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• Latency in ffmpeg while live streaming

  22 janvier 2014, par mustafa.yavuz

  I am working on a project which has real time jobs. I should process a live stream with almost no latency. As you know when we play a live stream with ffplay there is some latency since it buffers some frames before playing it. Is there same problem with ffmpeg, that is when I connect a live stream and do some process like resampling and other calculations, does it have latency as ffplay has. How can I be sure there is no latency ?

• Cron job using database results

  9 mars 2013, par Fibericon

  I need help in creating a cron job script. Basically, I want to grab the next scheduled item and run it through ffmpeg to stream. This would be the mysql query (I'm using PHP variables to indicate what should go there - I don't actually know how variables work in cron jobs) :

  SELECT show.file FROM show, schedule
  
  WHERE channel = 1 AND start_time <= $current_time;

  This would be the ffmpeg command :

  ffmpeg -re -i $file http://127.0.0.1:8090/feed.ffm

  How would I create a cron job to execute these commands ?

• How to Match ASS Subtitle Font Size with Flutter Text Size for a 1080p Video ?

  16 décembre 2024, par Mostafa Fathi

  I’m working on a project where I need to synchronize the font size of ASS subtitles with the text size a user sees in a Flutter application. The goal is to ensure that the text size in a 1080p video matches what the user sees in the app.

  


  What I've Tried :

  


  

  1. Calculating font size using height ratio (PlayResY/DeviceHeight) :

  


  


  

  • I used the formula :

  


  


  FontSize_ASS = FontSize_Flutter * (PlayResY / DeviceHeight)


  


  

  • While the result seemed logical, the final output in the video was smaller than expected.

  


  


  

  2. Adding a scaling factor :

  


  


  

  • I introduced a scaling factor (around 3.0) to address the size discrepancy.

  


  • This improved the result but still felt inconsistent and lacked precision.

  


  


  

  3. Using force_style in FFmpeg :

  


  


  

  • I applied the force_style parameter to control the font size in FFmpeg directly.

  


  


  ffmpeg -i input.mp4 -vf "subtitles=subtitle.ass:force_style='FontSize=90'" -c:a copy output.mp4


  


  

  • While it produced better results, it’s not an ideal solution as it bypasses the calculations in the ASS file.

  


  


  

  4. Aligning PlayResX and PlayResY in the ASS file :
I ensured that these parameters matched the target video resolution (1920×1080) :

  


  


  PlayResX: 1920
PlayResY: 1080


  


  

  • Despite this adjustment, the font size didn’t align perfectly with the Flutter app text size.

  


  


  

  5. Reading font metrics from the font file dynamically :
To improve precision, I wrote a function in Flutter that reads font metrics (units per EM, ascender, and descender) from the TTF font file and calculates a more accurate scaling factor :

  


  


  Future readFontMetrics(
      String fontFilePath, 
      double originalFontSize,
) async {
  final fontData = await File(fontFilePath).readAsBytes();
  final fontBytes = fontData.buffer.asUint8List();
  final byteData = ByteData.sublistView(fontBytes);

  int numTables = readUInt16BE(byteData, 4);
  int offsetTableStart = 12;
  Map> tables = {};

  for (int i = 0; i < numTables; i++) {
    int recordOffset = offsetTableStart + i * 16;
    String tag =
        utf8.decode(fontBytes.sublist(recordOffset, recordOffset          + 4));
    int offset = readUInt32BE(byteData, recordOffset + 8);
    int length = readUInt32BE(byteData, recordOffset + 12);

    tables[tag] = {
      'offset': offset,
      'length': length,
    };
  }

  if (!tables.containsKey('head') || !tables.containsKey('hhea'){
    print('Required tables not found in the font file.');
    return null;
  }

  int headOffset = tables['head']!['offset']!;
  int unitsPerEm = readUInt16BE(byteData, headOffset + 18);

  int hheaOffset = tables['hhea']!['offset']!;
  int ascender = readInt16BE(byteData, hheaOffset + 4);
  int descender = readInt16BE(byteData, hheaOffset + 6);

  print('unitsPerEm: $unitsPerEm');
  print('ascender: $ascender');
  print('descender: $descender');

  int nominalSize = unitsPerEm;
  int realDimensionSize = ascender - descender;
  double scaleFactor = realDimensionSize / nominalSize;
  double realFontSize = originalFontSize * scaleFactor;

  print('Scale Factor: $scaleFactor');
  print('Real Font Size: $realFontSize');

  return realFontSize;
}


  


  

  • This function dynamically reads the font properties (ascender, descender, and unitsPerEM) and calculates a scale factor to get the real font size. Despite this effort, discrepancies persist when mapping it to the ASS font size.

  


  


  Question :
How can I ensure that the font size in the ASS file accurately reflects the size the user sees in Flutter ? Is there a reliable method to calculate or align the sizes correctly across both systems ? Any insights or suggestions would be greatly appreciated.

  


  Thank you ! 🙏