JPEGView/Independent JPEG Group/jcsample.c

/*
 * jcsample.c
 *
 * Copyright (C) 1991-1994, 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 downsampling routines.
 *
 * Downsampling input data is counted in "row groups".  A row group
 * is defined to be max_v_samp_factor pixel 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.
 *
 * 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
 * pixels covered by the output pixel.  The hi-falutin sampling literature
 * refers to this as a "box filter".  In general the characteristics of a box
 * filter are not very good, but 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 pixel P is replaced by a weighted sum of itself and its
 * eight neighbors.  P'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"

#undef INPUT_SMOOTHING_SUPPORTED /* for now */
/* Input smoothing is currently unsupported since jcprepct.c doesn't
 * yet handle creation of context rows for downsampling.
 */


/* Pointer to routine to downsample a single component */
typedef JMETHOD(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 (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;
  register 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-1);
      pixval = GETJSAMPLE(*ptr++);
      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, JSAMPIMAGE output_buf,
		JDIMENSION out_row_group_index)
{
  my_downsample_ptr downsample = (my_downsample_ptr) cinfo->downsample;
  int ci;
  jpeg_component_info * compptr;
  JSAMPARRAY out_ptr;

  for (ci = 0, compptr = cinfo->comp_info; ci < cinfo->num_components;
       ci++, compptr++) {
    out_ptr = output_buf[ci] + (out_row_group_i