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