jquant1.c

/*
 * jquant1.c
 *
 * This file was part of the Independent JPEG Group's software:
 * Copyright (C) 1991-1996, Thomas G. Lane.
 * libjpeg-turbo Modifications:
 * Copyright (C) 2009, D. R. Commander
 * For conditions of distribution and use, see the accompanying README file.
 *
 * This file contains 1-pass color quantization (color mapping) routines.
 * These routines provide mapping to a fixed color map using equally spaced
 * color values.  Optional Floyd-Steinberg or ordered dithering is available.
 */

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

#ifdef QUANT_1PASS_SUPPORTED


/*
 * The main purpose of 1-pass quantization is to provide a fast, if not very
 * high quality, colormapped output capability.  A 2-pass quantizer usually
 * gives better visual quality; however, for quantized grayscale output this
 * quantizer is perfectly adequate.  Dithering is highly recommended with this
 * quantizer, though you can turn it off if you really want to.
 *
 * In 1-pass quantization the colormap must be chosen in advance of seeing the
 * image.  We use a map consisting of all combinations of Ncolors[i] color
 * values for the i'th component.  The Ncolors[] values are chosen so that
 * their product, the total number of colors, is no more than that requested.
 * (In most cases, the product will be somewhat less.)
 *
 * Since the colormap is orthogonal, the representative value for each color
 * component can be determined without considering the other components;
 * then these indexes can be combined into a colormap index by a standard
 * N-dimensional-array-subscript calculation.  Most of the arithmetic involved
 * can be precalculated and stored in the lookup table colorindex[].
 * colorindex[i][j] maps pixel value j in component i to the nearest
 * representative value (grid plane) for that component; this index is
 * multiplied by the array stride for component i, so that the
 * index of the colormap entry closest to a given pixel value is just
 *    sum( colorindex[component-number][pixel-component-value] )
 * Aside from being fast, this scheme allows for variable spacing between
 * representative values with no additional lookup cost.
 *
 * If gamma correction has been applied in color conversion, it might be wise
 * to adjust the color grid spacing so that the representative colors are
 * equidistant in linear space.  At this writing, gamma correction is not
 * implemented by jdcolor, so nothing is done here.
 */


/* Declarations for ordered dithering.
 *
 * We use a standard 16x16 ordered dither array.  The basic concept of ordered
 * dithering is described in many references, for instance Dale Schumacher's
 * chapter II.2 of Graphics Gems II (James Arvo, ed. Academic Press, 1991).
 * In place of Schumacher's comparisons against a "threshold" value, we add a
 * "dither" value to the input pixel and then round the result to the nearest
 * output value.  The dither value is equivalent to (0.5 - threshold) times
 * the distance between output values.  For ordered dithering, we assume that
 * the output colors are equally spaced; if not, results will probably be
 * worse, since the dither may be too much or too little at a given point.
 *
 * The normal calculation would be to form pixel value + dither, range-limit
 * this to 0..MAXJSAMPLE, and then index into the colorindex table as usual.
 * We can skip the separate range-limiting step by extending the colorindex
 * table in both directions.
 */

#define ODITHER_SIZE  16        /* dimension of dither matrix */
/* NB: if ODITHER_SIZE is not a power of 2, ODITHER_MASK uses will break */
#define ODITHER_CELLS (ODITHER_SIZE*ODITHER_SIZE)       /* # cells in matrix */
#define ODITHER_MASK  (ODITHER_SIZE-1) /* mask for wrapping around counters */

typedef int ODITHER_MATRIX[ODITHER_SIZE][ODITHER_SIZE];
typedef int (*ODITHER_MATRIX_PTR)[ODITHER_SIZE];

static const UINT8 base_dither_matrix[ODITHER_SIZE][ODITHER_SIZE] = {
  /* Bayer's order-4 dither array.  Generated by the code given in
   * Stephen Hawley's article "Ordered Dithering" in Graphics Gems I.
   * The values in this array must range from 0 to ODITHER_CELLS-1.
   */
  {   0,192, 48,240, 12,204, 60,252,  3,195, 51,243, 15,207, 63,255 },
  { 128, 64,176,112,140, 76,188,124,131, 67,179,115,143, 79,191,127 },
  {  32,224, 16,208, 44,236, 28,220, 35,227, 19,211, 47,239, 31,223 },
  { 160, 96,144, 80,172,108,156, 92,163, 99,147, 83,175,111,159, 95 },
  {   8,200, 56,248,  4,196, 52,244, 11,203, 59,251,  7,199, 55,247 },
  { 136, 72,184,120,132, 68,180,116,139, 75,187,123,135, 71,183,119 },
  {  40,232, 24,216, 36,228, 20,212, 43,235, 27,219, 39,231, 23,215 },
  { 168,104,152, 88,164,100,148, 84,171,107,155, 91,167,103,151, 87 },
  {   2,194, 50,242, 14,206, 62,254,  1,193, 49,241, 13,205, 61,253 },
  { 130, 66,178,114,142, 78,190,126,129, 65,177,113,141, 77,189,125 },
  {  34,226, 18,210, 46,238, 30,222, 33,225, 17,209, 45,237, 29,221 },
  { 162, 98,146, 82,174,110,158, 94,161, 97,145, 81,173,109,157, 93 },
  {  10,202, 58,250,  6,198, 54,246,  9,201, 57,249,  5,197, 53,245 },
  { 138, 74,186,122,134, 70,182,118,137, 73,185,121,133, 69,181,117 },
  {  42,234, 26,218, 38,230, 22,214, 41,233, 25,217, 37,229, 21,213 },
  { 170,106,154, 90,166,102,150, 86,169,105,153, 89,165,101,149, 85 }
};


