/src/libjpeg-turbo/src/jcsample.c

Source
/*
 * jcsample.c
 *
 * This file was part of the Independent JPEG Group's software:
 * Copyright (C) 1991-1996, Thomas G. Lane.
 * Lossless JPEG Modifications:
 * Copyright (C) 1999, Ken Murchison.
 * libjpeg-turbo Modifications:
 * Copyright 2009 Pierre Ossman <ossman@cendio.se> for Cendio AB
 * Copyright (C) 2015, 2019, 2022, 2024-2026, D. R. Commander.
 * For conditions of distribution and use, see the accompanying README.ijg
 * file.
 *
 * This file contains downsampling routines.
 *
 * Downsampling input data is counted in "row groups".  A row group is defined
 * to be max_v_samp_factor rows of each component, from which the downsampler
 * produces v_samp_factor sample rows.  A single row group is processed in each
 * call to the downsampler module.
 *
 * The downsampler is responsible for edge-expansion of its output data
 * to fill an integral number of DCT blocks horizontally.  The source buffer
 * may be modified if it is helpful for this purpose (the source buffer is
 * allocated wide enough to correspond to the desired output width).
 * The caller (the prep controller) is responsible for vertical padding.
 *
 * The downsampler may request "context rows" by setting need_context_rows
 * during startup.  In this case, the input arrays will contain at least one
 * row group's worth of components above and below the passed-in data; the
 * caller will create dummy rows at image top and bottom by replicating the
 * first or last real component row(s).
 *
 * An excellent reference for image resampling is
 *   Digital Image Warping, George Wolberg, 1990.
 *   Pub. by IEEE Computer Society Press, Los Alamitos, CA. ISBN 0-8186-8944-7.
 *
 * The downsampling algorithm used here is a simple average of the source
 * components covered by the output sample.  The hi-falutin sampling literature
 * refers to this as a "box filter".  In general, the characteristics of a box
 * filter are not very good.  However, for the specific cases we normally use
 * (1:1 and 2:1 ratios), the box is equivalent to a "triangle filter", which is
 * not nearly so bad.  If you intend to use other sampling ratios, you'd be
 * well advised to improve this code.
 *
 * A simple input-smoothing capability is provided.  This is mainly intended
 * for cleaning up color-dithered GIF input files.  (If you find it inadequate,
 * we suggest using an external filtering program such as pnmconvol).  When
 * enabled, each input component C is replaced by a weighted sum of itself and
 * its eight neighbors.  C's weight is 1-8*SF, and each neighbor's weight is
 * SF, where SF = (smoothing_factor / 1024).
 * Currently, smoothing is only supported for 2h2v sampling factors.
 */

#define JPEG_INTERNALS
#include "jinclude.h"
#include "jpeglib.h"
#ifdef WITH_SIMD
#include "../simd/jsimd.h"
#endif
#include "jsamplecomp.h"


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

/* Pointer to routine to downsample a single component */
typedef void (*downsample1_ptr) (j_compress_ptr cinfo,
                                 jpeg_component_info *compptr,
                                 _JSAMPARRAY input_data,
                                 _JSAMPARRAY output_data);

/* Private subobject */

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

  /* Downsampling method pointers, one per component */
  downsample1_ptr methods[MAX_COMPONENTS];
} my_downsampler;

typedef my_downsampler *my_downsample_ptr;


/*
 * Initialize for a downsampling pass.
 */

METHODDEF(void)
start_pass_downsample(j_compress_ptr cinfo)
{
  /* no work for now */
}


/*
 * Expand a component horizontally from width input_cols to width output_cols,
 * by duplicating the rightmost samples.
 */

LOCAL(void)
expand_right_edge(_JSAMPARRAY image_data, int num_rows, JDIMENSION input_cols,
                  JDIMENSION output_cols)
{
  register _JSAMPROW ptr;
  register _JSAMPLE pixval;
  register int count;
  int row;
  int numcols = (int)(output_cols - input_cols);

  if (numcols > 0) {
    for (row = 0; row < num_rows; row++) {
      ptr = image_data[row] + input_cols;
      pixval = ptr[-1];
      for (count = numcols; count > 0; count--)
        *ptr++ = pixval;
    }
  }
}


/*
 * Do downsampling for a whole row group (all components).
 *
 * In this version we simply downsample each component independently.
 */

