jquant2.cpp File Reference

#include "jinclude.h"
#include "jpeglib.h"

Include dependency graph for jquant2.cpp:

Classes
struct	my_cquantizer
struct	box

Macros
#define	JPEG_INTERNALS
#define	R_SCALE   2 /* scale R distances by this much */
#define	G_SCALE   3 /* scale G distances by this much */
#define	B_SCALE   1 /* and B by this much */
#define	C0_SCALE   R_SCALE
#define	C1_SCALE   G_SCALE
#define	C2_SCALE   B_SCALE
#define	MAXNUMCOLORS   (MAXJSAMPLE+1) /* maximum size of colormap */
#define	HIST_C0_BITS   5 /* bits of precision in R/B histogram */
#define	HIST_C1_BITS   6 /* bits of precision in G histogram */
#define	HIST_C2_BITS   5 /* bits of precision in B/R histogram */
#define	HIST_C0_ELEMS   (1<<HIST_C0_BITS)
#define	HIST_C1_ELEMS   (1<<HIST_C1_BITS)
#define	HIST_C2_ELEMS   (1<<HIST_C2_BITS)
#define	C0_SHIFT   (BITS_IN_JSAMPLE-HIST_C0_BITS)
#define	C1_SHIFT   (BITS_IN_JSAMPLE-HIST_C1_BITS)
#define	C2_SHIFT   (BITS_IN_JSAMPLE-HIST_C2_BITS)
#define	BOX_C0_LOG   (HIST_C0_BITS-3)
#define	BOX_C1_LOG   (HIST_C1_BITS-3)
#define	BOX_C2_LOG   (HIST_C2_BITS-3)
#define	BOX_C0_ELEMS   (1<<BOX_C0_LOG) /* # of hist cells in update box */
#define	BOX_C1_ELEMS   (1<<BOX_C1_LOG)
#define	BOX_C2_ELEMS   (1<<BOX_C2_LOG)
#define	BOX_C0_SHIFT   (C0_SHIFT + BOX_C0_LOG)
#define	BOX_C1_SHIFT   (C1_SHIFT + BOX_C1_LOG)
#define	BOX_C2_SHIFT   (C2_SHIFT + BOX_C2_LOG)
#define	STEP_C0   ((1 << C0_SHIFT) * C0_SCALE)
#define	STEP_C1   ((1 << C1_SHIFT) * C1_SCALE)
#define	STEP_C2   ((1 << C2_SHIFT) * C2_SCALE)
#define	STEPSIZE   ((MAXJSAMPLE+1)/16)

Typedefs
typedef uint16_t	histcell
typedef histcell *	histptr
typedef histcell	hist1d[(1<< 5)]
typedef hist1d *	hist2d
typedef hist2d *	hist3d
typedef int16_t	FSERROR
typedef int	LOCFSERROR
typedef FSERROR *	FSERRPTR
typedef my_cquantizer *	my_cquantize_ptr
typedef box *	boxptr

Functions
static void	prescan_quantize (j_decompress_ptr cinfo, JSAMPARRAY input_buf, JSAMPARRAY output_buf, int num_rows)
static boxptr	find_biggest_color_pop (boxptr boxlist, int numboxes)
static boxptr	find_biggest_volume (boxptr boxlist, int numboxes)
static void	update_box (j_decompress_ptr cinfo, boxptr boxp)
static int	median_cut (j_decompress_ptr cinfo, boxptr boxlist, int numboxes, int desired_colors)
static void	compute_color (j_decompress_ptr cinfo, boxptr boxp, int icolor)
static void	select_colors (j_decompress_ptr cinfo, int desired_colors)
static int	find_nearby_colors (j_decompress_ptr cinfo, int minc0, int minc1, int minc2, JSAMPLE colorlist[])
static void	find_best_colors (j_decompress_ptr cinfo, int minc0, int minc1, int minc2, int numcolors, JSAMPLE colorlist[], JSAMPLE bestcolor[])
static void	fill_inverse_cmap (j_decompress_ptr cinfo, int c0, int c1, int c2)
static void	pass2_no_dither (j_decompress_ptr cinfo, JSAMPARRAY input_buf, JSAMPARRAY output_buf, int num_rows)
static void	pass2_fs_dither (j_decompress_ptr cinfo, JSAMPARRAY input_buf, JSAMPARRAY output_buf, int num_rows)
static void	init_error_limit (j_decompress_ptr cinfo)
static void	finish_pass1 (j_decompress_ptr cinfo)
static void	finish_pass2 (j_decompress_ptr cinfo)
static void	start_pass_2_quant (j_decompress_ptr cinfo, boolean is_pre_scan)
static void	new_color_map_2_quant (j_decompress_ptr cinfo)
void	jinit_2pass_quantizer (j_decompress_ptr cinfo)

Macro Definition Documentation

#define JPEG_INTERNALS

#define R_SCALE   2 /* scale R distances by this much */

#define G_SCALE   3 /* scale G distances by this much */

#define B_SCALE   1 /* and B by this much */

#define C0_SCALE   R_SCALE

#define C1_SCALE   G_SCALE

#define C2_SCALE   B_SCALE

#define MAXNUMCOLORS   (MAXJSAMPLE+1) /* maximum size of colormap */

#define HIST_C0_BITS   5 /* bits of precision in R/B histogram */

#define HIST_C1_BITS   6 /* bits of precision in G histogram */

#define HIST_C2_BITS   5 /* bits of precision in B/R histogram */

#define HIST_C0_ELEMS   (1<<HIST_C0_BITS)

#define HIST_C1_ELEMS   (1<<HIST_C1_BITS)

#define HIST_C2_ELEMS   (1<<HIST_C2_BITS)

#define C0_SHIFT   (BITS_IN_JSAMPLE-HIST_C0_BITS)

#define C1_SHIFT   (BITS_IN_JSAMPLE-HIST_C1_BITS)

