No code, or other documentation files, are touched by this change.
Signed-off-by: Jim DeLaHunt <from+ffmpeg-...@jdlh.com>
---
doc/filters.texi | 167 ++++++++++++++++++++++++++++++++++++++++++-----
1 file changed, 149 insertions(+), 18 deletions(-)
diff --git a/doc/filters.texi b/doc/filters.texi
index 71a6787289..bd4a1ad2a9 100644
--- a/doc/filters.texi
+++ b/doc/filters.texi
@@ -11139,27 +11139,34 @@ format=pix_fmts=yuv420p|yuv444p|yuv410p
@anchor{fps}
@section fps
-Convert the video to specified constant frame rate by duplicating or dropping
-frames as necessary.
+Generate a video, having the specified constant frame rate, from the frames of
+the input video, by copying or duplicating or dropping input frames based on
+their input presentation time stamps (PTSs). The output video has new PTSs. You
+can choose the method for rounding from input PTS to output PTS.
It accepts the following parameters:
@table @option
@item fps
-The desired output frame rate. The default is @code{25}.
+The output frame rate, as a number of frames per second. This value can be an
+integer, real, or rational number, or an abbreviation. The default is @code{25}.
@item start_time
-Assume the first PTS should be the given value, in seconds. This allows for
-padding/trimming at the start of stream. By default, no assumption is made
-about the first frame's expected PTS, so no padding or trimming is done.
-For example, this could be set to 0 to pad the beginning with duplicates of
-the first frame if a video stream starts after the audio stream or to trim any
-frames with a negative PTS.
+The time, in seconds from the start of the input stream, which is converted to
+an input starting PTS value and an output starting PTS value.
+If set, @var{fps} drops input frames
+which have PTS values less than the input starting PTS. If not set, the input
+and output starting PTS values are zero, but @var{fps} drops no input frames based
+on PTS.
+(See details below.)
@item round
-Timestamp (PTS) rounding method.
+Rounding method to use when calculating output Presentation Timestamp
+(PTS) integer values from input PTS values. If the calculated output PTS value
+is not exactly an integer, then the method determines which of the two
+neighbouring integer values to choose.
-Possible values are:
+Possible method names are:
@table @option
@item zero
round towards 0
@@ -11170,43 +11177,167 @@ round towards -infinity
@item up
round towards +infinity
@item near
-round to nearest
+round to nearest (and if exactly at midpoint, away from 0)
@end table
The default is @code{near}.
@item eof_action
-Action performed when reading the last frame.
+Action which @var{fps} takes with the final input frame. The input video passes
+in an ending input PTS, which @var{fps} converts to an ending output PTS.
+@var{fps} drops any input frames with a PTS at or after this ending PTS.
Possible values are:
@table @option
@item round
-Use same timestamp rounding method as used for other frames.
+Use same rounding method as for other frames, to convert the ending input PTS
+to output PTS.
+
@item pass
-Pass through last frame if input duration has not been reached yet.
+Round the ending input PTS using @code{up}. This can have the effect of passing
+through one last input frame.
@end table
+
The default is @code{round}.
@end table
-Alternatively, the options can be specified as a flat string:
+Alternatively, the options may be specified as a flat string:
@var{fps}[:@var{start_time}[:@var{round}]].
+@var{fps} generates an output video with integer Presentation Time Stamp (PTS)
+values which increment by one each output frame, and with a time base set to
+the inverse of the given frame rate. @var{fps} copies, duplicates, or drops
+input frames, in sequence, to the output video. It does so according to their
+input PTS values, as converted to seconds (via the input time base), then
+rounded to output PTS values.
+
+@var{fps} sets output PTS values in terms of a time line which starts at
+zero. The integer PTS value multipled by the output time base gives a point
+in seconds of that output frame on that timeline. If the @var{start_time}
+parameter is not set, or is zero, the first output frame's PTS value is zero.
+Otherwise, the first PTS value is the output starting PTS value calculated
+from the @var{start_time} parameter.
+
+@var{fps} interprets input PTS values in terms of the same time line. It
+multiplies the input PTS value by the input time base time, to get a frame
+position in seconds on the time line. It rounds that position to an integer
+output PTS value. For example, if the input video has a frame rate
+of 30 fps, a time base of 1/30 seconds, and its first frame has a
+PTS of 300, then @var{fps} treats that frame as occurring 10 seconds (300 * 1/30
+seconds) after the start of the video, even though it is the first frame.
+
+Setting a @code{start_time} value allows for padding/trimming at the
+start of stream. For example, @code{start_time} could be set to 0 to pad the
+beginning with duplicates of the first frame if a video stream starts after
+the audio stream or to trim any frames with a negative PTS. When
+@code{start_time} is not set, the @var{fps} filter makes no assumption about
+the first frame's expected PTS, and does not pad or trim input frames which
+have a PTS set.
