Method DecodeScanComp

ImgDecode.cpp:1604–1835

Decode a single component for one block of an MCU - Pull bits from the main buffer - Find matching huffman codes - Fill in the IDCT matrix (m_anDctBlock[]) - Perform the IDCT to create spatial domain INPUT: - nTblDhtDc = DHT table ID for DC component - nTblDhtAc = DHT talbe ID for AC component - nMcuX = UNUSED - nMcuY = UNUSED RETURN: - Indicate if the decode was successful POST

// FIXME: Consider adding checks for DHT table like in ReadScanVal()
//
bool CimgDecode::DecodeScanComp(unsigned nTblDhtDc,unsigned nTblDhtAc,unsigned nTblDqt,unsigned nMcuX,unsigned nMcuY)
{
	nMcuX;	// Unreferenced param
	nMcuY;	// Unreferenced param
	unsigned		nZrl;
	signed			nVal;
	bool			bDone = false;
	bool			bDC = true;	// Start with DC coeff

	teRsvRet	eRsvRet;	// Return value from ReadScanVal()

	unsigned nNumCoeffs = 0;
	//unsigned nDctMax = 0;			// Maximum DCT coefficient to use for IDCT
	unsigned nSavedBufPos = 0;
	unsigned nSavedBufErr = SCANBUF_OK;
	unsigned nSavedBufAlign = 0;

	// Profiling: No difference noted
	DecodeIdctClear();

	while (!bDone) {
		BuffTopup();

		// Note that once we perform ReadScanVal(), then GetScanBufPos() will be
		// after the decoded VLC
		// Save old file position info in case we want accurate error positioning
		nSavedBufPos   = m_anScanBuffPtr_pos[0];
		nSavedBufErr   = m_nScanBuffLatchErr;
		nSavedBufAlign = m_nScanBuffPtr_align;

		// ReadScanVal return values:
		// - RSV_OK			OK
		// - RSV_EOB		End of block
		// - RSV_UNDERFLOW	Ran out of data in buffer
		// - RSV_RST_TERM	No huffman code found, but restart marker seen
		// Assume nTblDht just points to DC tables, adjust for AC
		// e.g. nTblDht = 0,2,4
		eRsvRet = ReadScanVal(bDC?0:1,bDC?nTblDhtDc:nTblDhtAc,nZrl,nVal);

		// Handle Restart marker first.
		if (eRsvRet == RSV_RST_TERM) {
			// Assume that m_bRestartRead is TRUE
			// No huffman code found because either we ran out of bits
			// in the scan buffer or the bits padded with 1's didn't result
			// in a valid VLC code.

			// Steps:
			//   1) Reset the decoder state (DC values)
			//   2) Advance the buffer pointer (might need to handle the
			//      case of perfect alignment to byte boundary separately)
			//   3) Flush the Scan Buffer
			//   4) Clear m_bRestartRead
			//   5) Refill Scan Buffer with BuffTopUp()
			//   6) Re-invoke ReadScanVal()

			// Step 1:
			DecodeRestartDcState();