#define C2_SHIFT   (BITS_IN_JSAMPLE-HIST_C2_BITS)

#define BOX_C0_LOG   (HIST_C0_BITS-3)

#define BOX_C1_LOG   (HIST_C1_BITS-3)

#define BOX_C2_LOG   (HIST_C2_BITS-3)

#define BOX_C0_ELEMS   (1<<BOX_C0_LOG) /* # of hist cells in update box */

#define BOX_C1_ELEMS   (1<<BOX_C1_LOG)

#define BOX_C2_ELEMS   (1<<BOX_C2_LOG)

#define BOX_C0_SHIFT   (C0_SHIFT + BOX_C0_LOG)

#define BOX_C1_SHIFT   (C1_SHIFT + BOX_C1_LOG)

#define BOX_C2_SHIFT   (C2_SHIFT + BOX_C2_LOG)

#define STEP_C0   ((1 << C0_SHIFT) * C0_SCALE)

#define STEP_C1   ((1 << C1_SHIFT) * C1_SCALE)

#define STEP_C2   ((1 << C2_SHIFT) * C2_SCALE)

#define STEPSIZE   ((MAXJSAMPLE+1)/16)

Typedef Documentation

typedef uint16_t histcell

typedef histcell* histptr

typedef histcell hist1d[(1<< 5)]

typedef hist1d* hist2d

typedef hist2d* hist3d

typedef int16_t FSERROR

typedef int LOCFSERROR

typedef FSERROR* FSERRPTR

typedef my_cquantizer* my_cquantize_ptr

typedef box* boxptr

Function Documentation

static void prescan_quantize ( j_decompress_ptr  cinfo, JSAMPARRAY  input_buf, JSAMPARRAY  output_buf, int  num_rows  )

static

{
  my_cquantize_ptr cquantize = (my_cquantize_ptr) cinfo->cquantize;
  register JSAMPROW ptr;
  register histptr histp;
  register hist3d histogram = cquantize->histogram;
  int row;
  JDIMENSION col;
  JDIMENSION width = cinfo->output_width;

  for (row = 0; row < num_rows; row++) {
    ptr = input_buf[row];
    for (col = width; col > 0; col--) {
      /* get pixel value and index into the histogram */
      histp = & histogram[GETJSAMPLE(ptr[0]) >> C0_SHIFT]
             [GETJSAMPLE(ptr[1]) >> C1_SHIFT]
             [GETJSAMPLE(ptr[2]) >> C2_SHIFT];
      /* increment, check for overflow and undo increment if so. */
      if (++(*histp) <= 0)
    (*histp)--;
      ptr += 3;
    }
  }
}

static boxptr find_biggest_color_pop ( boxptr  boxlist, int  numboxes  )

static

{
  register boxptr boxp;
  register int i;
  register long maxc = 0;
  boxptr which = NULL;
  
  for (i = 0, boxp = boxlist; i < numboxes; i++, boxp++) {
    if (boxp->colorcount > maxc && boxp->volume > 0) {
      which = boxp;
      maxc = boxp->colorcount;
    }
  }
  return which;
}

static boxptr find_biggest_volume ( boxptr  boxlist, int  numboxes  )

static

{
  register boxptr boxp;
  register int i;
  register int32_t maxv = 0;
  boxptr which = NULL;
  
  for (i = 0, boxp = boxlist; i < numboxes; i++, boxp++) {
    if (boxp->volume > maxv) {
      which = boxp;
      maxv = boxp->volume;
    }
  }
  return which;
}

static void update_box ( j_decompress_ptr  cinfo, boxptr  boxp  )

static

{
  my_cquantize_ptr cquantize = (my_cquantize_ptr) cinfo->cquantize;
  hist3d histogram = cquantize->histogram;
  histptr histp;
  int c0,c1,c2;
  int c0min,c0max,c1min,c1max,c2min,c2max;
  int32_t dist0,dist1,dist2;
  long ccount;
  
  c0min = boxp->c0min;  c0max = boxp->c0max;
  c1min = boxp->c1min;  c1max = boxp->c1max;
  c2min = boxp->c2min;  c2max = boxp->c2max;
  
  if (c0max > c0min)
    for (c0 = c0min; c0 <= c0max; c0++)
      for (c1 = c1min; c1 <= c1max; c1++) {
    histp = & histogram[c0][c1][c2min];
    for (c2 = c2min; c2 <= c2max; c2++)
      if (*histp++ != 0) {
        boxp->c0min = c0min = c0;
        goto have_c0min;
      }
      }
 have_c0min:
  if (c0max > c0min)
    for (c0 = c0max; c0 >= c0min; c0--)
      for (c1 = c1min; c1 <= c1max; c1++) {
    histp = & histogram[c0][c1][c2min];
    for (c2 = c2min; c2 <= c2max; c2++)
      if (*histp++ != 0) {
        boxp->c0max = c0max = c0;
        goto have_c0max;
      }
      }
 have_c0max:
  if (c1max > c1min)
    for (c1 = c1min; c1 <= c1max; c1++)
      for (c0 = c0min; c0 <= c0max; c0++) {
    histp = & histogram[c0][c1][c2min];
    for (c2 = c2min; c2 <= c2max; c2++)
      if (*histp++ != 0) {
        boxp->c1min = c1min = c1;
        goto have_c1min;
      }
      }
 have_c1min:
  if (c1max > c1min)
    for (c1 = c1max; c1 >= c1min; c1--)
      for (c0 = c0min; c0 <= c0max; c0++) {
    histp = & histogram[c0][c1][c2min];
    for (c2 = c2min; c2 <= c2max; c2++)
      if (*histp++ != 0) {
        boxp->c1max = c1max = c1;
        goto have_c1max;
      }
      }
 have_c1max:
  if (c2max > c2min)
    for (c2 = c2min; c2 <= c2max; c2++)
      for (c0 = c0min; c0 <= c0max; c0++) {
    histp = & histogram[c0][c1min][c2];
    for (c1 = c1min; c1 <= c1max; c1++, histp += HIST_C2_ELEMS)
      if (*histp != 0) {
        boxp->c2min = c2min = c2;
        goto have_c2min;
      }
      }
 have_c2min:
  if (c2max > c2min)
    for (c2 = c2max; c2 >= c2min; c2--)
      for (c0 = c0min; c0 <= c0max; c0++) {
    histp = & histogram[c0][c1min][c2];
    for (c1 = c1min; c1 <= c1max; c1++, histp += HIST_C2_ELEMS)
      if (*histp != 0) {
        boxp->c2max = c2max = c2;
        goto have_c2max;
      }
      }
 have_c2max:

