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• libFLAC : CPUID detecion improvements.

  28 juin 2014, par Erik de Castro Lopo

  libFLAC : CPUID detecion improvements.
  According to docs, it’s incorrect to just call CPUID with EAX=1.
  
  One must to ensure that this value is supported.
  CPUs that don’t support CPUID level 1 are very old, but...
  
  if FLAC tests CPUID presence it should also test CPUID level support.
  Also the function FLAC__cpu_have_cpuid_asm_ia32 was simplified
  
  according to the docs at Intel website and in Wikipedia.
  Patch-from : lvqcl <lvqcl.mail@gmail.com>
  

  • [DH] src/libFLAC/cpu.c
  • [DH] src/libFLAC/ia32/cpu_asm.nasm

• avcodec/vc1dec : Re-order init to avoid initting hwaccel too early

  8 août 2015, par Philip Langdale

  avcodec/vc1dec : Re-order init to avoid initting hwaccel too early
  At least for vdpau, the hwaccel init code tries to check the video
  
  profile and ensure that there is a matching vdpau profile available.
  If it can’t find a match, it will fail to initialise.
  In the case of wmv3/vc1, I observed initialisation to fail all the
  
  time. It turns out that this is due to the hwaccel being initialised
  
  very early in the codec init, before the profile has been extracted
  
  and set.
  Conceptually, it’s a simple fix to reorder the init code, but it gets
  
  messy really fast because ff_get_format(), which is what implicitly
  
  trigger hwaccel init, is called multiple times through various shared
  
  init calls from h263, etc. It’s incredibly hard to prove to my own
  
  satisfaction that it’s safe to move the vc1 specific init code
  
  ahead of this generic code, but all the vc1 fate tests pass, and I’ve
  
  visually inspected a couple of samples and things seem correct.
  Signed-off-by : Philip Langdale <philipl@overt.org>
  

  • [DH] libavcodec/vc1dec.c

• 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 ! 🙏