METHODDEF(void)
sep_downsample(j_compress_ptr cinfo, _JSAMPIMAGE input_buf,
               JDIMENSION in_row_index, _JSAMPIMAGE output_buf,
               JDIMENSION out_row_group_index)
{
  my_downsample_ptr downsample = (my_downsample_ptr)cinfo->downsample;
  int ci;
  jpeg_component_info *compptr;
  _JSAMPARRAY in_ptr, out_ptr;

  for (ci = 0, compptr = cinfo->comp_info; ci < cinfo->num_components;
       ci++, compptr++) {
    in_ptr = input_buf[ci] + in_row_index;
    out_ptr = output_buf[ci] + (out_row_group_index * compptr->v_samp_factor);
    (*downsample->methods[ci]) (cinfo, compptr, in_ptr, out_ptr);
  }
}


/*
 * Downsample components from a single plane.
 * One row group is processed per call.
 * This version handles arbitrary integral sampling ratios, without smoothing.
 * Note that this version is not actually used for customary sampling ratios.
 */

METHODDEF(void)
int_downsample(j_compress_ptr cinfo, jpeg_component_info *compptr,
               _JSAMPARRAY input_data, _JSAMPARRAY output_data)
{
  int inrow, outrow, h_expand, v_expand, numpix, numpix2, h, v;
  JDIMENSION outcol, outcol_h;  /* outcol_h == outcol*h_expand */
  int data_unit = cinfo->master->lossless ? 1 : DCTSIZE;
  JDIMENSION output_cols = compptr->width_in_blocks * data_unit;
  _JSAMPROW inptr, outptr;
  JLONG outvalue;

  h_expand = cinfo->max_h_samp_factor / compptr->h_samp_factor;
  v_expand = cinfo->max_v_samp_factor / compptr->v_samp_factor;
  numpix = h_expand * v_expand;
  numpix2 = numpix / 2;

  /* Expand input data enough to let all the output samples be generated
   * by the standard loop.  Special-casing padded output would be more
   * efficient.
   */
  expand_right_edge(input_data, cinfo->max_v_samp_factor, cinfo->image_width,
                    output_cols * h_expand);

  inrow = 0;
  for (outrow = 0; outrow < compptr->v_samp_factor; outrow++) {
    outptr = output_data[outrow];
    for (outcol = 0, outcol_h = 0; outcol < output_cols;
         outcol++, outcol_h += h_expand) {
      outvalue = 0;
      for (v = 0; v < v_expand; v++) {
        inptr = input_data[inrow + v] + outcol_h;
        for (h = 0; h < h_expand; h++) {
          outvalue += (JLONG)(*inptr++);
        }
      }
      *outptr++ = (_JSAMPLE)((outvalue + numpix2) / numpix);
    }
    inrow += v_expand;
  }
}


/*
 * Downsample components from a single plane.
 * This version handles the special case of a full-size component,
 * without smoothing.
 */

METHODDEF(void)
fullsize_downsample(j_compress_ptr cinfo, jpeg_component_info *compptr,
                    _JSAMPARRAY input_data, _JSAMPARRAY output_data)
{
  int data_unit = cinfo->master->lossless ? 1 : DCTSIZE;

  /* Copy the data */
  _jcopy_sample_rows(input_data, 0, output_data, 0, cinfo->max_v_samp_factor,
                     cinfo->image_width);
  /* Edge-expand */
  expand_right_edge(output_data, cinfo->max_v_samp_factor, cinfo->image_width,
                    compptr->width_in_blocks * data_unit);
}


/*
 * Downsample components from a single plane.
 * This version handles the common case of 2:1 horizontal and 1:1 vertical,
 * without smoothing.
 *
 * A note about the "bias" calculations: when rounding fractional values to
 * integer, we do not want to always round 0.5 up to the next integer.
 * If we did that, we'd introduce a noticeable bias towards larger values.
 * Instead, this code is arranged so that 0.5 will be rounded up or down at
 * alternate pixel locations (a simple ordered dither pattern).
 */

METHODDEF(void)
h2v1_downsample(j_compress_ptr cinfo, jpeg_component_info *compptr,
                _JSAMPARRAY input_data, _JSAMPARRAY output_data)
{
  int outrow;
  JDIMENSION outcol;
  int data_unit = cinfo->master->lossless ? 1 : DCTSIZE;
  JDIMENSION output_cols = compptr->width_in_blocks * data_unit;
  register _JSAMPROW inptr, outptr;
  register int bias;

