/src/libjpeg-turbo.main/src/jcprepct.c

Source
/*
 * jcprepct.c
 *
 * This file was part of the Independent JPEG Group's software:
 * Copyright (C) 1994-1996, Thomas G. Lane.
 * Lossless JPEG Modifications:
 * Copyright (C) 1999, Ken Murchison.
 * libjpeg-turbo Modifications:
 * Copyright (C) 2022, 2024, 2026, D. R. Commander.
 * For conditions of distribution and use, see the accompanying README.ijg
 * file.
 *
 * This file contains the compression preprocessing controller.
 * This controller manages the color conversion, downsampling,
 * and edge expansion steps.
 *
 * Most of the complexity here is associated with buffering input rows
 * as required by the downsampler.  See the comments at the head of
 * jcsample.c for the downsampler's needs.
 */

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


#if BITS_IN_JSAMPLE != 16 || defined(C_LOSSLESS_SUPPORTED)

/* At present, jcsample.c can request context rows only for smoothing.
 * In the future, we might also need context rows for CCIR601 sampling
 * or other more-complex downsampling procedures.  The code to support
 * context rows should be compiled only if needed.
 */
#ifdef INPUT_SMOOTHING_SUPPORTED
#define CONTEXT_ROWS_SUPPORTED
#endif


/*
 * For the simple (no-context-row) case, we just need to buffer one row group's
 * worth of components for the downsampling step.  At the bottom of the image,
 * we pad to a full row group by replicating the last component row(s).  The
 * downsampler's last output row is then replicated if needed to pad out to a
 * full iMCU row.
 *
 * When providing context rows, we must buffer three row groups' worth of
 * components.  Three row groups are physically allocated, but the row pointer
 * arrays are made five row groups high, with the extra pointers above and
 * below "wrapping around" to point to the last and first real row groups.
 * This allows the downsampler to access the proper context rows.  At the top
 * and bottom of the image, we create dummy context rows by copying the first
 * or last real component row(s).  This copying could be avoided by pointer
 * hacking, as is done in jdmainct.c, but it doesn't seem worth the trouble on
 * the compression side.
 */


/* Private buffer controller object */

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

  /* Downsampling input buffer.  This buffer holds color-converted data
   * until we have enough to do a downsample step.
   */
  _JSAMPARRAY color_buf[MAX_COMPONENTS];

  JDIMENSION rows_to_go;        /* counts rows remaining in source image */
  int next_buf_row;             /* index of next row to store in color_buf */

#ifdef CONTEXT_ROWS_SUPPORTED   /* only needed for context case */
  int this_row_group;           /* starting row index of group to process */
  int next_buf_stop;            /* downsample when we reach this index */
#endif
} my_prep_controller;

typedef my_prep_controller *my_prep_ptr;


/*
 * Initialize for a processing pass.
 */

METHODDEF(void)
start_pass_prep(j_compress_ptr cinfo, J_BUF_MODE pass_mode)
{
  my_prep_ptr prep = (my_prep_ptr)cinfo->prep;

  if (pass_mode != JBUF_PASS_THRU)
    ERREXIT(cinfo, JERR_BAD_BUFFER_MODE);

  /* Initialize total-height counter for detecting bottom of image */
  prep->rows_to_go = cinfo->image_height;
  /* Mark the conversion buffer empty */
  prep->next_buf_row = 0;
#ifdef CONTEXT_ROWS_SUPPORTED
  /* Preset additional state variables for context mode.
   * These aren't used in non-context mode, so we needn't test which mode.
   */
  prep->this_row_group = 0;
  /* Set next_buf_stop to stop after two row groups have been read in. */
  prep->next_buf_stop = 2 * cinfo->max_v_samp_factor;
#endif
}


/*
 * Expand an image vertically from height input_rows to height output_rows,
 * by duplicating the bottom row.
 */