/* Declarations for Floyd-Steinberg dithering.
 *
 * Errors are accumulated into the array fserrors[], at a resolution of
 * 1/16th of a pixel count.  The error at a given pixel is propagated
 * to its not-yet-processed neighbors using the standard F-S fractions,
 *              ...     (here)  7/16
 *              3/16    5/16    1/16
 * We work left-to-right on even rows, right-to-left on odd rows.
 *
 * We can get away with a single array (holding one row's worth of errors)
 * by using it to store the current row's errors at pixel columns not yet
 * processed, but the next row's errors at columns already processed.  We
 * need only a few extra variables to hold the errors immediately around the
 * current column.  (If we are lucky, those variables are in registers, but
 * even if not, they're probably cheaper to access than array elements are.)
 *
 * The fserrors[] array is indexed [component#][position].
 * We provide (#columns + 2) entries per component; the extra entry at each
 * end saves us from special-casing the first and last pixels.
 *
 * Note: on a wide image, we might not have enough room in a PC's near data
 * segment to hold the error array; so it is allocated with alloc_large.
 */

#if BITS_IN_JSAMPLE == 8
typedef INT16 FSERROR;          /* 16 bits should be enough */
typedef int LOCFSERROR;         /* use 'int' for calculation temps */
#else
typedef INT32 FSERROR;          /* may need more than 16 bits */
typedef INT32 LOCFSERROR;       /* be sure calculation temps are big enough */
#endif

typedef FSERROR FAR *FSERRPTR;  /* pointer to error array (in FAR storage!) */


/* Private subobject */

#define MAX_Q_COMPS 4           /* max components I can handle */

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

  /* Initially allocated colormap is saved here */
  JSAMPARRAY sv_colormap;       /* The color map as a 2-D pixel array */
  int sv_actual;                /* number of entries in use */

  JSAMPARRAY colorindex;        /* Precomputed mapping for speed */
  /* colorindex[i][j] = index of color closest to pixel value j in component i,
   * premultiplied as described above.  Since colormap indexes must fit into
   * JSAMPLEs, the entries of this array will too.
   */
  boolean is_padded;            /* is the colorindex padded for odither? */

  int Ncolors[MAX_Q_COMPS];     /* # of values alloced to each component */

  /* Variables for ordered dithering */
  int row_index;                /* cur row's vertical index in dither matrix */
  ODITHER_MATRIX_PTR odither[MAX_Q_COMPS]; /* one dither array per component */

  /* Variables for Floyd-Steinberg dithering */
  FSERRPTR fserrors[MAX_Q_COMPS]; /* accumulated errors */
  boolean on_odd_row;           /* flag to remember which row we are on */
} my_cquantizer;

typedef my_cquantizer * my_cquantize_ptr;


/*
 * Policy-making subroutines for create_colormap and create_colorindex.
 * These routines determine the colormap to be used.  The rest of the module
 * only assumes that the colormap is orthogonal.
 *
 *  * select_ncolors decides how to divvy up the available colors
 *    among the components.
 *  * output_value defines the set of representative values for a component.
 *  * largest_input_value defines the mapping from input values to
 *    representative values for a component.
 * Note that the latter two routines may impose different policies for
 * different components, though this is not currently done.
 */


LOCAL(int)
select_ncolors (j_decompress_ptr cinfo, int Ncolors[])
/* Determine allocation of desired colors to components, */
/* and fill in Ncolors[] array to indicate choice. */
/* Return value is total number of colors (product of Ncolors[] values). */
{
  int nc = cinfo->out_color_components; /* number of color components */
  int max_colors = cinfo->desired_number_of_colors;
  int total_colors, iroot, i, j;
  boolean changed;
  long temp;
  int RGB_order[3] = { RGB_GREEN, RGB_RED, RGB_BLUE };
  RGB_order[0] = rgb_green[cinfo->out_color_space];
  RGB_order[1] = rgb_red[cinfo->out_color_space];
  RGB_order[2] = rgb_blue[cinfo->out_color_space];

  /* We can allocate at least the nc'th root of max_colors per component. */
  /* Compute floor(nc'th root of max_colors). */
  iroot = 1;
  do {
    iroot++;
    temp = iroot;               /* set temp = iroot ** nc */
    for (i = 1; i < nc; i++)
      temp *= iroot;
  } while (temp <= (long) max_colors); /* repeat till iroot exceeds root */
  iroot--;                      /* now iroot = floor(root) */