  /* Update box volume.
   * We use 2-norm rather than real volume here; this biases the method
   * against making long narrow boxes, and it has the side benefit that
   * a box is splittable iff norm > 0.
   * Since the differences are expressed in histogram-cell units,
   * we have to shift back to JSAMPLE units to get consistent distances;
   * after which, we scale according to the selected distance scale factors.
   */
  dist0 = ((c0max - c0min) << C0_SHIFT) * C0_SCALE;
  dist1 = ((c1max - c1min) << C1_SHIFT) * C1_SCALE;
  dist2 = ((c2max - c2min) << C2_SHIFT) * C2_SCALE;
  boxp->volume = dist0*dist0 + dist1*dist1 + dist2*dist2;
  
  /* Now scan remaining volume of box and compute population */
  ccount = 0;
  for (c0 = c0min; c0 <= c0max; c0++)
    for (c1 = c1min; c1 <= c1max; c1++) {
      histp = & histogram[c0][c1][c2min];
      for (c2 = c2min; c2 <= c2max; c2++, histp++)
    if (*histp != 0) {
      ccount++;
    }
    }
  boxp->colorcount = ccount;
}

static int median_cut ( j_decompress_ptr  cinfo, boxptr  boxlist, int  numboxes, int  desired_colors  )

static

{
  int n,lb;
  int c0,c1,c2,cmax;
  register boxptr b1,b2;

  while (numboxes < desired_colors) {
    /* Select box to split.
     * Current algorithm: by population for first half, then by volume.
     */
    if (numboxes*2 <= desired_colors) {
      b1 = find_biggest_color_pop(boxlist, numboxes);
    } else {
      b1 = find_biggest_volume(boxlist, numboxes);
    }
    if (b1 == NULL)     /* no splittable boxes left! */
      break;
    b2 = &boxlist[numboxes];    /* where new box will go */
    /* Copy the color bounds to the new box. */
    b2->c0max = b1->c0max; b2->c1max = b1->c1max; b2->c2max = b1->c2max;
    b2->c0min = b1->c0min; b2->c1min = b1->c1min; b2->c2min = b1->c2min;
    /* Choose which axis to split the box on.
     * Current algorithm: longest scaled axis.
     * See notes in update_box about scaling distances.
     */
    c0 = ((b1->c0max - b1->c0min) << C0_SHIFT) * C0_SCALE;
    c1 = ((b1->c1max - b1->c1min) << C1_SHIFT) * C1_SCALE;
    c2 = ((b1->c2max - b1->c2min) << C2_SHIFT) * C2_SCALE;
    /* We want to break any ties in favor of green, then red, blue last.
     * This code does the right thing for R,G,B or B,G,R color orders only.
     */
#if RGB_RED == 0
    cmax = c1; n = 1;
    if (c0 > cmax) { cmax = c0; n = 0; }
    if (c2 > cmax) { n = 2; }
#else
    cmax = c1; n = 1;
    if (c2 > cmax) { cmax = c2; n = 2; }
    if (c0 > cmax) { n = 0; }
#endif
    /* Choose split point along selected axis, and update box bounds.
     * Current algorithm: split at halfway point.
     * (Since the box has been shrunk to minimum volume,
     * any split will produce two nonempty subboxes.)
     * Note that lb value is max for lower box, so must be < old max.
     */
    switch (n) {
    case 0:
      lb = (b1->c0max + b1->c0min) / 2;
      b1->c0max = lb;
      b2->c0min = lb+1;
      break;
    case 1:
      lb = (b1->c1max + b1->c1min) / 2;
      b1->c1max = lb;
      b2->c1min = lb+1;
      break;
    case 2:
      lb = (b1->c2max + b1->c2min) / 2;
      b1->c2max = lb;
      b2->c2min = lb+1;
      break;
    }
    /* Update stats for boxes */
    update_box(cinfo, b1);
    update_box(cinfo, b2);
    numboxes++;
  }
  return numboxes;
}

static void compute_color ( j_decompress_ptr  cinfo, boxptr  boxp, int  icolor  )

static

{
  /* Current algorithm: mean weighted by pixels (not colors) */
  /* Note it is important to get the rounding correct! */
  my_cquantize_ptr cquantize = (my_cquantize_ptr) cinfo->cquantize;
  hist3d histogram = cquantize->histogram;
  histptr histp;
  int c0,c1,c2;
  int c0min,c0max,c1min,c1max,c2min,c2max;
  long count;
  long total = 0;
  long c0total = 0;
  long c1total = 0;
  long c2total = 0;
  
  c0min = boxp->c0min;  c0max = boxp->c0max;
  c1min = boxp->c1min;  c1max = boxp->c1max;
  c2min = boxp->c2min;  c2max = boxp->c2max;
  
  for (c0 = c0min; c0 <= c0max; c0++)
    for (c1 = c1min; c1 <= c1max; c1++) {
      histp = & histogram[c0][c1][c2min];
      for (c2 = c2min; c2 <= c2max; c2++) {
    if ((count = *histp++) != 0) {
      total += count;
      c0total += ((c0 << C0_SHIFT) + ((1<<C0_SHIFT)>>1)) * count;
      c1total += ((c1 << C1_SHIFT) + ((1<<C1_SHIFT)>>1)) * count;
      c2total += ((c2 << C2_SHIFT) + ((1<<C2_SHIFT)>>1)) * count;
    }
      }
    }
  
  cinfo->colormap[0][icolor] = (JSAMPLE) ((c0total + (total>>1)) / total);
  cinfo->colormap[1][icolor] = (JSAMPLE) ((c1total + (total>>1)) / total);
  cinfo->colormap[2][icolor] = (JSAMPLE) ((c2total + (total>>1)) / total);
}

static void select_colors ( j_decompress_ptr  cinfo, int  desired_colors  )

static

{
  boxptr boxlist;
  int numboxes;
  int i;

