jdhuff.c

一套图像处理程序,支持三种图像文件格式,我调试过了,很好用

  state->cur.get_buffer = get_buffer;
  state->cur.bits_left = bits_left;

  return TRUE;
}


/*
 * These macros provide the in-line portion of bit fetching.
 * Use check_bit_buffer to ensure there are N bits in get_buffer
 * before using get_bits, peek_bits, or drop_bits.
 *	check_bit_buffer(state,n,action);
 *		Ensure there are N bits in get_buffer; if suspend, take action.
 *      val = get_bits(state,n);
 *		Fetch next N bits.
 *      val = peek_bits(state,n);
 *		Fetch next N bits without removing them from the buffer.
 *	drop_bits(state,n);
 *		Discard next N bits.
 * The value N should be a simple variable, not an expression, because it
 * is evaluated multiple times.
 */

#define check_bit_buffer(state,nbits,action) \
	{ if ((state).cur.bits_left < (nbits))  \
	    if (! fill_bit_buffer(&(state), nbits))  \
	      { action; } }

#define get_bits(state,nbits) \
	(((int) ((state).cur.get_buffer >> ((state).cur.bits_left -= (nbits)))) & ((1<<(nbits))-1))

#define peek_bits(state,nbits) \
	(((int) ((state).cur.get_buffer >> ((state).cur.bits_left -  (nbits)))) & ((1<<(nbits))-1))

#define drop_bits(state,nbits) \
	((state).cur.bits_left -= (nbits))


/*
 * Code for extracting next Huffman-coded symbol from input bit stream.
 * We use a lookahead table to process codes of up to HUFF_LOOKAHEAD bits
 * without looping.  Usually, more than 95% of the Huffman codes will be 8
 * or fewer bits long.  The few overlength codes are handled with a loop.
 * The primary case is made a macro for speed reasons; the secondary
 * routine slow_DECODE is rarely entered and need not be inline code.
 *
 * Notes about the huff_DECODE macro:
 * 1. Near the end of the data segment, we may fail to get enough bits
 *    for a lookahead.  In that case, we do it the hard way.
 * 2. If the lookahead table contains no entry, the next code must be
 *    more than HUFF_LOOKAHEAD bits long.
 * 3. slow_DECODE returns -1 if forced to suspend.
 */

#define huff_DECODE(result,state,htbl,donelabel) \
{ if (state.cur.bits_left < HUFF_LOOKAHEAD) {  \
    if (! fill_bit_buffer(&state, 0)) return FALSE;  \
    if (state.cur.bits_left < HUFF_LOOKAHEAD) {  \
      if ((result = slow_DECODE(&state, htbl, 1)) < 0) return FALSE;  \
      goto donelabel;  \
    }  \
  }  \
  { register int nb, look;  \
    look = peek_bits(state, HUFF_LOOKAHEAD);  \
    if ((nb = htbl->look_nbits[look]) != 0) {  \
      drop_bits(state, nb);  \
      result = htbl->look_sym[look];  \
    } else {  \
      if ((result = slow_DECODE(&state, htbl, HUFF_LOOKAHEAD+1)) < 0)  \
	return FALSE;  \
    }  \
  }  \
donelabel:;  \
}

  
LOCAL int
slow_DECODE (working_state * state, D_DERIVED_TBL * htbl, int min_bits)
{
  register int l = min_bits;
  register INT32 code;

  /* huff_DECODE has determined that the code is at least min_bits */
  /* bits long, so fetch that many bits in one swoop. */

  check_bit_buffer(*state, l, return -1);
  code = get_bits(*state, l);

  /* Collect the rest of the Huffman code one bit at a time. */
  /* This is per Figure F.16 in the JPEG spec. */

  while (code > htbl->maxcode[l]) {
    code <<= 1;
    check_bit_buffer(*state, 1, return -1);
    code |= get_bits(*state, 1);
    l++;
  }

  /* With garbage input we may reach the sentinel value l = 17. */

  if (l > 16) {
    WARNMS(state->cinfo, JWRN_HUFF_BAD_CODE);
    return 0;			/* fake a zero as the safest result */
  }

  return htbl->pub->huffval[ htbl->valptr[l] +
			    ((int) (code - htbl->mincode[l])) ];
}


/* Figure F.12: extend sign bit.
 * On some machines, a shift and add will be faster than a table lookup.
 */