LOCAL(void)
expand_bottom_edge(_JSAMPARRAY image_data, JDIMENSION num_cols, int input_rows,
                   int output_rows)
{
  register int row;

  for (row = input_rows; row < output_rows; row++) {
    _jcopy_sample_rows(image_data, input_rows - 1, image_data, row, 1,
                       num_cols);
  }
}


/*
 * Process some data in the simple no-context case.
 *
 * Preprocessor output data is counted in "row groups".  A row group
 * is defined to be v_samp_factor sample rows of each component.
 * Downsampling will produce this much data from each max_v_samp_factor
 * input rows.
 */

METHODDEF(void)
pre_process_data(j_compress_ptr cinfo, _JSAMPARRAY input_buf,
                 JDIMENSION *in_row_ctr, JDIMENSION in_rows_avail,
                 _JSAMPIMAGE output_buf, JDIMENSION *out_row_group_ctr,
                 JDIMENSION out_row_groups_avail)
{
  my_prep_ptr prep = (my_prep_ptr)cinfo->prep;
  int numrows, ci;
  JDIMENSION inrows;
  jpeg_component_info *compptr;
  int data_unit = cinfo->master->lossless ? 1 : DCTSIZE;

  while (*in_row_ctr < in_rows_avail &&
         *out_row_group_ctr < out_row_groups_avail) {
    /* Do color conversion to fill the conversion buffer. */
    inrows = in_rows_avail - *in_row_ctr;
    numrows = cinfo->max_v_samp_factor - prep->next_buf_row;
    numrows = (int)MIN((JDIMENSION)numrows, inrows);
    (*cinfo->cconvert->_color_convert) (cinfo, input_buf + *in_row_ctr,
                                        prep->color_buf,
                                        (JDIMENSION)prep->next_buf_row,
                                        numrows);
    *in_row_ctr += numrows;
    prep->next_buf_row += numrows;
    prep->rows_to_go -= numrows;
    /* If at bottom of image, pad to fill the conversion buffer. */
    if (prep->rows_to_go == 0 &&
        prep->next_buf_row < cinfo->max_v_samp_factor) {
      for (ci = 0; ci < cinfo->num_components; ci++) {
        expand_bottom_edge(prep->color_buf[ci], cinfo->image_width,
                           prep->next_buf_row, cinfo->max_v_samp_factor);
      }
      prep->next_buf_row = cinfo->max_v_samp_factor;
    }
    /* If we've filled the conversion buffer, empty it. */
    if (prep->next_buf_row == cinfo->max_v_samp_factor) {
      (*cinfo->downsample->_downsample) (cinfo,
                                         prep->color_buf, (JDIMENSION)0,
                                         output_buf, *out_row_group_ctr);
      prep->next_buf_row = 0;
      (*out_row_group_ctr)++;
    }
    /* If at bottom of image, pad the output to a full iMCU height.
     * Note we assume the caller is providing a one-iMCU-height output buffer!
     */
    if (prep->rows_to_go == 0 && *out_row_group_ctr < out_row_groups_avail) {
      for (ci = 0, compptr = cinfo->comp_info; ci < cinfo->num_components;
           ci++, compptr++) {
        expand_bottom_edge(output_buf[ci],
                           compptr->width_in_blocks * data_unit,
                           (int)(*out_row_group_ctr * compptr->v_samp_factor),
                           (int)(out_row_groups_avail * compptr->v_samp_factor));
      }
      *out_row_group_ctr = out_row_groups_avail;
      break;                    /* can exit outer loop without test */
    }
  }
}