  /* Allocate workspace for box list */
  boxlist = (boxptr) (*cinfo->mem->alloc_small)
    ((j_common_ptr) cinfo, JPOOL_IMAGE, desired_colors * SIZEOF(box));
  /* Initialize one box containing whole space */
  numboxes = 1;
  boxlist[0].c0min = 0;
  boxlist[0].c0max = MAXJSAMPLE >> C0_SHIFT;
  boxlist[0].c1min = 0;
  boxlist[0].c1max = MAXJSAMPLE >> C1_SHIFT;
  boxlist[0].c2min = 0;
  boxlist[0].c2max = MAXJSAMPLE >> C2_SHIFT;
  /* Shrink it to actually-used volume and set its statistics */
  update_box(cinfo, & boxlist[0]);
  /* Perform median-cut to produce final box list */
  numboxes = median_cut(cinfo, boxlist, numboxes, desired_colors);
  /* Compute the representative color for each box, fill colormap */
  for (i = 0; i < numboxes; i++)
    compute_color(cinfo, & boxlist[i], i);
  cinfo->actual_number_of_colors = numboxes;
  TRACEMS1(cinfo, 1, JTRC_QUANT_SELECTED, numboxes);
}

static int find_nearby_colors ( j_decompress_ptr  cinfo, int  minc0, int  minc1, int  minc2, JSAMPLE  colorlist[]  )

static

{
  int numcolors = cinfo->actual_number_of_colors;
  int maxc0, maxc1, maxc2;
  int centerc0, centerc1, centerc2;
  int i, x, ncolors;
  int32_t minmaxdist, min_dist, max_dist, tdist;
  int32_t mindist[MAXNUMCOLORS];    /* min distance to colormap entry i */

  /* Compute true coordinates of update box's upper corner and center.
   * Actually we compute the coordinates of the center of the upper-corner
   * histogram cell, which are the upper bounds of the volume we care about.
   * Note that since ">>" rounds down, the "center" values may be closer to
   * min than to max; hence comparisons to them must be "<=", not "<".
   */
  maxc0 = minc0 + ((1 << BOX_C0_SHIFT) - (1 << C0_SHIFT));
  centerc0 = (minc0 + maxc0) >> 1;
  maxc1 = minc1 + ((1 << BOX_C1_SHIFT) - (1 << C1_SHIFT));
  centerc1 = (minc1 + maxc1) >> 1;
  maxc2 = minc2 + ((1 << BOX_C2_SHIFT) - (1 << C2_SHIFT));
  centerc2 = (minc2 + maxc2) >> 1;

  /* For each color in colormap, find:
   *  1. its minimum squared-distance to any point in the update box
   *     (zero if color is within update box);
   *  2. its maximum squared-distance to any point in the update box.
   * Both of these can be found by considering only the corners of the box.
   * We save the minimum distance for each color in mindist[];
   * only the smallest maximum distance is of interest.
   */
  minmaxdist = 0x7FFFFFFFL;

  for (i = 0; i < numcolors; i++) {
    /* We compute the squared-c0-distance term, then add in the other two. */
    x = GETJSAMPLE(cinfo->colormap[0][i]);
    if (x < minc0) {
      tdist = (x - minc0) * C0_SCALE;
      min_dist = tdist*tdist;
      tdist = (x - maxc0) * C0_SCALE;
      max_dist = tdist*tdist;
    } else if (x > maxc0) {
      tdist = (x - maxc0) * C0_SCALE;
      min_dist = tdist*tdist;
      tdist = (x - minc0) * C0_SCALE;
      max_dist = tdist*tdist;
    } else {
      /* within cell range so no contribution to min_dist */
      min_dist = 0;
      if (x <= centerc0) {
    tdist = (x - maxc0) * C0_SCALE;
    max_dist = tdist*tdist;
      } else {
    tdist = (x - minc0) * C0_SCALE;
    max_dist = tdist*tdist;
      }
    }

    x = GETJSAMPLE(cinfo->colormap[1][i]);
    if (x < minc1) {
      tdist = (x - minc1) * C1_SCALE;
      min_dist += tdist*tdist;
      tdist = (x - maxc1) * C1_SCALE;
      max_dist += tdist*tdist;
    } else if (x > maxc1) {
      tdist = (x - maxc1) * C1_SCALE;
      min_dist += tdist*tdist;
      tdist = (x - minc1) * C1_SCALE;
      max_dist += tdist*tdist;
    } else {
      /* within cell range so no contribution to min_dist */
      if (x <= centerc1) {
    tdist = (x - maxc1) * C1_SCALE;
    max_dist += tdist*tdist;
      } else {
    tdist = (x - minc1) * C1_SCALE;
    max_dist += tdist*tdist;
      }
    }

    x = GETJSAMPLE(cinfo->colormap[2][i]);
    if (x < minc2) {
      tdist = (x - minc2) * C2_SCALE;
      min_dist += tdist*tdist;
      tdist = (x - maxc2) * C2_SCALE;
      max_dist += tdist*tdist;
    } else if (x > maxc2) {
      tdist = (x - maxc2) * C2_SCALE;
      min_dist += tdist*tdist;
      tdist = (x - minc2) * C2_SCALE;
      max_dist += tdist*tdist;
    } else {
      /* within cell range so no contribution to min_dist */
      if (x <= centerc2) {
    tdist = (x - maxc2) * C2_SCALE;
    max_dist += tdist*tdist;
      } else {
    tdist = (x - minc2) * C2_SCALE;
    max_dist += tdist*tdist;
      }
    }

    mindist[i] = min_dist;  /* save away the results */
    if (max_dist < minmaxdist)
      minmaxdist = max_dist;
  }