#ifdef AVOID_TABLES

#define huff_EXTEND(x,s)  ((x) < (1<<((s)-1)) ? (x) + (((-1)<<(s)) + 1) : (x))

#else

#define huff_EXTEND(x,s)  ((x) < extend_test[s] ? (x) + extend_offset[s] : (x))

static const int extend_test[16] =   /* entry n is 2**(n-1) */
  { 0, 0x0001, 0x0002, 0x0004, 0x0008, 0x0010, 0x0020, 0x0040, 0x0080,
    0x0100, 0x0200, 0x0400, 0x0800, 0x1000, 0x2000, 0x4000 };

static const int extend_offset[16] = /* entry n is (-1 << n) + 1 */
  { 0, ((-1)<<1) + 1, ((-1)<<2) + 1, ((-1)<<3) + 1, ((-1)<<4) + 1,
    ((-1)<<5) + 1, ((-1)<<6) + 1, ((-1)<<7) + 1, ((-1)<<8) + 1,
    ((-1)<<9) + 1, ((-1)<<10) + 1, ((-1)<<11) + 1, ((-1)<<12) + 1,
    ((-1)<<13) + 1, ((-1)<<14) + 1, ((-1)<<15) + 1 };

#endif /* AVOID_TABLES */


/*
 * Check for a restart marker & resynchronize decoder.
 * Returns FALSE if must suspend.
 */

LOCAL boolean
process_restart (j_decompress_ptr cinfo)
{
  huff_entropy_ptr entropy = (huff_entropy_ptr) cinfo->entropy;
  int ci;

  /* Throw away any unused bits remaining in bit buffer; */
  /* include any full bytes in next_marker's count of discarded bytes */
  cinfo->marker->discarded_bytes += entropy->saved.bits_left / 8;
  entropy->saved.bits_left = 0;

  /* Advance past the RSTn marker */
  if (! (*cinfo->marker->read_restart_marker) (cinfo))
    return FALSE;

  /* Re-initialize DC predictions to 0 */
  for (ci = 0; ci < cinfo->comps_in_scan; ci++)
    entropy->saved.last_dc_val[ci] = 0;

  /* Reset restart counter */
  entropy->restarts_to_go = cinfo->restart_interval;

  entropy->printed_eod = FALSE; /* next segment can get another warning */

  return TRUE;
}


/* ZAG[i] is the natural-order position of the i'th element of zigzag order.
 * If the incoming data is corrupted, decode_mcu could attempt to
 * reference values beyond the end of the array.  To avoid a wild store,
 * we put some extra zeroes after the real entries.
 */

static const int ZAG[DCTSIZE2+16] = {
  0,  1,  8, 16,  9,  2,  3, 10,
 17, 24, 32, 25, 18, 11,  4,  5,
 12, 19, 26, 33, 40, 48, 41, 34,
 27, 20, 13,  6,  7, 14, 21, 28,
 35, 42, 49, 56, 57, 50, 43, 36,
 29, 22, 15, 23, 30, 37, 44, 51,
 58, 59, 52, 45, 38, 31, 39, 46,
 53, 60, 61, 54, 47, 55, 62, 63,
  0,  0,  0,  0,  0,  0,  0,  0, /* extra entries in case k>63 below */
  0,  0,  0,  0,  0,  0,  0,  0
};


/*
 * Decode and return one MCU's worth of Huffman-compressed coefficients.
 * The coefficients are reordered from zigzag order into natural array order,
 * but are not dequantized.
 *
 * The i'th block of the MCU is stored into the block pointed to by
 * MCU_data[i].  WE ASSUME THIS AREA HAS BEEN ZEROED BY THE CALLER.
 * (Wholesale zeroing is usually a little faster than retail...)
 *
 * Returns FALSE if data source requested suspension.  In that case no
 * changes have been made to permanent state.  (Exception: some output
 * coefficients may already have been assigned.  This is harmless for
 * this module, but would not work for decoding progressive JPEG.)
 */

METHODDEF boolean
decode_mcu (j_decompress_ptr cinfo, JBLOCKROW *MCU_data)
{
  huff_entropy_ptr entropy = (huff_entropy_ptr) cinfo->entropy;
  register int s, k, r;
  int blkn, ci;
  JBLOCKROW block;
  working_state state;
  D_DERIVED_TBL * dctbl;
  D_DERIVED_TBL * actbl;
  jpeg_component_info * compptr;