#ifdef CONTEXT_ROWS_SUPPORTED

/*
 * Process some data in the context case.
 */

METHODDEF(void)
pre_process_context(j_compress_ptr cinfo, _JSAMPARRAY input_buf,
                    JDIMENSION *in_row_ctr, JDIMENSION in_rows_avail,
                    _JSAMPIMAGE output_buf, JDIMENSION *out_row_group_ctr,
                    JDIMENSION out_row_groups_avail)
{
  my_prep_ptr prep = (my_prep_ptr)cinfo->prep;
  int numrows, ci;
  int buf_height = cinfo->max_v_samp_factor * 3;
  JDIMENSION inrows;

  while (*out_row_group_ctr < out_row_groups_avail) {
    if (*in_row_ctr < in_rows_avail) {
      /* Do color conversion to fill the conversion buffer. */
      inrows = in_rows_avail - *in_row_ctr;
      numrows = prep->next_buf_stop - prep->next_buf_row;
      numrows = (int)MIN((JDIMENSION)numrows, inrows);
      (*cinfo->cconvert->_color_convert) (cinfo, input_buf + *in_row_ctr,
                                          prep->color_buf,
                                          (JDIMENSION)prep->next_buf_row,
                                          numrows);
      /* Pad at top of image, if first time through */
      if (prep->rows_to_go == cinfo->image_height) {
        for (ci = 0; ci < cinfo->num_components; ci++) {
          int row;
          for (row = 1; row <= cinfo->max_v_samp_factor; row++) {
            _jcopy_sample_rows(prep->color_buf[ci], 0, prep->color_buf[ci],
                               -row, 1, cinfo->image_width);
          }
        }
      }
      *in_row_ctr += numrows;
      prep->next_buf_row += numrows;
      prep->rows_to_go -= numrows;
    } else {
      /* Return for more data, unless we are at the bottom of the image. */
      if (prep->rows_to_go != 0)
        break;
      /* When at bottom of image, pad to fill the conversion buffer. */
      if (prep->next_buf_row < prep->next_buf_stop) {
        for (ci = 0; ci < cinfo->num_components; ci++) {
          expand_bottom_edge(prep->color_buf[ci], cinfo->image_width,
                             prep->next_buf_row, prep->next_buf_stop);
        }
        prep->next_buf_row = prep->next_buf_stop;
      }
    }
    /* If we've gotten enough data, downsample a row group. */
    if (prep->next_buf_row == prep->next_buf_stop) {
      (*cinfo->downsample->_downsample) (cinfo, prep->color_buf,
                                         (JDIMENSION)prep->this_row_group,
                                         output_buf, *out_row_group_ctr);
      (*out_row_group_ctr)++;
      /* Advance pointers with wraparound as necessary. */
      prep->this_row_group += cinfo->max_v_samp_factor;
      if (prep->this_row_group >= buf_height)
        prep->this_row_group = 0;
      if (prep->next_buf_row >= buf_height)
        prep->next_buf_row = 0;
      prep->next_buf_stop = prep->next_buf_row + cinfo->max_v_samp_factor;
    }
  }
}


/*
 * Create the wrapped-around downsampling input buffer needed for context mode.
 */

LOCAL(void)
create_context_buffer(j_compress_ptr cinfo)
{
  my_prep_ptr prep = (my_prep_ptr)cinfo->prep;
  int rgroup_height = cinfo->max_v_samp_factor;
  int ci, i;
  jpeg_component_info *compptr;
  _JSAMPARRAY true_buffer, fake_buffer;
  int data_unit = cinfo->master->lossless ? 1 : DCTSIZE;

  /* Grab enough space for fake row pointers for all the components;
   * we need five row groups' worth of pointers for each component.
   */
  fake_buffer = (_JSAMPARRAY)
    (*cinfo->mem->alloc_small) ((j_common_ptr)cinfo, JPOOL_IMAGE,
                                (cinfo->num_components * 5 * rgroup_height) *
                                sizeof(_JSAMPROW));