  /* Expand input data enough to let all the output samples be generated
   * by the standard loop.  Special-casing padded output would be more
   * efficient.
   */
  expand_right_edge(input_data, cinfo->max_v_samp_factor, cinfo->image_width,
                    output_cols * 2);

  for (outrow = 0; outrow < compptr->v_samp_factor; outrow++) {
    outptr = output_data[outrow];
    inptr = input_data[outrow];
    bias = 0;                   /* bias = 0,1,0,1,... for successive samples */
    for (outcol = 0; outcol < output_cols; outcol++) {
      *outptr++ = (_JSAMPLE)((inptr[0] + inptr[1] + bias) >> 1);
      bias ^= 1;                /* 0=>1, 1=>0 */
      inptr += 2;
    }
  }
}


/*
 * Downsample components from a single plane.
 * This version handles the standard case of 2:1 horizontal and 2:1 vertical,
 * without smoothing.
 */

METHODDEF(void)
h2v2_downsample(j_compress_ptr cinfo, jpeg_component_info *compptr,
                _JSAMPARRAY input_data, _JSAMPARRAY output_data)
{
  int inrow, outrow;
  JDIMENSION outcol;
  int data_unit = cinfo->master->lossless ? 1 : DCTSIZE;
  JDIMENSION output_cols = compptr->width_in_blocks * data_unit;
  register _JSAMPROW inptr0, inptr1, outptr;
  register int bias;

  /* Expand input data enough to let all the output samples be generated
   * by the standard loop.  Special-casing padded output would be more
   * efficient.
   */
  expand_right_edge(input_data, cinfo->max_v_samp_factor, cinfo->image_width,
                    output_cols * 2);

  inrow = 0;
  for (outrow = 0; outrow < compptr->v_samp_factor; outrow++) {
    outptr = output_data[outrow];
    inptr0 = input_data[inrow];
    inptr1 = input_data[inrow + 1];
    bias = 1;                   /* bias = 1,2,1,2,... for successive samples */
    for (outcol = 0; outcol < output_cols; outcol++) {
      *outptr++ = (_JSAMPLE)
        ((inptr0[0] + inptr0[1] + inptr1[0] + inptr1[1] + bias) >> 2);
      bias ^= 3;                /* 1=>2, 2=>1 */
      inptr0 += 2;  inptr1 += 2;
    }
    inrow += 2;
  }
}


#ifdef INPUT_SMOOTHING_SUPPORTED

/*
 * Downsample components from a single plane.
 * This version handles the standard case of 2:1 horizontal and 2:1 vertical,
 * with smoothing.  One row of context is required.
 */

METHODDEF(void)
h2v2_smooth_downsample(j_compress_ptr cinfo, jpeg_component_info *compptr,
                       _JSAMPARRAY input_data, _JSAMPARRAY output_data)
{
  int inrow, outrow;
  JDIMENSION colctr;
  int data_unit = cinfo->master->lossless ? 1 : DCTSIZE;
  JDIMENSION output_cols = compptr->width_in_blocks * data_unit;
  register _JSAMPROW inptr0, inptr1, above_ptr, below_ptr, outptr;
  JLONG membersum, neighsum, memberscale, neighscale;

  /* Expand input data enough to let all the output samples be generated
   * by the standard loop.  Special-casing padded output would be more
   * efficient.
   */
  expand_right_edge(input_data - 1, cinfo->max_v_samp_factor + 2,
                    cinfo->image_width, output_cols * 2);

  /* We don't bother to form the individual "smoothed" input component values;
   * we can directly compute the output which is the average of the four
   * smoothed values.  Each of the four member components contributes a
   * fraction (1-8*SF) to its own smoothed image and a fraction SF to each of
   * the three other smoothed components, therefore a total fraction (1-5*SF)/4
   * to the final output.  The four corner-adjacent neighbor components
   * contribute a fraction SF to just one smoothed component, or SF/4 to the
   * final output; while the eight edge-adjacent neighbors contribute SF to
   * each of two smoothed components, or SF/2 overall.  In order to use integer
   * arithmetic, these factors are scaled by 2^16 = 65536.  Also recall that
   * SF = smoothing_factor / 1024.
   */

  memberscale = 16384 - cinfo->smoothing_factor * 80; /* scaled (1-5*SF)/4 */
  neighscale = cinfo->smoothing_factor * 16; /* scaled SF/4 */

  inrow = 0;
  for (outrow = 0; outrow < compptr->v_samp_factor; outrow++) {
    outptr = output_data[outrow];
    inptr0 = input_data[inrow];
    inptr1 = input_data[inrow + 1];
    above_ptr = input_data[inrow - 1];
    below_ptr = input_data[inrow + 2];