  /* Now we know that no cell in the update box is more than minmaxdist
   * away from some colormap entry.  Therefore, only colors that are
   * within minmaxdist of some part of the box need be considered.
   */
  ncolors = 0;
  for (i = 0; i < numcolors; i++) {
    if (mindist[i] <= minmaxdist)
      colorlist[ncolors++] = (JSAMPLE) i;
  }
  return ncolors;
}

static void find_best_colors ( j_decompress_ptr  cinfo, int  minc0, int  minc1, int  minc2, int  numcolors, JSAMPLE  colorlist[], JSAMPLE  bestcolor[]  )

static

{
  int ic0, ic1, ic2;
  int i, icolor;
  register int32_t * bptr;  /* pointer into bestdist[] array */
  JSAMPLE * cptr;       /* pointer into bestcolor[] array */
  int32_t dist0, dist1;     /* initial distance values */
  register int32_t dist2;       /* current distance in inner loop */
  int32_t xx0, xx1;     /* distance increments */
  register int32_t xx2;
  int32_t inc0, inc1, inc2; /* initial values for increments */
  /* This array holds the distance to the nearest-so-far color for each cell */
  int32_t bestdist[BOX_C0_ELEMS * BOX_C1_ELEMS * BOX_C2_ELEMS];

  /* Initialize best-distance for each cell of the update box */
  bptr = bestdist;
  for (i = BOX_C0_ELEMS*BOX_C1_ELEMS*BOX_C2_ELEMS-1; i >= 0; i--)
    *bptr++ = 0x7FFFFFFFL;
  
  /* For each color selected by find_nearby_colors,
   * compute its distance to the center of each cell in the box.
   * If that's less than best-so-far, update best distance and color number.
   */
  
  /* Nominal steps between cell centers ("x" in Thomas article) */
#define STEP_C0  ((1 << C0_SHIFT) * C0_SCALE)
#define STEP_C1  ((1 << C1_SHIFT) * C1_SCALE)
#define STEP_C2  ((1 << C2_SHIFT) * C2_SCALE)
  
  for (i = 0; i < numcolors; i++) {
    icolor = GETJSAMPLE(colorlist[i]);
    /* Compute (square of) distance from minc0/c1/c2 to this color */
    inc0 = (minc0 - GETJSAMPLE(cinfo->colormap[0][icolor])) * C0_SCALE;
    dist0 = inc0*inc0;
    inc1 = (minc1 - GETJSAMPLE(cinfo->colormap[1][icolor])) * C1_SCALE;
    dist0 += inc1*inc1;
    inc2 = (minc2 - GETJSAMPLE(cinfo->colormap[2][icolor])) * C2_SCALE;
    dist0 += inc2*inc2;
    /* Form the initial difference increments */
    inc0 = inc0 * (2 * STEP_C0) + STEP_C0 * STEP_C0;
    inc1 = inc1 * (2 * STEP_C1) + STEP_C1 * STEP_C1;
    inc2 = inc2 * (2 * STEP_C2) + STEP_C2 * STEP_C2;
    /* Now loop over all cells in box, updating distance per Thomas method */
    bptr = bestdist;
    cptr = bestcolor;
    xx0 = inc0;
    for (ic0 = BOX_C0_ELEMS-1; ic0 >= 0; ic0--) {
      dist1 = dist0;
      xx1 = inc1;
      for (ic1 = BOX_C1_ELEMS-1; ic1 >= 0; ic1--) {
    dist2 = dist1;
    xx2 = inc2;
    for (ic2 = BOX_C2_ELEMS-1; ic2 >= 0; ic2--) {
      if (dist2 < *bptr) {
        *bptr = dist2;
        *cptr = (JSAMPLE) icolor;
      }
      dist2 += xx2;
      xx2 += 2 * STEP_C2 * STEP_C2;
      bptr++;
      cptr++;
    }
    dist1 += xx1;
    xx1 += 2 * STEP_C1 * STEP_C1;
      }
      dist0 += xx0;
      xx0 += 2 * STEP_C0 * STEP_C0;
    }
  }
}

static void fill_inverse_cmap ( j_decompress_ptr  cinfo, int  c0, int  c1, int  c2  )

static

{
  my_cquantize_ptr cquantize = (my_cquantize_ptr) cinfo->cquantize;
  hist3d histogram = cquantize->histogram;
  int minc0, minc1, minc2;  /* lower left corner of update box */
  int ic0, ic1, ic2;
  register JSAMPLE * cptr;  /* pointer into bestcolor[] array */
  register histptr cachep;  /* pointer into main cache array */
  /* This array lists the candidate colormap indexes. */
  JSAMPLE colorlist[MAXNUMCOLORS];
  int numcolors;        /* number of candidate colors */
  /* This array holds the actually closest colormap index for each cell. */
  JSAMPLE bestcolor[BOX_C0_ELEMS * BOX_C1_ELEMS * BOX_C2_ELEMS];

  /* Convert cell coordinates to update box ID */
  c0 >>= BOX_C0_LOG;
  c1 >>= BOX_C1_LOG;
  c2 >>= BOX_C2_LOG;

  /* Compute true coordinates of update box's origin corner.
   * Actually we compute the coordinates of the center of the corner
   * histogram cell, which are the lower bounds of the volume we care about.
   */
  minc0 = (c0 << BOX_C0_SHIFT) + ((1 << C0_SHIFT) >> 1);
  minc1 = (c1 << BOX_C1_SHIFT) + ((1 << C1_SHIFT) >> 1);
  minc2 = (c2 << BOX_C2_SHIFT) + ((1 << C2_SHIFT) >> 1);
  