  /* Process restart marker if needed; may have to suspend */
  if (cinfo->restart_interval) {
    if (entropy->restarts_to_go == 0)
      if (! process_restart(cinfo))
	return FALSE;
  }

  /* Load up working state */
  state.unread_marker = cinfo->unread_marker;
  state.next_input_byte = cinfo->src->next_input_byte;
  state.bytes_in_buffer = cinfo->src->bytes_in_buffer;
  ASSIGN_STATE(state.cur, entropy->saved);
  state.cinfo = cinfo;

  /* Outer loop handles each block in the MCU */

  for (blkn = 0; blkn < cinfo->blocks_in_MCU; blkn++) {
    block = MCU_data[blkn];
    ci = cinfo->MCU_membership[blkn];
    compptr = cinfo->cur_comp_info[ci];
    dctbl = entropy->dc_derived_tbls[compptr->dc_tbl_no];
    actbl = entropy->ac_derived_tbls[compptr->ac_tbl_no];

    /* Decode a single block's worth of coefficients */

    /* Section F.2.2.1: decode the DC coefficient difference */
    huff_DECODE(s, state, dctbl, label1);
    if (s) {
      check_bit_buffer(state, s, return FALSE);
      r = get_bits(state, s);
      s = huff_EXTEND(r, s);
    }

    /* Shortcut if component's values are not interesting */
    if (! compptr->component_needed)
      goto skip_ACs;

    /* Convert DC difference to actual value, update last_dc_val */
    s += state.cur.last_dc_val[ci];
    state.cur.last_dc_val[ci] = s;
    /* Output the DC coefficient (assumes ZAG[0] = 0) */
    (*block)[0] = (JCOEF) s;

    /* Do we need to decode the AC coefficients for this component? */
    if (compptr->DCT_scaled_size > 1) {

      /* Section F.2.2.2: decode the AC coefficients */
      /* Since zeroes are skipped, output area must be cleared beforehand */
      for (k = 1; k < DCTSIZE2; k++) {
	huff_DECODE(s, state, actbl, label2);
      
	r = s >> 4;
	s &= 15;
      
	if (s) {
	  k += r;
	  check_bit_buffer(state, s, return FALSE);
	  r = get_bits(state, s);
	  s = huff_EXTEND(r, s);
	  /* Output coefficient in natural (dezigzagged) order */
	  (*block)[ZAG[k]] = (JCOEF) s;
	} else {
	  if (r != 15)
	    break;
	  k += 15;
	}
      }

    } else {
skip_ACs:

      /* Section F.2.2.2: decode the AC coefficients */
      /* In this path we just discard the values */
      for (k = 1; k < DCTSIZE2; k++) {
	huff_DECODE(s, state, actbl, label3);
      
	r = s >> 4;
	s &= 15;
      
	if (s) {
	  k += r;
	  check_bit_buffer(state, s, return FALSE);
	  drop_bits(state, s);
	} else {
	  if (r != 15)
	    break;
	  k += 15;
	}
      }

    }
  }

  /* Completed MCU, so update state */
  cinfo->unread_marker = state.unread_marker;
  cinfo->src->next_input_byte = state.next_input_byte;
  cinfo->src->bytes_in_buffer = state.bytes_in_buffer;
  ASSIGN_STATE(entropy->saved, state.cur);

  /* Account for restart interval (no-op if not using restarts) */
  entropy->restarts_to_go--;

  return TRUE;
}


/*
 * Module initialization routine for Huffman entropy decoding.
 */

GLOBAL void
jinit_huff_decoder (j_decompress_ptr cinfo)
{
  huff_entropy_ptr entropy;
  int i;

  entropy = (huff_entropy_ptr)
    (*cinfo->mem->alloc_small) ((j_common_ptr) cinfo, JPOOL_IMAGE,
				SIZEOF(huff_entropy_decoder));
  cinfo->entropy = (struct jpeg_entropy_decoder *) entropy;
  entropy->pub.start_pass = start_pass_huff_decoder;
  entropy->pub.decode_mcu = decode_mcu;

  /* Mark tables unallocated */
  for (i = 0; i < NUM_HUFF_TBLS; i++) {
    entropy->dc_derived_tbls[i] = entropy->ac_derived_tbls[i] = NULL;
  }
}