R/vis-adjust_playback_time.r

In jsta/glatos: A package for the Great Lakes Acoustic Telemetry Observation System

##' Modify playback time of video
##'
##' Speed up or slow down playback of video using FFmpeg software.
##' 
##' @param scale_factor multiplicative factor changes duration of
##'   video playback. See details.
##'
##' @param input character, path to video file (any FFmpeg supported
##'   file type- *.mp4, *.wmv, see \link{make_frames})
##'
##' @param output_dir character, output directory, default is working
##'   directory
##'
##' @param output character, output file name
##' 
##' @param overwrite logical, default is \code{overwrite = TRUE}
##'
##' @param ffmpeg A file path (characer) to FFmpeg executable. This
##'   argument is only needed if ffmpeg is not added to your system
##'   path. For Windows machines, path must point to 'ffmpeg.exe',
##'   located in the bin subfolder within the ffmpeg folder.  For
##'   example on Windows machines,
##'   "C:\\Users\\Username\\Documents\\ffmpeg-3.4.1-win64-static\\bin\\ffmpeg.exe").
##'   On Mac, path must point to 'ffmpeg' within the 'bin'
##'   subfolder "/home/directory/Documents/bin/ffmpeg".  see \link{make_video}
##'
##' @param diagnostic_mode Logical (default = FALSE). If true, return value
##'  is a character vector with FFMPEG output.
##'  
##' @details \code{adjust_playback_time} adjusts playback speed of
##'   video.  \code{scale_factor} controls the magnitude of speed-up
##'   or slow-down by modifying the presentation timestamp of each
##'   video frame. For more information, see 
##'   \href{https://trac.ffmpeg.org/wiki}{How to speed up/slow down a video}.  
##'   Values of \code{scale_factor} < 1 speed up playback and >1 slow
##'   down video. In addition to changing playback, function can
##'   change output format by specifying a different file extension in
##'   \code{output}.
##' 
##' @return One video animation will be written to \code{output_dir}
##'
##' @author Todd Hayden, Tom Binder, Chris Holbrook
##'
##' @examples
##' 
##' \dontrun{
##' 
##' # load example frames 
##' frames <- system.file("extdata", "frames", package = "glatos")
##'
##' # make video animation 
##' make_video(dir = frames, pattern = "%02d.png", output = "animation.mp4" )
##'
##' # slow video down by a factor of 10 
##' path <- file.path(getwd(), "animation.mp4")
##' adjust_playback_time(scale_factor = 10, input = path)
##'
##' # slow video down by a factor of 10 and change format of output video
##' adjust_playback_time(scale_factor = 10, input = path, output = "slow.wmv")
##' 
##' # speed up video 
##' adjust_playback_time(scale_factor=0.5, input = path, output = "faster.mp4")
##' }
##' @export
##'


adjust_playback_time <- function(scale_factor = 1,
                                 input, 
                                 output_dir = getwd(),
                                 output = "new.mp4",
                                 overwrite = FALSE,
                                 ffmpeg = NA,
                                 diagnostic_mode = FALSE){

  # test ffmpeg and get path
  ffmpeg <- get_ffmpeg_path(ffmpeg)
    
  cmd <- ifelse(is.na(ffmpeg), 'ffmpeg', ffmpeg)
  
  input <- shQuote(input)
  output_file <- file.path(output_dir, output)
  out <- shQuote(output_file)
  ffcall <- sprintf('-i %s -filter:v "setpts=%f*PTS" %s %s', input,
                    scale_factor, out, (ifelse(overwrite, "-y", "-n")))
  
  #check if output file exists
  if(file.exists(output_file) & overwrite == FALSE) {
    warning("No video file written because output file already exists and ",
      "overwrite = FALSE.")
    return()
  }
  
  msg_i <-  system2(cmd, ffcall, stdout = TRUE)

  if(diagnostic_mode) {
    message("[diagnostic mode]: See return object for ffmpeg output.")
    return(msg_i)
  }
  
  message("Video file written to ", output_file, ".")
  return(output_file)

}