Pdfium.Net.Free 一个免费的Pdfium的 .net包装器--创建字符子集

项目地址：

Pdfium.Net：https://github.com/1000374/Pdfium.Net

PdfiumViewer：https://github.com/1000374/PdfiumViewer

Pdfium.Net.Free 一个免费的Pdfium的 .net包装器--加载字体 

接上篇，怎么创建字符子集呢？获取字符集内的字形符号需要引用wpf下PresentationCore.dll，根据比对传入字符和字符集中的字形获取对应的字形字节，从而生成新的字符集，

如需要生成ttf文件，则直接byte[]保存成ttf即可

  /// <summary>
  /// create SubCharacterSet
  /// </summary>
  /// <param name="fontPath">font path</param>
  /// <param name="sourceText"></param>
  /// <returns></returns>
  public static byte[] CreateSubSet(this string fontPath, string sourceText)
  {
      if (!File.Exists(fontPath))
          throw new ArgumentException($"{fontPath} not find");
      var glyphTypeface = new GlyphTypeface(new Uri(fontPath, UriKind.RelativeOrAbsolute));
      var Index = new List<ushort>();
      var sourceTextBytes = Encoding.Unicode.GetBytes(sourceText);
      var sourceTextChars = Encoding.Unicode.GetChars(sourceTextBytes);
      for (var charPos = 0; charPos <= (sourceTextChars.Length - 1); charPos++)
      {
          var sourceTextCharVal = (int)sourceTextChars[charPos];
          var glyphIndex = glyphTypeface.CharacterToGlyphMap[sourceTextCharVal];
          if (!Index.Contains(glyphIndex))
              Index.Add(glyphIndex);
      }
      return glyphTypeface.ComputeSubset(Index);
  }