    /* Special case for first column: pretend column -1 is same as column 0 */
    membersum = inptr0[0] + inptr0[1] + inptr1[0] + inptr1[1];
    neighsum = above_ptr[0] + above_ptr[1] + below_ptr[0] + below_ptr[1] +
               inptr0[0] + inptr0[2] + inptr1[0] + inptr1[2];
    neighsum += neighsum;
    neighsum += above_ptr[0] + above_ptr[2] + below_ptr[0] + below_ptr[2];
    membersum = membersum * memberscale + neighsum * neighscale;
    *outptr++ = (_JSAMPLE)((membersum + 32768) >> 16);
    inptr0 += 2;  inptr1 += 2;  above_ptr += 2;  below_ptr += 2;

    for (colctr = output_cols - 2; colctr > 0; colctr--) {
      /* sum of components directly mapped to this output element */
      membersum = inptr0[0] + inptr0[1] + inptr1[0] + inptr1[1];
      /* sum of edge-neighbor components */
      neighsum = above_ptr[0] + above_ptr[1] + below_ptr[0] + below_ptr[1] +
                 inptr0[-1] + inptr0[2] + inptr1[-1] + inptr1[2];
      /* The edge-neighbors count twice as much as corner-neighbors */
      neighsum += neighsum;
      /* Add in the corner-neighbors */
      neighsum += above_ptr[-1] + above_ptr[2] + below_ptr[-1] + below_ptr[2];
      /* form final output scaled up by 2^16 */
      membersum = membersum * memberscale + neighsum * neighscale;
      /* round, descale and output it */
      *outptr++ = (_JSAMPLE)((membersum + 32768) >> 16);
      inptr0 += 2;  inptr1 += 2;  above_ptr += 2;  below_ptr += 2;
    }

    /* Special case for last column */
    membersum = inptr0[0] + inptr0[1] + inptr1[0] + inptr1[1];
    neighsum = above_ptr[0] + above_ptr[1] + below_ptr[0] + below_ptr[1] +
               inptr0[-1] + inptr0[1] + inptr1[-1] + inptr1[1];
    neighsum += neighsum;
    neighsum += above_ptr[-1] + above_ptr[1] + below_ptr[-1] + below_ptr[1];
    membersum = membersum * memberscale + neighsum * neighscale;
    *outptr = (_JSAMPLE)((membersum + 32768) >> 16);

    inrow += 2;
  }
}


/*
 * Downsample components from a single plane.
 * This version handles the special case of a full-size component,
 * with smoothing.  One row of context is required.
 */

METHODDEF(void)
fullsize_smooth_downsample(j_compress_ptr cinfo, jpeg_component_info *compptr,
                           _JSAMPARRAY input_data, _JSAMPARRAY output_data)
{
  int outrow;
  JDIMENSION colctr;
  int data_unit = cinfo->master->lossless ? 1 : DCTSIZE;
  JDIMENSION output_cols = compptr->width_in_blocks * data_unit;
  register _JSAMPROW inptr, above_ptr, below_ptr, outptr;
  JLONG membersum, neighsum, memberscale, neighscale;
  int colsum, lastcolsum, nextcolsum;

  /* Expand input data enough to let all the output samples be generated
   * by the standard loop.  Special-casing padded output would be more
   * efficient.
   */
  expand_right_edge(input_data - 1, cinfo->max_v_samp_factor + 2,
                    cinfo->image_width, output_cols);

  /* Each of the eight neighbor components contributes a fraction SF to the
   * smoothed component, while the main component contributes (1-8*SF).  In
   * order to use integer arithmetic, these factors are multiplied by
   * 2^16 = 65536.  Also recall that SF = smoothing_factor / 1024.
   */

  memberscale = 65536L - cinfo->smoothing_factor * 512L; /* scaled 1-8*SF */
  neighscale = cinfo->smoothing_factor * 64; /* scaled SF */

  for (outrow = 0; outrow < compptr->v_samp_factor; outrow++) {
    outptr = output_data[outrow];
    inptr = input_data[outrow];
    above_ptr = input_data[outrow - 1];
    below_ptr = input_data[outrow + 1];