  /* Determine which colormap entries are close enough to be candidates
   * for the nearest entry to some cell in the update box.
   */
  numcolors = find_nearby_colors(cinfo, minc0, minc1, minc2, colorlist);

  /* Determine the actually nearest colors. */
  find_best_colors(cinfo, minc0, minc1, minc2, numcolors, colorlist,
           bestcolor);

  /* Save the best color numbers (plus 1) in the main cache array */
  c0 <<= BOX_C0_LOG;        /* convert ID back to base cell indexes */
  c1 <<= BOX_C1_LOG;
  c2 <<= BOX_C2_LOG;
  cptr = bestcolor;
  for (ic0 = 0; ic0 < BOX_C0_ELEMS; ic0++) {
    for (ic1 = 0; ic1 < BOX_C1_ELEMS; ic1++) {
      cachep = & histogram[c0+ic0][c1+ic1][c2];
      for (ic2 = 0; ic2 < BOX_C2_ELEMS; ic2++) {
    *cachep++ = (histcell) (GETJSAMPLE(*cptr++) + 1);
      }
    }
  }
}

static void pass2_no_dither ( j_decompress_ptr  cinfo, JSAMPARRAY  input_buf, JSAMPARRAY  output_buf, int  num_rows  )

static

{
  my_cquantize_ptr cquantize = (my_cquantize_ptr) cinfo->cquantize;
  hist3d histogram = cquantize->histogram;
  register JSAMPROW inptr, outptr;
  register histptr cachep;
  register int c0, c1, c2;
  int row;
  JDIMENSION col;
  JDIMENSION width = cinfo->output_width;

  for (row = 0; row < num_rows; row++) {
    inptr = input_buf[row];
    outptr = output_buf[row];
    for (col = width; col > 0; col--) {
      /* get pixel value and index into the cache */
      c0 = GETJSAMPLE(*inptr++) >> C0_SHIFT;
      c1 = GETJSAMPLE(*inptr++) >> C1_SHIFT;
      c2 = GETJSAMPLE(*inptr++) >> C2_SHIFT;
      cachep = & histogram[c0][c1][c2];
      /* If we have not seen this color before, find nearest colormap entry */
      /* and update the cache */
      if (*cachep == 0)
    fill_inverse_cmap(cinfo, c0,c1,c2);
      /* Now emit the colormap index for this cell */
      *outptr++ = (JSAMPLE) (*cachep - 1);
    }
  }
}

static void pass2_fs_dither ( j_decompress_ptr  cinfo, JSAMPARRAY  input_buf, JSAMPARRAY  output_buf, int  num_rows  )

static

{
  my_cquantize_ptr cquantize = (my_cquantize_ptr) cinfo->cquantize;
  hist3d histogram = cquantize->histogram;
  register LOCFSERROR cur0, cur1, cur2; /* current error or pixel value */
  LOCFSERROR belowerr0, belowerr1, belowerr2; /* error for pixel below cur */
  LOCFSERROR bpreverr0, bpreverr1, bpreverr2; /* error for below/prev col */
  register FSERRPTR errorptr;   /* => fserrors[] at column before current */
  JSAMPROW inptr;       /* => current input pixel */
  JSAMPROW outptr;      /* => current output pixel */
  histptr cachep;
  int dir;          /* +1 or -1 depending on direction */
  int dir3;         /* 3*dir, for advancing inptr & errorptr */
  int row;
  JDIMENSION col;
  JDIMENSION width = cinfo->output_width;
  JSAMPLE *range_limit = cinfo->sample_range_limit;
  int *error_limit = cquantize->error_limiter;
  JSAMPROW colormap0 = cinfo->colormap[0];
  JSAMPROW colormap1 = cinfo->colormap[1];
  JSAMPROW colormap2 = cinfo->colormap[2];
  SHIFT_TEMPS

  for (row = 0; row < num_rows; row++) {
    inptr = input_buf[row];
    outptr = output_buf[row];
    if (cquantize->on_odd_row) {
      /* work right to left in this row */
      inptr += (width-1) * 3;   /* so point to rightmost pixel */
      outptr += width-1;
      dir = -1;
      dir3 = -3;
      errorptr = cquantize->fserrors + (width+1)*3; /* => entry after last column */
      cquantize->on_odd_row = FALSE; /* flip for next time */
    } else {
      /* work left to right in this row */
      dir = 1;
      dir3 = 3;
      errorptr = cquantize->fserrors; /* => entry before first real column */
      cquantize->on_odd_row = TRUE; /* flip for next time */
    }
    /* Preset error values: no error propagated to first pixel from left */
    cur0 = cur1 = cur2 = 0;
    /* and no error propagated to row below yet */
    belowerr0 = belowerr1 = belowerr2 = 0;
    bpreverr0 = bpreverr1 = bpreverr2 = 0;

