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• Formatting ffmpeg arguments correctly in Swift

  22 octobre 2019, par NCrusher

  I deleted my previous post about this because I’ve gone through a lot more trial and error on it and I needed to be sure I was giving current and relevant information.

  I’m trying to create a very simple audio conversion app for MacOS using ffmpeg. Because it’s geared at audiobooks, the audio options are pretty basic.

  func conversionSelection() {
  
      if inputFileUrl != nil {
  
          let conversionChoice = conversionOptionsPopup.indexOfSelectedItem
  
          switch conversionChoice {
  
              case 1 :
  
                  outputExtension = ".mp3"
  
                  ffmpegFilters = ["-c:a libmp3lame", "-ac 1", "-ar 22050", "-q:a 9"]
  
              case 2 :
  
                  outputExtension = ".mp3"
  
                  ffmpegFilters = ["-c:a libmp3lame", "-ac 2", "-ar 44100", "-q:a 5"]
  
              case 3 :
  
                  outputExtension = ".mp3"
  
                  ffmpegFilters = ["-c:a libmp3lame", "-ac 1", "-ar 22050", "-b:a 32k"]
  
              case 4:
  
                  outputExtension = ".flac"
  
                  ffmpegFilters = ["-c:a flac"]
  
              default :
  
                  outputExtension = ".m4b"
  
                  ffmpegFilters = ["-c copy"]
  
          }
  
      }
  
  }

  I don’t want to inundate this post with code, but I’m not sure what all is necessary here for people to help me troubleshoot this problem. This is the code that allows me to get my input and output paths set up :

  func updateOutputText(outputString: String, outputField: NSTextField) {
  
      if inputFileUrl != nil && outputDirectoryUrl == nil {
  
          // derive output path and filename from input and tack on a new extension if a different conversion format is chosen
  
          let outputFileUrl = inputFileUrl!.deletingPathExtension()
  
          let outputPath = outputFileUrl.path
  
          outputFilePath = outputPath + "\(outputExtension)"
  
      } else if inputFileUrl != nil && outputDirectoryUrl != nil {
  
          // derive output directory from outputBrowseButton action, derive filename from input file, and derive output format from conversionSelection
  
          let outputFile = inputFileUrl!.deletingPathExtension()
  
          let outputFilename = outputFile.lastPathComponent
  
          let outputDirectory = outputDirectoryUrl!.path
  
          outputFilePath = outputDirectory + "/" + outputFilename + "\(outputExtension)"
  
      }
  
      outputTextDisplay.stringValue = outputFilePath
  
  }
  
  
  
  // update input and output text fields
  
  func updateInputText(inputString: String, inputField: NSTextField) {
  
      conversionSelection()
  
      inputTextDisplay.stringValue = inputFilePath
  
  }

  Everything on the Swift side appears to be working. The input and output Browse buttons work fine. The input and output file paths are written to text fields exactly as they should be. When I select a different conversion option, it updates the file extension for my output file.

  Here are my methods for actually launching ffmpeg :

  func ffmpegConvert(inputPath: String, filters: String, outputPath: String) {
  
      guard let launchPath = Bundle.main.path(forResource: "ffmpeg", ofType: "") else { return }
  
      do {
  
          let convertTask: Process = Process()
  
          convertTask.launchPath = launchPath
  
          convertTask.arguments = [
  
              "-i", inputPath,
  
              filters,
  
              outputPath
  
          ]
  
          convertTask.standardInput = FileHandle.nullDevice
  
          convertTask.launch()
  
          convertTask.waitUntilExit()
  
      }
  
  }
  
  @IBAction func startConversionClicked(_ sender: Any) {
  
      ffmpegConvert(inputPath: inputFilePath, filters: ffmpegFilters.joined(), outputPath: "outputFilePath")
  
  }

  The errors are coming from ffmpeg. But I’m fairly certain the PROBLEM is that I haven’t figured out how to write them so that Swift will pass them on to ffmpeg properly.

  If I use the argument arrays exactly as they are formatted above, these are my results :

  When I choose the default option (.m4b, "-c copy") I get this error :

  Unrecognized option ’c copy’. Error splitting the argument list :
  Option not found

  If I choose any of the other options (mp3 or flac), I get both a warning that reads

  Trailing options were found on the commandline.

  Then it will actually read in the metadata of my input file (so I know my input path works at least) and then it will tell me :

  At least one output file must be specified

  So while it’s reading my input file correctly, perhaps it’s not reading my output file path ?

  Moving on.

  I then put double hyphens in the argument strings, thinking that perhaps the reason ffmpeg was reading "-c copy" as "c copy" is because the hyphen is a mathematical operator. My results are as follows :

  (Still doesn’t read metadata)
  Unrecognized option ’-c copy’.
  Error splitting the argument list : Option not found

  (choosing .mp3 output options)
  (doesn’t read metadata this time)
  Unrecognized option ’-c:a libmp3lame—ac 1—ar 22050—q:a 9’.
  Error splitting the argument list : Option not found

  (choosing .flac output option)
  (doesn’t read metadata this time)
  Unrecognized option ’-c:a flac’.
  Error splitting the argument list : Option not found

  So. No help there. Time for a different approach.

  This time, I added a whitespace in front of my arguments. Results :

  (reads metadata)
  [NULL @ 0x106800000] Unable to find a suitable output format for ’ -c copy’
  -c copy : Invalid argument

  (reads metadata)
  [NULL @ 0x10b004400] Unable to find a suitable output format for ’ -c:a libmp3lame -ac 1 -ar 22050 -q:a 9’
  -c:a libmp3lame -ac 1 -ar 22050 -q:a 9 : Invalid argument

  (reads metadata)
  NULL @ 0x10580c400] Unable to find a suitable output format for ’ -c:a flac’
  -c:a flac : Invalid argument

  So, again, I have fairly decent confirmation that it’s reading my input file correctly, but possibly not my output file. Which might make sense in that the process of stringing together my output filepath is a lot more complex than the process for my input file, but I have visual confirmation that my output file path is accurate displayed in my outputTextDisplay box, which displays the outputFilePath perfectly.

  I really have no idea what isn’t working here. I’m sorry for the long post, but I’m not able to narrow the pertinent information down any further.

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