    /* Special case for first column */
    colsum = (*above_ptr++) + (*below_ptr++) + inptr[0];
    membersum = *inptr++;
    nextcolsum = above_ptr[0] + below_ptr[0] + inptr[0];
    neighsum = colsum + (colsum - membersum) + nextcolsum;
    membersum = membersum * memberscale + neighsum * neighscale;
    *outptr++ = (_JSAMPLE)((membersum + 32768) >> 16);
    lastcolsum = colsum;  colsum = nextcolsum;

    for (colctr = output_cols - 2; colctr > 0; colctr--) {
      membersum = *inptr++;
      above_ptr++;  below_ptr++;
      nextcolsum = above_ptr[0] + below_ptr[0] + inptr[0];
      neighsum = lastcolsum + (colsum - membersum) + nextcolsum;
      membersum = membersum * memberscale + neighsum * neighscale;
      *outptr++ = (_JSAMPLE)((membersum + 32768) >> 16);
      lastcolsum = colsum;  colsum = nextcolsum;
    }

    /* Special case for last column */
    membersum = *inptr;
    neighsum = lastcolsum + (colsum - membersum) + colsum;
    membersum = membersum * memberscale + neighsum * neighscale;
    *outptr = (_JSAMPLE)((membersum + 32768) >> 16);

  }
}

#endif /* INPUT_SMOOTHING_SUPPORTED */


/*
 * Module initialization routine for downsampling.
 * Note that we must select a routine for each component.
 */

GLOBAL(void)
_jinit_downsampler(j_compress_ptr cinfo)
{
  my_downsample_ptr downsample;
  int ci;
  jpeg_component_info *compptr;
#ifdef INPUT_SMOOTHING_SUPPORTED
  boolean smoothok = TRUE;
#endif

#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);
  }

  downsample = (my_downsample_ptr)
    (*cinfo->mem->alloc_small) ((j_common_ptr)cinfo, JPOOL_IMAGE,
                                sizeof(my_downsampler));
  cinfo->downsample = (struct jpeg_downsampler *)downsample;
  downsample->pub.start_pass = start_pass_downsample;
  downsample->pub._downsample = sep_downsample;
  downsample->pub.need_context_rows = FALSE;

  if (cinfo->CCIR601_sampling)
    ERREXIT(cinfo, JERR_CCIR601_NOTIMPL);

  /* Verify we can handle the sampling factors, and set up method pointers */
  for (ci = 0, compptr = cinfo->comp_info; ci < cinfo->num_components;
       ci++, compptr++) {
    if (compptr->h_samp_factor == cinfo->max_h_samp_factor &&
        compptr->v_samp_factor == cinfo->max_v_samp_factor) {
#ifdef INPUT_SMOOTHING_SUPPORTED
      if (cinfo->smoothing_factor) {
        downsample->methods[ci] = fullsize_smooth_downsample;
        downsample->pub.need_context_rows = TRUE;
      } else
#endif
        downsample->methods[ci] = fullsize_downsample;
    } else if (compptr->h_samp_factor * 2 == cinfo->max_h_samp_factor &&
               compptr->v_samp_factor == cinfo->max_v_samp_factor) {
#ifdef INPUT_SMOOTHING_SUPPORTED
      smoothok = FALSE;
#endif
#ifdef WITH_SIMD
      if (jsimd_set_h2v1_downsample(cinfo))
        downsample->methods[ci] = jsimd_h2v1_downsample;
      else
#endif
        downsample->methods[ci] = h2v1_downsample;
    } else if (compptr->h_samp_factor * 2 == cinfo->max_h_samp_factor &&
               compptr->v_samp_factor * 2 == cinfo->max_v_samp_factor) {
#ifdef INPUT_SMOOTHING_SUPPORTED
      if (cinfo->smoothing_factor) {
        downsample->methods[ci] = h2v2_smooth_downsample;
        downsample->pub.need_context_rows = TRUE;
      } else
#endif
      {
#ifdef WITH_SIMD
        if (jsimd_set_h2v2_downsample(cinfo))
          downsample->methods[ci] = jsimd_h2v2_downsample;
        else
#endif
          downsample->methods[ci] = h2v2_downsample;
      }
    } else if ((cinfo->max_h_samp_factor % compptr->h_samp_factor) == 0 &&
               (cinfo->max_v_samp_factor % compptr->v_samp_factor) == 0) {
#ifdef INPUT_SMOOTHING_SUPPORTED
      smoothok = FALSE;
#endif
      downsample->methods[ci] = int_downsample;
    } else
      ERREXIT(cinfo, JERR_FRACT_SAMPLE_NOTIMPL);
  }

#ifdef INPUT_SMOOTHING_SUPPORTED
  if (cinfo->smoothing_factor && !smoothok)
    TRACEMS(cinfo, 0, JTRC_SMOOTH_NOTIMPL);
#endif
}

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

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Created: 2026-04-12 06:58