    for (col = width; col > 0; col--) {
      /* curN holds the error propagated from the previous pixel on the
       * current line.  Add the error propagated from the previous line
       * to form the complete error correction term for this pixel, and
       * round the error term (which is expressed * 16) to an integer.
       * RIGHT_SHIFT rounds towards minus infinity, so adding 8 is correct
       * for either sign of the error value.
       * Note: errorptr points to *previous* column's array entry.
       */
      cur0 = RIGHT_SHIFT(cur0 + errorptr[dir3+0] + 8, 4);
      cur1 = RIGHT_SHIFT(cur1 + errorptr[dir3+1] + 8, 4);
      cur2 = RIGHT_SHIFT(cur2 + errorptr[dir3+2] + 8, 4);
      /* Limit the error using transfer function set by init_error_limit.
       * See comments with init_error_limit for rationale.
       */
      cur0 = error_limit[cur0];
      cur1 = error_limit[cur1];
      cur2 = error_limit[cur2];
      /* Form pixel value + error, and range-limit to 0..MAXJSAMPLE.
       * The maximum error is +- MAXJSAMPLE (or less with error limiting);
       * this sets the required size of the range_limit array.
       */
      cur0 += GETJSAMPLE(inptr[0]);
      cur1 += GETJSAMPLE(inptr[1]);
      cur2 += GETJSAMPLE(inptr[2]);
      cur0 = GETJSAMPLE(range_limit[cur0]);
      cur1 = GETJSAMPLE(range_limit[cur1]);
      cur2 = GETJSAMPLE(range_limit[cur2]);
      /* Index into the cache with adjusted pixel value */
      cachep = & histogram[cur0>>C0_SHIFT][cur1>>C1_SHIFT][cur2>>C2_SHIFT];
      /* If we have not seen this color before, find nearest colormap */
      /* entry and update the cache */
      if (*cachep == 0)
    fill_inverse_cmap(cinfo, cur0>>C0_SHIFT,cur1>>C1_SHIFT,cur2>>C2_SHIFT);
      /* Now emit the colormap index for this cell */
      { register int pixcode = *cachep - 1;
    *outptr = (JSAMPLE) pixcode;
    /* Compute representation error for this pixel */
    cur0 -= GETJSAMPLE(colormap0[pixcode]);
    cur1 -= GETJSAMPLE(colormap1[pixcode]);
    cur2 -= GETJSAMPLE(colormap2[pixcode]);
      }
      /* Compute error fractions to be propagated to adjacent pixels.
       * Add these into the running sums, and simultaneously shift the
       * next-line error sums left by 1 column.
       */
      { register LOCFSERROR bnexterr, delta;

    bnexterr = cur0;    /* Process component 0 */
    delta = cur0 * 2;
    cur0 += delta;      /* form error * 3 */
    errorptr[0] = (FSERROR) (bpreverr0 + cur0);
    cur0 += delta;      /* form error * 5 */
    bpreverr0 = belowerr0 + cur0;
    belowerr0 = bnexterr;
    cur0 += delta;      /* form error * 7 */
    bnexterr = cur1;    /* Process component 1 */
    delta = cur1 * 2;
    cur1 += delta;      /* form error * 3 */
    errorptr[1] = (FSERROR) (bpreverr1 + cur1);
    cur1 += delta;      /* form error * 5 */
    bpreverr1 = belowerr1 + cur1;
    belowerr1 = bnexterr;
    cur1 += delta;      /* form error * 7 */
    bnexterr = cur2;    /* Process component 2 */
    delta = cur2 * 2;
    cur2 += delta;      /* form error * 3 */
    errorptr[2] = (FSERROR) (bpreverr2 + cur2);
    cur2 += delta;      /* form error * 5 */
    bpreverr2 = belowerr2 + cur2;
    belowerr2 = bnexterr;
    cur2 += delta;      /* form error * 7 */
      }
      /* At this point curN contains the 7/16 error value to be propagated
       * to the next pixel on the current line, and all the errors for the
       * next line have been shifted over.  We are therefore ready to move on.
       */
      inptr += dir3;        /* Advance pixel pointers to next column */
      outptr += dir;
      errorptr += dir3;     /* advance errorptr to current column */
    }
    /* Post-loop cleanup: we must unload the final error values into the
     * final fserrors[] entry.  Note we need not unload belowerrN because
     * it is for the dummy column before or after the actual array.
     */
    errorptr[0] = (FSERROR) bpreverr0; /* unload prev errs into array */
    errorptr[1] = (FSERROR) bpreverr1;
    errorptr[2] = (FSERROR) bpreverr2;
  }
}

static void init_error_limit ( j_decompress_ptr  cinfo )

static

{
  my_cquantize_ptr cquantize = (my_cquantize_ptr) cinfo->cquantize;
  int * table;
  int in, out;

  table = (int *) (*cinfo->mem->alloc_small)
    ((j_common_ptr) cinfo, JPOOL_IMAGE, (MAXJSAMPLE*2+1) * SIZEOF(int));
  table += MAXJSAMPLE;      /* so can index -MAXJSAMPLE .. +MAXJSAMPLE */
  cquantize->error_limiter = table;

#define STEPSIZE ((MAXJSAMPLE+1)/16)
  /* Map errors 1:1 up to +- MAXJSAMPLE/16 */
  out = 0;
  for (in = 0; in < STEPSIZE; in++, out++) {
    table[in] = out; table[-in] = -out;
  }
  /* Map errors 1:2 up to +- 3*MAXJSAMPLE/16 */
  for (; in < STEPSIZE*3; in++, out += (in&1) ? 0 : 1) {
    table[in] = out; table[-in] = -out;
  }
  /* Clamp the rest to final out value (which is (MAXJSAMPLE+1)/8) */
  for (; in <= MAXJSAMPLE; in++) {
    table[in] = out; table[-in] = -out;
  }
#undef STEPSIZE
}

static void finish_pass1 ( j_decompress_ptr  cinfo )

static

{
  my_cquantize_ptr cquantize = (my_cquantize_ptr) cinfo->cquantize;

  /* Select the representative colors and fill in cinfo->colormap */
  cinfo->colormap = cquantize->sv_colormap;
  select_colors(cinfo, cquantize->desired);
  /* Force next pass to zero the color index table */
  cquantize->needs_zeroed = TRUE;
}

static void finish_pass2 ( j_decompress_ptr  cinfo )

static

{
  /* no work */
}

static void start_pass_2_quant ( j_decompress_ptr  cinfo, boolean  is_pre_scan  )

static

{
  my_cquantize_ptr cquantize = (my_cquantize_ptr) cinfo->cquantize;
  hist3d histogram = cquantize->histogram;
  int i;

