F-IN-BOX Blog

How to get a width and a height from an SWF

• Author: Artem Razin
• 2 May, 2007
• Category: SWF format

Many users ask me how to get dimensions of a movie. The answer is simple: just decode an SWF header. All Macromedia Flash (SWF) files begin with the following header (see SWF specification for more information):

If an SWF file begins with “CWS”, the header is zipped. Unzip it. Use zlib, for example. The sample code:

SIZE GetFlashMovieSize(LPCTSTR szSWFPath)
{
	SIZE size = { 0 };
	HANDLE hFile = CreateFile(szSWFPath,
                                  GENERIC_READ,
                                  FILE_SHARE_READ,
                                  NULL,
                                  OPEN_EXISTING,
                                  0,
                                  NULL); 

	if (INVALID_HANDLE_VALUE != hFile)
	{
		DWORD dwNumberOfBytesRead; 

		// Read the first byte
		char chFirstByte;
		ReadFile(hFile,
                         (LPVOID)&chFirstByte,
                         sizeof(chFirstByte),
                         &dwNumberOfBytesRead,
                         NULL); 

		// Skip signature (3 bytes), version (1 byte) and file size (4 bytes)
		const DWORD dwOffset = 3 + 1 + 4;
		SetFilePointer(hFile, dwOffset, NULL, FILE_BEGIN); 

		char outbuffer[256]; 

		if (67 == chFirstByte)
			// Compressed
		{
			char buffer[256];
			DWORD dwNumberOfBytesRead;
			ReadFile(hFile,
                                 buffer,
                                 sizeof(buffer),
                                 &dwNumberOfBytesRead,
                                 NULL); 

			z_stream stream = { 0 };
			stream.next_in = (Bytef*)buffer;
			stream.avail_in = 256;
			stream.next_out = (Bytef*)outbuffer;
			stream.avail_out = 256;
			inflateInit(&stream); 

			inflate(&stream, 1); 

			inflateEnd(&stream);
		}
		else
		{
			DWORD dwNumberOfBytesRead;
			ReadFile(hFile,
                                 outbuffer,
                                 sizeof(outbuffer),
                                 &dwNumberOfBytesRead,
                                 NULL);
		} 

		unsigned char nByte = outbuffer[0]; 

		// How many bits each number occupy
		unsigned char intBits = nByte >> 3; 

		int intTotalBits = intBits * 4 + 5;
		int intTotalBytes = (intTotalBits + 7) / 8; 

		unsigned char * pnBytes = new unsigned char [intTotalBytes];
		CopyMemory(pnBytes, outbuffer, min(intTotalBytes, sizeof(outbuffer))); 

		int intOffset = 5;
		int intXMin =
                    GetIntegerFromBits(pnBytes, intOffset, intBits) / 20;
		int intXMax =
                    GetIntegerFromBits(pnBytes, intOffset + intBits, intBits) / 20;
		int intYMin =
                    GetIntegerFromBits(pnBytes, intOffset + 2 * intBits, intBits) / 20;
		int intYMax =
                    GetIntegerFromBits(pnBytes, intOffset + 3 * intBits, intBits) / 20; 

		size.cx = intXMax - intXMin;
		size.cy = intYMax - intYMin; 

		delete[] pnBytes; 

		CloseHandle(hFile);
	} 

	return size;
} 

int GetIntegerFromBits(LPVOID lpData,
                       int intOffset,
                       int intLength)
{
	int intResult = 0;
	LPBYTE lpBytes = static_cast(lpData); 

	int intBitIndex = 0;
	int intSign; 

	for (int intBitPos = intOffset;
	     intBitPos < intOffset + intLength;
             intBitPos++, intBitIndex++)
	{
		int intByteIndex = intBitPos / 8;
		__int8 nByte = lpBytes[intByteIndex];
		int intLocalBitIndex = 7 - intBitPos % 8;
		int intBitValue = ((nByte >> intLocalBitIndex) & 1); 

		if (0 == intBitIndex)
			intSign = 1 == intBitValue ? -1 : +1;
		else
		{
			intResult <<= 1;
			intResult += intBitValue;
		}
	} 

	intResult *= intSign; 

	return intResult;
}

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