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• 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 ! 🙏

  


• FFMPEG conversion from MOV to MP4 Issue : Only 1st second is converted

  24 avril 2012, par Jeff

  i'm trying to convert an MOV file to MP4.
  I've tried so many options but the output file still only 1 second length.

  This is one of the FFMPEG comand I used :

  ffmpeg -i NY_BTS1PastryHQnew.mov -f mp4 -vcodec copy -acodec copy
  output.mp4

  The output.mp4 is only 1sec long.
  The output of the FFMPEG is :

  ffmpeg version git-2012-02-22-534a82a Copyright (c) 2000-2012 the FFmpeg developers
  
    built on Feb 22 2012 14:44:38 with gcc 4.4.5
  
    configuration: --enable-gpl --enable-libfaac --enable-libmp3lame --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libtheora --enable-libvorbis --enable-libx264 --enable-nonfree --enable-postproc --enable-version3 --enable-x11grab
  
    libavutil      51. 40.100 / 51. 40.100
  
    libavcodec     54.  4.100 / 54.  4.100
  
    libavformat    54.  1.100 / 54.  1.100
  
    libavdevice    53.  4.100 / 53.  4.100
  
    libavfilter     2. 62.101 /  2. 62.101
  
    libswscale      2.  1.100 /  2.  1.100
  
    libswresample   0.  7.100 /  0.  7.100
  
    libpostproc    52.  0.100 / 52.  0.100
  
  Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'NY_BTS1PastryHQnew.mov':
  
    Metadata:
  
      major_brand     : qt  
  
      minor_version   : 512
  
      compatible_brands: qt  
  
      creation_time   : 2011-06-17 01:38:11
  
      encoder         : Lavf54.1.100
  
    Duration: 00:00:01.45, start: 0.000000, bitrate: 225 kb/s
  
      Stream #0:0(eng): Video: h264 (Main) (avc1 / 0x31637661), yuv420p, 1280x720, 277 kb/s, 25 fps, 25 tbr, 2500 tbn, 5k tbc
  
      Metadata:
  
        creation_time   : 2011-06-17 01:38:11
  
        handler_name    : 
  
                          DataHandler
  
      Stream #0:1(eng): Audio: aac (mp4a / 0x6134706D), 48000 Hz, stereo, s16, 122 kb/s
  
      Metadata:
  
        creation_time   : 2011-06-17 01:38:11
  
        handler_name    : 
  
                          DataHandler
  
  File 'output.mp4' already exists. Overwrite ? [y/N] y
  
  Output #0, mp4, to 'output.mp4':
  
    Metadata:
  
      major_brand     : qt  
  
      minor_version   : 512
  
      compatible_brands: qt  
  
      creation_time   : 2011-06-17 01:38:11
  
      encoder         : Lavf54.1.100
  
      Stream #0:0(eng): Video: h264 (![0][0][0] / 0x0021), yuv420p, 1280x720, q=2-31, 277 kb/s, 25 fps, 2500 tbn, 2500 tbc
  
      Metadata:
  
        creation_time   : 2011-06-17 01:38:11
  
        handler_name    : 
  
                          DataHandler
  
      Stream #0:1(eng): Audio: aac (@[0][0][0] / 0x0040), 48000 Hz, stereo, 122 kb/s
  
      Metadata:
  
        creation_time   : 2011-06-17 01:38:11
  
        handler_name    : 
  
                          DataHandler
  
  Stream mapping:
  
    Stream #0:0 -> #0:0 (copy)
  
    Stream #0:1 -> #0:1 (copy)
  
  Press [q] to stop, [?] for help
  
  frame=   12 fps=  0 q=-1.0 Lsize=      40kB time=00:00:00.40 bitrate= 814.6kbits/s    
  
  video:16kB audio:22kB global headers:0kB muxing overhead 4.766554%

  I can't see any error, do you have a clue ?

  Thanks

• ffmpeg on fb0 from Nexus Galaxy error : "could not get frame filename number 2"

  3 mars 2012, par user848106

  I use ffmpeg to convert fb0 files from Androids and produce screenshots. For some reason this does not work with the Nexus Galaxy.

  I get this error :

  [image2 @ 0000000001E0E350] Could not get frame filename number 2 from pattern '
  
  image.png'
  
  av_interleaved_write_frame(): Invalid argument

  Here is the process :

      C:\dev\scripts>adb pull /dev/graphics/fb0
  
  3292 KB/s (16777216 bytes in 4.976s)
  
  
  
  C:\dev\scripts>ffmpeg -vframes 1 -vcodec rawvideo -f rawvideo -pix
  
  _fmt rgb32 -s 720x1080 -i fb0 -f image2 -vcodec png image.png
  
  ffmpeg version N-36635-gceb0dd9 Copyright (c) 2000-2012 the FFmpeg developers
  
    built on Jan  9 2012 17:45:55 with gcc 4.6.2
  
    configuration: --enable-gpl --enable-version3 --disable-w32threads --enable-ru
  
  ntime-cpudetect --enable-avisynth --enable-bzlib --enable-frei0r --enable-libope
  
  ncore-amrnb --enable-libopencore-amrwb --enable-libfreetype --enable-libgsm --en
  
  able-libmp3lame --enable-libopenjpeg --enable-librtmp --enable-libschroedinger -
  
  -enable-libspeex --enable-libtheora --enable-libvo-aacenc --enable-libvo-amrwben
  
  c --enable-libvorbis --enable-libvpx --enable-libx264 --enable-libxavs --enable-
  
  libxvid --enable-zlib
  
    libavutil      51. 34.100 / 51. 34.100
  
    libavcodec     53. 54.100 / 53. 54.100
  
    libavformat    53. 29.100 / 53. 29.100
  
    libavdevice    53.  4.100 / 53.  4.100
  
    libavfilter     2. 58.100 /  2. 58.100
  
    libswscale      2.  1.100 /  2.  1.100
  
    libswresample   0.  6.100 /  0.  6.100
  
    libpostproc    51.  2.100 / 51.  2.100
  
  [rawvideo @ 000000000037D5A0] Estimating duration from bitrate, this may be inac
  
  curate
  
  Input #0, rawvideo, from 'fb0':
  
    Duration: N/A, start: 0.000000, bitrate: N/A
  
      Stream #0:0: Video: rawvideo (BGRA / 0x41524742), bgra, 720x1080, 25 tbr, 25
  
   tbn, 25 tbc
  
  [buffer @ 000000000037D420] w:720 h:1080 pixfmt:bgra tb:1/1000000 sar:0/1 sws_pa
  
  ram:
  
  Output #0, image2, to 'image.png':
  
    Metadata:
  
      encoder         : Lavf53.29.100
  
      Stream #0:0: Video: png, bgra, 720x1080, q=2-31, 200 kb/s, 90k tbn, 25 tbc
  
  Stream mapping:
  
    Stream #0:0 -> #0:0 (rawvideo -> png)
  
  Press [q] to stop, [?] for help
  
  [image2 @ 0000000001E0E350] Could not get frame filename number 2 from pattern '
  
  image.png'
  
  av_interleaved_write_frame(): Invalid argument