  /* Only F-S dithering or no dithering is supported. */
  /* If user asks for ordered dither, give him F-S. */
  if (cinfo->dither_mode != JDITHER_NONE)
    cinfo->dither_mode = JDITHER_FS;

  if (is_pre_scan) {
    /* Set up method pointers */
    cquantize->pub.color_quantize = prescan_quantize;
    cquantize->pub.finish_pass = finish_pass1;
    cquantize->needs_zeroed = TRUE; /* Always zero histogram */
  } else {
    /* Set up method pointers */
    if (cinfo->dither_mode == JDITHER_FS)
      cquantize->pub.color_quantize = pass2_fs_dither;
    else
      cquantize->pub.color_quantize = pass2_no_dither;
    cquantize->pub.finish_pass = finish_pass2;

    /* Make sure color count is acceptable */
    i = cinfo->actual_number_of_colors;
    if (i < 1)
      ERREXIT1(cinfo, JERR_QUANT_FEW_COLORS, 1);
    if (i > MAXNUMCOLORS)
      ERREXIT1(cinfo, JERR_QUANT_MANY_COLORS, MAXNUMCOLORS);

    if (cinfo->dither_mode == JDITHER_FS) {
      size_t arraysize = (size_t) ((cinfo->output_width + 2) *
                   (3 * SIZEOF(FSERROR)));
      /* Allocate Floyd-Steinberg workspace if we didn't already. */
      if (cquantize->fserrors == NULL)
    cquantize->fserrors = (FSERRPTR) (*cinfo->mem->alloc_large)
      ((j_common_ptr) cinfo, JPOOL_IMAGE, arraysize);
      /* Initialize the propagated errors to zero. */
      jzero_far((void FAR *) cquantize->fserrors, arraysize);
      /* Make the error-limit table if we didn't already. */
      if (cquantize->error_limiter == NULL)
    init_error_limit(cinfo);
      cquantize->on_odd_row = FALSE;
    }

  }
  /* Zero the histogram or inverse color map, if necessary */
  if (cquantize->needs_zeroed) {
    for (i = 0; i < HIST_C0_ELEMS; i++) {
      jzero_far((void FAR *) histogram[i],
        HIST_C1_ELEMS*HIST_C2_ELEMS * SIZEOF(histcell));
    }
    cquantize->needs_zeroed = FALSE;
  }
}

static void new_color_map_2_quant ( j_decompress_ptr  cinfo )

static

{
  my_cquantize_ptr cquantize = (my_cquantize_ptr) cinfo->cquantize;

  /* Reset the inverse color map */
  cquantize->needs_zeroed = TRUE;
}

void jinit_2pass_quantizer

(

j_decompress_ptr

cinfo

)

{
  my_cquantize_ptr cquantize;
  int i;

  cquantize = (my_cquantize_ptr)
    (*cinfo->mem->alloc_small) ((j_common_ptr) cinfo, JPOOL_IMAGE,
                SIZEOF(my_cquantizer));
  cinfo->cquantize = (struct jpeg_color_quantizer *) cquantize;
  cquantize->pub.start_pass = start_pass_2_quant;
  cquantize->pub.new_color_map = new_color_map_2_quant;
  cquantize->fserrors = NULL;   /* flag optional arrays not allocated */
  cquantize->error_limiter = NULL;

  /* Make sure jdmaster didn't give me a case I can't handle */
  if (cinfo->out_color_components != 3)
    ERREXIT(cinfo, JERR_NOTIMPL);

  /* Allocate the histogram/inverse colormap storage */
  cquantize->histogram = (hist3d) (*cinfo->mem->alloc_small)
    ((j_common_ptr) cinfo, JPOOL_IMAGE, HIST_C0_ELEMS * SIZEOF(hist2d));
  for (i = 0; i < HIST_C0_ELEMS; i++) {
    cquantize->histogram[i] = (hist2d) (*cinfo->mem->alloc_large)
      ((j_common_ptr) cinfo, JPOOL_IMAGE,
       HIST_C1_ELEMS*HIST_C2_ELEMS * SIZEOF(histcell));
  }
  cquantize->needs_zeroed = TRUE; /* histogram is garbage now */

  /* Allocate storage for the completed colormap, if required.
   * We do this now since it is FAR storage and may affect
   * the memory manager's space calculations.
   */
  if (cinfo->enable_2pass_quant) {
    /* Make sure color count is acceptable */
    int desired = cinfo->desired_number_of_colors;
    /* Lower bound on # of colors ... somewhat arbitrary as long as > 0 */
    if (desired < 8)
      ERREXIT1(cinfo, JERR_QUANT_FEW_COLORS, 8);
    /* Make sure colormap indexes can be represented by JSAMPLEs */
    if (desired > MAXNUMCOLORS)
      ERREXIT1(cinfo, JERR_QUANT_MANY_COLORS, MAXNUMCOLORS);
    cquantize->sv_colormap = (*cinfo->mem->alloc_sarray)
      ((j_common_ptr) cinfo,JPOOL_IMAGE, (JDIMENSION) desired, (JDIMENSION) 3);
    cquantize->desired = desired;
  } else
    cquantize->sv_colormap = NULL;

  /* Only F-S dithering or no dithering is supported. */
  /* If user asks for ordered dither, give him F-S. */
  if (cinfo->dither_mode != JDITHER_NONE)
    cinfo->dither_mode = JDITHER_FS;

  /* Allocate Floyd-Steinberg workspace if necessary.
   * This isn't really needed until pass 2, but again it is FAR storage.
   * Although we will cope with a later change in dither_mode,
   * we do not promise to honor max_memory_to_use if dither_mode changes.
   */
  if (cinfo->dither_mode == JDITHER_FS) {
    cquantize->fserrors = (FSERRPTR) (*cinfo->mem->alloc_large)
      ((j_common_ptr) cinfo, JPOOL_IMAGE,
       (size_t) ((cinfo->output_width + 2) * (3 * SIZEOF(FSERROR))));
    /* Might as well create the error-limiting table too. */
    init_error_limit(cinfo);
  }
}