+
See also the @ref{setpts} filter.
+@subsection Details
+
+@var{fps} emits exactly one frame for each output PTS value. If there is
+exactly one input frame with an input PTS which converts to the current output
+PTS value, @var{fps} emits that frame. If there are multiple frames which
+convert to the same output PTS value, @var{fps} emits the final frame of that
+group, and drops the previous frames. If no input frame PTS converts to a given
+output PTS value, @var{fps} emits another copy of the previously emitted frame.
+When the first input frame converts to an output PTS after the first frame, then
+@var{fps} emits copies of that first frame until the output PTS reaches the
+converted value of that first frame's input PTS.
+
+@var{fps} always drops input frames which have no PTS value set, regardless
+of the @var{start_time} parameter.
+
+The @var{frame rate} value may be provided in a variety of forms. Each form is
+converted into a rational number, with an integer numerator and denominator.
+Each value must be zero or greater.
+
+@itemize
+
+@item
+An integer number, e.g. @code{25}. This converts to the rational number
+@code{25/1}.
+
+@item
+A real number, e.g. @code{3.14145926}. This converts to a rational number,
+e.g. @code{954708/303893}
+
+@item
+A rational number. The numerator and denominator may be either integers or real
+numbers. e.g. @code{30/1.001} or @code{30000/1001}, which both convert to
+@code{30000/1001}.
+
+@item
+An abbreviation. e.g @code{ntsc} as @code{30000/1001},
+@code{ntsc-film} as @code{24000/1001}. See the complete list at
+@ref{Video rate,,the "Video rate" section in the ffmpeg-utils(1) manual,ffmpeg-utils}.
+
+@end itemize
+
+@var{fps} defines a sync point, where one input PTS value is treated as
occuring
+at the same moment as one output PTS value. IT calcluates other PTS values
+as positive or negative time offsets from this sync point.
+This affects the details of how rounding works. If @var{start_time} is set,
+then the input and output PTS values which @var{fps} calculates from this
+become the sync point. Otherwise, input and output PTS values of zero are
+the sync point.
+
+Note that @var{fps} does not assume that input frames are separated by exactly
+1/frame_rate seconds. It takes the input PTS values literally. If the increment
+of PTS between frames varies along the video, fps will treat those frames as
+happening at varying time intervals.
+
+As a consequence, if you have a video which is supposed to be at a certain frame
+rate, but in reality the frames were not always captured at the exactly right
+moment, and if the input PTS values reflect that variation, then you can pass
+this video through an @var{fps} filter set to the same frame rate. A @var{round}
+method of @code{near} will pass through each input frame exactly once, and
+the output video will have new PTS values which reflect the exact time interval
+(1/frame rate) between frames.
+
+Because @var{fps} treats input PTS values at face value when converting them to
+time on the time line, and because that time line starts at zero, an input
+video with PTS values that do not start at zero might yield unexpected results.
+Suppose the input PTS values start with the value 300, which converts to 10
+seconds after the sync point. Then @var{fps} will repeat the first frame of the
+input video to fill the first 10 seconds of the output video. (However,
+@command{ffmpeg} may suppress those repeated frames, depending on the
+@option{-vsync} setting.) If you set @var{start_time} to 10 seconds, then
+@var{fps} sets the sync point to the PTS values converted from the 10 second
+moment on the time line. It no longer repeats the first frame. And, it starts
+the output PTS at the value corresponding to 10 seconds, instead of zero.
+
@subsection Examples
@itemize
@item
-A typical usage in order to set the fps to 25:
+A typical usage in order to emit a video with a frame rate of 25 frames per
second:
@example
fps=fps=25
@end example
@item
-Sets the fps to 24, using abbreviation and rounding method to round to nearest:
+Emit a video with a frame rate of 24 frames per second, using an abbreviation,
+and rounding method to round to nearest:
@example
fps=fps=film:round=near
@end example
+
+@item
+If an input video is supposed to have a frame rate of 29.97 frames per second
+(NTSC standard), but the time base is 3003/90000, and the PTS values increment
+variably at slightly more and less than that rate, this emits a video with the
+same frames, but with a new time base and PTS values that increment at exactly
+the NTSC rate. If some frames were dropped by the recorder, but the PTS values
+still reflect when the remaining frames were captured, this will also repeat
+frames to fill the gaps from the dropped frames.
+@example
+fps=fps=30/1.001:round=near
+@end example
+
@end itemize
@section framepack
--
2.26.2
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