JPEGView/Independent JPEG Group/jdcoefct.c

/*
 * jdcoefct.c
 *
 * Copyright (C) 1994, Thomas G. Lane.
 * This file is part of the Independent JPEG Group's software.
 * For conditions of distribution and use, see the accompanying README file.
 *
 * This file contains the coefficient buffer controller for decompression.
 * This controller is the top level of the JPEG decompressor proper.
 * The coefficient buffer lies between entropy decoding and inverse-DCT steps.
 *
 * For now, this file does not support a full-image coefficient buffer.
 * One will be necessary to handle noninterleaved or progressive JPEG files.
 */

#define JPEG_INTERNALS
#include "jinclude.h"
#include "jpeglib.h"


/* Private buffer controller object */

typedef struct {
  struct jpeg_d_coef_controller pub; /* public fields */

  JDIMENSION MCU_col_num;	/* saves next MCU column to process */
  JDIMENSION MCU_row_num;	/* keep track of MCU row # within image */

  /* When not doing block smoothing, it's sufficient to buffer just one MCU
   * (although this may prove a bit slow in practice).  We allocate a
   * workspace of MAX_BLOCKS_IN_MCU coefficient blocks, and let the entropy
   * decoder write into that workspace each time.  On 80x86, the workspace
   * is FAR even though it's not really very big; this is to keep the
   * module interfaces unchanged when a large coefficient buffer is necessary.
   */
  JBLOCKROW MCU_buffer[MAX_BLOCKS_IN_MCU];
} my_coef_controller;

typedef my_coef_controller * my_coef_ptr;


/*
 * Initialize for a processing pass.
 */

METHODDEF void
start_pass (j_decompress_ptr cinfo, J_BUF_MODE pass_mode)
{
  my_coef_ptr coef = (my_coef_ptr) cinfo->coef;

  switch (pass_mode) {
  case JBUF_PASS_THRU:
    break;
  default:
    ERREXIT(cinfo, JERR_BAD_BUFFER_MODE);
    break;
  }
  coef->MCU_col_num = 0;
  coef->MCU_row_num = 0;
}


/*
 * Process some data.
 * Always attempts to emit one fully interleaved MCU row ("iMCU" row).
 * Returns TRUE if it completed a row, FALSE if not (suspension).
 *
 * NB: output_buf contains a plane for each component in image.
 * For now, we assume this is the same as the components in the scan.
 */

METHODDEF boolean
decompress_data (j_decompress_ptr cinfo, JSAMPIMAGE output_buf)
{
  my_coef_ptr coef = (my_coef_ptr) cinfo->coef;
  JDIMENSION MCU_col_num;	/* index of current MCU within row */
  JDIMENSION last_MCU_col = cinfo->MCUs_per_row - 1;
  JDIMENSION last_MCU_row = cinfo->MCU_rows_in_scan - 1;
  int blkn, ci, xindex, yindex, useful_width;
  JSAMPARRAY output_ptr;
  JDIMENSION start_col, output_col;
  jpeg_component_info *compptr;
  inverse_DCT_method_ptr inverse_DCT;

  /* Loop to process as much as one whole MCU row */

  for (MCU_col_num = coef->MCU_col_num; MCU_col_num <= last_MCU_col;
       MCU_col_num++) {

    /* Try to fetch an MCU.  Entropy decoder expects buffer to be zeroed. */
    jzero_far((void FAR *) coef->MCU_buffer[0],
	      (size_t) (cinfo->blocks_in_MCU * SIZEOF(JBLOCK)));
    if (! (*cinfo->entropy->decode_mcu) (cinfo, coef->MCU_buffer)) {
      /* Suspension forced; return with row unfinished */
      coef->MCU_col_num = MCU_col_num; /* update my state */
      return FALSE;
    }

    /* Determine where data should go in output_buf and do the IDCT thing.
     * We skip dummy blocks at the right and bottom edges (but blkn gets
     * incremented past them!).  Note the inner loop relies on having
     * allocated the MCU_buffer[] blocks sequentially.
     */
    blkn = 0;			/* index of current DCT block within MCU */
    for (ci = 0; ci < cinfo->comps_in_scan; ci++) {
      compptr = cinfo->cur_comp_info[ci];
      /* Don't bother to IDCT an uninteresting component. */
      if (! compptr->component_needed) {
	blkn += compptr->MCU_blocks;
	continue;
      }
      inverse_DCT = cinfo->idct->inverse_DCT[compptr->component_index];
      if (MCU_col_num < last_MCU_col)
	useful_width = compptr->MCU_width;
      else
	useful_width = compptr->last_col_width;
      output_ptr = output_buf[ci];
      start_col = MCU_col_num * compptr->MCU_sample_width;
      for (yindex = 0; yindex < compptr->MCU_height; yindex++) {
	if (coe