Way to create closed GOP around desired clip

[Travillion]

Hello all,

Is there a way (preferably with ffmpeg as that is what I'm familiar with) to specify a closed GOP around a clip.

Many of my recordings require cuts and it is very difficult to accurately cut clips with the common open GOP structure. I have tried for a long time to cut only on IDR frames but even this is unreliable because of open GOPs.

Then I had the idea, what if I could re-encode and force a closed GOP around my desired clips? Is that possible? I know ffmpeg has a forced keyframes flag, but that doesn't garauntee the clip between keyframes will even be a single GOP, let alone a closed one. I also don't think I need to convert the entire source to closed GOP, just between the frames I want to cut.

Any ideas? I should clarify, sources are h264.

[Budman1]

Unless I am mistaken and all the videos I cut and paste are not what I think they are, FFMpeg creates Closed GOP by default. Most of the hundreds I have downloaded are also closed GOP. Cutting on key frame is a specific method to assure :
1. The final frame is a P frame
2. The final 5? frames Pts times are the same interval
3. No following frames have to be decoded before the P frame
4. FFMpeg script has to be specific, I.E. -ss Pecedes or follows -i filename depending on whjat you want to cut

if you copy the following to a BAT file and then drag and drop a video on the it, you will have a lot of information you need.

setlocal enabledelayedexpansion
@echo off
::for %%a in (*.mp4,*.mpg,*.flv) Do (
for %%a in ("%~dpnx1") Do (
set /a count=0
cd %%~dpa
echo frame,media_type,stream_index,key_frame,pkt_pts,pkt_pts_time,pkt_dts,pkt_dts_time,best_effort_timestamp,^
best_effort_timestamp_time,pkt_duration,pkt_duration_time,pkt_pos,pkt_size,^
Width,Height,pix_fmt,sample_aspect_ratio,pict_type,coded_picture_number,display_picture_number,^
interlaced_frame,top_field_first,repeat_pict,color_range,color_space,color_primaries,color_transfer,^
chroma_location > "%%~na_ffprobe.csv"
echo No.  pts_time  type  Frame# > "%%~na_AllFrames.txt"
ver > nul
set /a Number=0
ffprobe.exe -v quiet -select_streams v:0 -print_format csv -show_entries frame "%%~nxa"  >> "%%~na_ffprobe.csv"
for /F "tokens=4,6,18,19,20 delims=," %%b in ('findstr "video" "%%~na_ffprobe.csv"') do (
set "x=%%b"
set "y=%%c"
set "z=%%d"
set "w=%%e"
set "v=%%f"
set "u=%%g"
set sort=!y:~0,-7!
::if !sort! GEQ 0 if !sort! LEQ 10 echo !count!  !y:~0,13!  !x! !w:~0,1! >> "%%~na_AllFrames.txt"
echo !count!    !y:~0,13!  !x! !w:~0,1!     !v! >> "%%~na_AllFrames.txt"
set /a count+=1
 set /a ekko=count%%100
 if !ekko! EQU 0 echo Frame !count! processed
)
)
rem pause

This will produce a CSV file with all information and a shorter text file such as what follows here. notice that not only does the frame before the next 'I' frame is a 'P' frame but no following frames have a lowere decode number, meaning it would have to be decoded before the 'P' frame, signifying Closed GOP. Cutting here works fine for all my cuts.

No. pts_time type Frame#
0 0.080000 1 I 0
1 0.120000 0 B 3
2 0.160000 0 B 2
3 0.200000 0 B 4
4 0.240000 0 P 1
5 0.280000 0 B 7
6 0.320000 0 B 6
7 0.360000 0 P 5

70 2.880000 0 P 66
71 2.920000 0 B 73
72 2.960000 0 B 72
73 3.000000 0 B 74
74 3.040000 0 P 71
75 3.080000 1 I 75
76 3.120000 0 B 78
77 3.160000 0 B 79
78 3.200000 0 B 77
79 3.240000 0 B 80
80 3.280000 0 P 76
81 3.320000 0 B 83
82 3.360000 0 B 84
83 3.400000 0 B 82
84 3.440000 0 B 85
85 3.480000 0 P 81

[Travillion]

This has got to be the most helpful response I've ever gotten, on any forum. Thank you budman, for such a detailed response!