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doc/ffmpeg: document value source for -force_key_frames 

Also clarify behaviour in case of specified timestamps
This commit is contained in:
Gyan Doshi 2019-04-20 14:16:09 +05:30
parent c3a17ffff6
commit fa677750de
1 changed files with 23 additions and 5 deletions
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doc/ffmpeg.texi
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@ -879,12 +879,19 @@ Deprecated see -bsf
@item -force_key_frames[:@var{stream_specifier}] @var{time}[,@var{time}...] (@emph{output,per-stream})
@item -force_key_frames[:@var{stream_specifier}] expr:@var{expr} (@emph{output,per-stream})
Force key frames at the specified timestamps, more precisely at the first
frames after each specified time.
@item -force_key_frames[:@var{stream_specifier}] source (@emph{output,per-stream})
If the argument is prefixed with @code{expr:}, the string @var{expr}
is interpreted like an expression and is evaluated for each frame. A
key frame is forced in case the evaluation is non-zero.
@var{force_key_frames} can take arguments of the following form:
@table @option
@item @var{time}[,@var{time}...]
If the argument consists of timestamps, ffmpeg will round the specified times to the nearest
output timestamp as per the encoder time base and force a keyframe at the first frame having
timestamp equal or greater than the computed timestamp. Note that if the encoder time base is too
coarse, then the keyframes may be forced on frames with timestamps lower than the specified time.
The default encoder time base is the inverse of the output framerate but may be set otherwise
via @code{-enc_time_base}.
If one of the times is "@code{chapters}[@var{delta}]", it is expanded into
the time of the beginning of all chapters in the file, shifted by
@ -898,6 +905,11 @@ before the beginning of every chapter:
-force_key_frames 0:05:00,chapters-0.1
@end example
@item expr:@var{expr}
If the argument is prefixed with @code{expr:}, the string @var{expr}
is interpreted like an expression and is evaluated for each frame. A
key frame is forced in case the evaluation is non-zero.
The expression in @var{expr} can contain the following constants:
@table @option
@item n
@ -925,6 +937,12 @@ starting from second 13:
-force_key_frames expr:if(isnan(prev_forced_t),gte(t,13),gte(t,prev_forced_t+5))
@end example
@item source
If the argument is @code{source}, ffmpeg will force a key frame if
the current frame being encoded is marked as a key frame in its source.
@end table
Note that forcing too many keyframes is very harmful for the lookahead
algorithms of certain encoders: using fixed-GOP options or similar
would be more efficient.