• A little theory
• Function GetSwfFileHeader

A little theory

Any SWF-file consists of 2 main parts: Header and Body. So reading of header is enough to get info about the movie. Header record is described by the following structure:

TSWFHeader = packed record
  Signature: array [0..2] of char;
  Version: byte;
  FileLength: cardinal;
  FrameSize: TSWFRect;
  FrameRate: byte;
  FrameRateRemainder: byte;
  FrameCount: cardinal;
end;

TSWFRect = packed record
  Xmin: integer;  // in twips
  Xmax: integer;  // in twips
  Ymin: integer;  // in twips
  Ymax: integer;  // in twips
end;

Header reading is complicated by two features:
1. Size of header record is not a fixed value because of the bit-packing techniques used for Rect record storing. Rect structure: Nbits, Xmin, Xmax, Ymin, Ymax. The unsigned Nbits field occupies the first five bits and indicates how long the next four signed fields are.
2. In compressed format a part of header is stored in the packed part of the file. Has been uncompressed using the open standard ZLIB. The data format used by the ZLIB library is described by Request for Comments (RFCs) documents 1950 to 1952.

Function GetSwfFileHeader

 function GetSwfFileHeader(const FileName: String; var Header: TSWFHeader): boolean;

Function returns true if the file is read successfully. Rect record fields are in twips (1 px = 20 twips). Frame rate consists of 2 parts: FrameRate is integer and FrameRateRemainder is fractional. The complete value is calculated from
FrameRate + FrameRateRemainder / 256.

How it works:
1. Reading first 8 bytes. This is non compressed part of header.
2. Signature comparing. Unpacking in case compressed format.
3. Reading the Rect bit structure and other data.

Function GetSwfFileHeader(const FileName: string; var Header: TSWFHeader): boolean;
const
  BuffSize = 25;
var
  Buffer: PByteArray;
  NBitsField: byte;
  Poz: longword;
  FileStream: TFileStream;
  MemStream: TMemoryStream;
  ZStream: TDecompressionStream;
begin
  Result:= FALSE;
  if not FileExists(FileName) then Exit;
  FileStream:= TFileStream.Create(FileName, fmOpenRead);
  try
    FileStream.Position:= 0;
    if FileStream.Size > 22 then
    begin
      GetMem(Buffer, BuffSize);
      try
        FileStream.Read(Header, 8);
        if(Header.Signature = 'CWS') and (Header.Version >= 6) then
        begin
          Result:= TRUE;
          MemStream:= TMemoryStream.Create;
          try
            MemStream.CopyFrom(FileStream, FileStream.Size-8);
            MemStream.Position:= 0;
            ZStream:= TDecompressionStream.Create(MemStream);
            try
              ZStream.Read(Buffer^, BuffSize);
            finally
              ZStream.Free;
            end;
          finally
            MemStream.Free;
          end;
        end
          else begin
            FileStream.Read(Buffer^, BuffSize);
            Result:= Header.Signature = 'FWS';
          end;

          if Result then
            with Header do
            begin
              Poz:= 0;
              NBitsField:= TBitWidth(ReadNBits(Buffer^, Poz, 5)); Inc(Poz, 5);
              FrameSize.Xmin:= Integer(ReadNBits(Buffer^, Poz, NBitsField)); Inc(Poz, NBitsField);
              FrameSize.Xmax:= Integer(ReadNBits(Buffer^, Poz, NBitsField)); Inc(Poz, NBitsField);
              FrameSize.Ymin:= Integer(ReadNBits(Buffer^, Poz, NBitsField)); Inc(Poz, NBitsField);
              FrameSize.Ymax:= Integer(ReadNBits(Buffer^, Poz, NBitsField)); Inc(Poz, NBitsField);
              NBitsField:= Poz mod 8;
              Poz:= Poz div 8;
              if (NBitsField > 0) then Inc(Poz);
              FrameRateRemainder:= Buffer^[Poz]; // 8.[8]
              FrameRate:= Buffer^[Poz+1];
              FrameCount:= Buffer^[Poz+2] or (Buffer^[Poz+3] shl 8);
            end;

       finally
         FreeMem(Buffer);
       end;
    end;
  finally
    FileStream.Free;
  end;
end;

The full source is SwfFileInfo.pas.

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