  for (ci = 0, compptr = cinfo->comp_info; ci < cinfo->num_components;
       ci++, compptr++) {
    /* Allocate the actual buffer space (3 row groups) for this component.
     * We make the buffer wide enough to allow the downsampler to edge-expand
     * horizontally within the buffer, if it so chooses.
     */
    true_buffer = (_JSAMPARRAY)(*cinfo->mem->alloc_sarray)
      ((j_common_ptr)cinfo, JPOOL_IMAGE,
       (JDIMENSION)(((long)compptr->width_in_blocks * data_unit *
                     cinfo->max_h_samp_factor) / compptr->h_samp_factor),
       (JDIMENSION)(3 * rgroup_height));
    /* Copy true buffer row pointers into the middle of the fake row array */
    memcpy(fake_buffer + rgroup_height, true_buffer,
           3 * rgroup_height * sizeof(_JSAMPROW));
    /* Fill in the above and below wraparound pointers */
    for (i = 0; i < rgroup_height; i++) {
      fake_buffer[i] = true_buffer[2 * rgroup_height + i];
      fake_buffer[4 * rgroup_height + i] = true_buffer[i];
    }
    prep->color_buf[ci] = fake_buffer + rgroup_height;
    fake_buffer += 5 * rgroup_height; /* point to space for next component */
  }
}

#endif /* CONTEXT_ROWS_SUPPORTED */


/*
 * Initialize preprocessing controller.
 */

GLOBAL(void)
_jinit_c_prep_controller(j_compress_ptr cinfo, boolean need_full_buffer)
{
  my_prep_ptr prep;
  int ci;
  jpeg_component_info *compptr;
  int data_unit = cinfo->master->lossless ? 1 : DCTSIZE;

#ifdef C_LOSSLESS_SUPPORTED
  if (cinfo->master->lossless) {
#if BITS_IN_JSAMPLE == 8
    if (cinfo->data_precision > BITS_IN_JSAMPLE || cinfo->data_precision < 2)
#else
    if (cinfo->data_precision > BITS_IN_JSAMPLE ||
        cinfo->data_precision < BITS_IN_JSAMPLE - 3)
#endif
      ERREXIT1(cinfo, JERR_BAD_PRECISION, cinfo->data_precision);
  } else
#endif
  {
    if (cinfo->data_precision != BITS_IN_JSAMPLE)
      ERREXIT1(cinfo, JERR_BAD_PRECISION, cinfo->data_precision);
  }

  if (need_full_buffer)         /* safety check */
    ERREXIT(cinfo, JERR_BAD_BUFFER_MODE);

  prep = (my_prep_ptr)
    (*cinfo->mem->alloc_small) ((j_common_ptr)cinfo, JPOOL_IMAGE,
                                sizeof(my_prep_controller));
  cinfo->prep = (struct jpeg_c_prep_controller *)prep;
  prep->pub.start_pass = start_pass_prep;

  /* Allocate the color conversion buffer.
   * We make the buffer wide enough to allow the downsampler to edge-expand
   * horizontally within the buffer, if it so chooses.
   */
  if (cinfo->downsample->need_context_rows) {
    /* Set up to provide context rows */
#ifdef CONTEXT_ROWS_SUPPORTED
    prep->pub._pre_process_data = pre_process_context;
    create_context_buffer(cinfo);
#else
    ERREXIT(cinfo, JERR_NOT_COMPILED);
#endif
  } else {
    /* No context, just make it tall enough for one row group */
    prep->pub._pre_process_data = pre_process_data;
    for (ci = 0, compptr = cinfo->comp_info; ci < cinfo->num_components;
         ci++, compptr++) {
      prep->color_buf[ci] = (_JSAMPARRAY)(*cinfo->mem->alloc_sarray)
        ((j_common_ptr)cinfo, JPOOL_IMAGE,
         (JDIMENSION)(((long)compptr->width_in_blocks * data_unit *
                       cinfo->max_h_samp_factor) / compptr->h_samp_factor),
         (JDIMENSION)cinfo->max_v_samp_factor);
    }
  }
}

#endif /* BITS_IN_JSAMPLE != 16 || defined(C_LOSSLESS_SUPPORTED) */

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Created: 2026-03-12 08:02