[FFmpeg-user] Copying MP4 Stream

[Rainer Dorsch]

Hi,

I am recording an MP4 stream using

ffmpeg -i rtsp:///Streaming/Channels/1 -r 20  -vcodec copy -an -t 3600 name.mp4

The first few seconds are always garbage. I assume because there is no keyframe 
in the stream, right when the recording starts. Recoding with -vcodec libx264 
does not show the problem, but the CPU it consumes it too high. I cannot tell 
if it does not have the problem, because it somehow manages to convince the 
codec of the camera to send a keyframe or if it just finds out which frames to 
discard.

What is somewhat strange is that the output of 

ffprobe -show_frames test-c3.mp4|grep key_frame|pastebinit

shows always keyframes at the beginning:

key_frame=1
key_frame=0
key_frame=1
key_frame=0
key_frame=1
key_frame=0
key_frame=0
key_frame=0
[...]
key_frame=0
key_frame=1
[...]

Here is the full output (w/o the grep):

	http://bokomoko.de/~rd/out.log

Assuming that the missing key frame is the problem in the copy mode, can I 
rather somehow discard the frames until the first real keyframe arrives in the 
mp4 stream (in the copy mode)? 

Is there a way to strip of the initial garbage of existing videos? 

Would determining the first frame with key_frame=1 with coded_picture_number>10 
in

ffprobe -show_frames test-c3.mp4

e.g.

[FRAME]
media_type=video
stream_index=0
key_frame=1
pkt_pts=142848
pkt_pts_time=13.950000
pkt_dts=142848
pkt_dts_time=13.950000
best_effort_timestamp=142848
best_effort_timestamp_time=13.950000
pkt_duration=512
pkt_duration_time=0.050000
pkt_pos=2091273
pkt_size=554015
width=2688
height=1520
pix_fmt=yuvj420p
sample_aspect_ratio=N/A
pict_type=I
coded_picture_number=285
display_picture_number=0
interlaced_frame=0
top_field_first=0
repeat_pict=0
color_range=pc
color_space=bt709
color_primaries=bt709
color_transfer=bt709
chroma_location=left
[/FRAME]

and use in there 

coded_picture_number=285

and throw away the first 285 frames (since it starts with 
coded_picture_number=0) ?

Many thanks
Rainer

-- 
Rainer Dorsch
http://bokomoko.de/