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):

Field
Type
Comment


Signature
UI8
Signature byte always ‘F’


Signature
UI8
Signature byte always ‘W’


Signature
UI8
Signature byte always ‘S’


Version
UI8
Single byte file version (e.g. 0×04F for SWF 4)


FileLength
UI32
Length of entire file in bytes


FrameSize
RECT
Frame size in twips


FrameRate
UI16
Frame delay in 8.8 fixed number of frames per second


FrameCount
UI16
Total number of frames in movie

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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