    Metadata:
      DURATION        : 00:00:06.720000000
[STREAM]
index=0
codec_name=h264
codec_long_name=H.264 / AVC / MPEG-4 AVC / MPEG-4 part 10
profile=High
codec_type=video
codec_tag_string=[0][0][0][0]
codec_tag=0x0000
width=1280
height=720
coded_width=1280
coded_height=720
closed_captions=0
has_b_frames=2
sample_aspect_ratio=1:1
display_aspect_ratio=16:9
pix_fmt=yuv420p
level=32
color_range=tv
color_space=bt709
color_transfer=bt709
color_primaries=bt709
chroma_location=left
field_order=progressive
refs=1
is_avc=true
nal_length_size=4
id=N/A
r_frame_rate=25/1
avg_frame_rate=25/1
time_base=1/1000
start_pts=0
start_time=0.000000
duration_ts=N/A
duration=N/A
bit_rate=N/A
max_bit_rate=N/A
bits_per_raw_sample=8
nb_frames=N/A
nb_read_frames=N/A
nb_read_packets=N/A
DISPOSITION:default=1
DISPOSITION:dub=0
DISPOSITION:original=0
DISPOSITION:comment=0
DISPOSITION:lyrics=0
DISPOSITION:karaoke=0
DISPOSITION:forced=0
DISPOSITION:hearing_impaired=0
DISPOSITION:visual_impaired=0
DISPOSITION:clean_effects=0
DISPOSITION:attached_pic=0
DISPOSITION:timed_thumbnails=0
TAG:DURATION=00:00:06.720000000
[/STREAM]