  /* Must have at least 2 color values per component */
  if (iroot < 2)
    ERREXIT1(cinfo, JERR_QUANT_FEW_COLORS, (int) temp);

  /* Initialize to iroot color values for each component */
  total_colors = 1;
  for (i = 0; i < nc; i++) {
    Ncolors[i] = iroot;
    total_colors *= iroot;
  }
  /* We may be able to increment the count for one or more components without
   * exceeding max_colors, though we know not all can be incremented.
   * Sometimes, the first component can be incremented more than once!
   * (Example: for 16 colors, we start at 2*2*2, go to 3*2*2, then 4*2*2.)
   * In RGB colorspace, try to increment G first, then R, then B.
   */
  do {
    changed = FALSE;
    for (i = 0; i < nc; i++) {
      j = (cinfo->out_color_space == JCS_RGB ? RGB_order[i] : i);
      /* calculate new total_colors if Ncolors[j] is incremented */
      temp = total_colors / Ncolors[j];
      temp *= Ncolors[j]+1;     /* done in long arith to avoid oflo */
      if (temp > (long) max_colors)
        break;                  /* won't fit, done with this pass */
      Ncolors[j]++;             /* OK, apply the increment */
      total_colors = (int) temp;
      changed = TRUE;
    }
  } while (changed);

  return total_colors;
}


LOCAL(int)
output_value (j_decompress_ptr cinfo, int ci, int j, int maxj)
/* Return j'th output value, where j will range from 0 to maxj */
/* The output values must fall in 0..MAXJSAMPLE in increasing order */
{
  /* We always provide values 0 and MAXJSAMPLE for each component;
   * any additional values are equally spaced between these limits.
   * (Forcing the upper and lower values to the limits ensures that
   * dithering can't produce a color outside the selected gamut.)
   */
  return (int) (((INT32) j * MAXJSAMPLE + maxj/2) / maxj);
}


LOCAL(int)
largest_input_value (j_decompress_ptr cinfo, int ci, int j, int maxj)
/* Return largest input value that should map to j'th output value */
/* Must have largest(j=0) >= 0, and largest(j=maxj) >= MAXJSAMPLE */
{
  /* Breakpoints are halfway between values returned by output_value */
  return (int) (((INT32) (2*j + 1) * MAXJSAMPLE + maxj) / (2*maxj));
}


/*
 * Create the colormap.
 */

LOCAL(void)
create_colormap (j_decompress_ptr cinfo)
{
  my_cquantize_ptr cquantize = (my_cquantize_ptr) cinfo->cquantize;
  JSAMPARRAY colormap;          /* Created colormap */
  int total_colors;             /* Number of distinct output colors */
  int i,j,k, nci, blksize, blkdist, ptr, val;

  /* Select number of colors for each component */
  total_colors = select_ncolors(cinfo, cquantize->Ncolors);

  /* Report selected color counts */
  if (cinfo->out_color_components == 3)
    TRACEMS4(cinfo, 1, JTRC_QUANT_3_NCOLORS,
             total_colors, cquantize->Ncolors[0],
             cquantize->Ncolors[1], cquantize->Ncolors[2]);
  else
    TRACEMS1(cinfo, 1, JTRC_QUANT_NCOLORS, total_colors);

  /* Allocate and fill in the colormap. */
  /* The colors are ordered in the map in standard row-major order, */
  /* i.e. rightmost (highest-indexed) color changes most rapidly. */

  colormap = (*cinfo->mem->alloc_sarray)
    ((j_common_ptr) cinfo, JPOOL_IMAGE,
     (JDIMENSION) total_colors, (JDIMENSION) cinfo->out_color_components);

  /* blksize is number of adjacent repeated entries for a component */
  /* blkdist is distance between groups of identical entries for a component */
  blkdist = total_colors;

  for (i = 0; i < cinfo->out_color_components; i++) {
    /* fill in colormap entries for i'th color component */
    nci = cquantize->Ncolors[i]; /* # of distinct values for this color */
    blksize = blkdist / nci;
    for (j = 0; j < nci; j++) {
      /* Compute j'th output value (out of nci) for component */
      val = output_value(cinfo, i, j, nci-1);
      /* Fill in all colormap entries that have this value of this component */
      for (ptr = j * blksize; ptr < total_colors; ptr += blkdist) {
        /* fill in blksize entries beginning at ptr */
        for (k = 0; k < blksize; k++)
          colormap[i][ptr+k] = (JSAMPLE) val;
      }
    }
    blkdist = blksize;          /* blksize of this color is blkdist of next */
  }

  /* Save the colormap in private storage,
   * where it will survive color quantization mode changes.
   */
  cquantize->sv_colormap = colormap;
  cquantize->sv_actual = total_colors;
}


/*
 * Create the color index table.
 */

LOCAL(void)
create_colorindex (j_decompress_ptr cinfo)
{
  my_cquantize_ptr cquantize = (my_cquantize_ptr) cinfo->cquantize;
  JSAMPROW indexptr;
  int i,j,k, nci, blksize, val, pad;

  /* For ordered dither, we pad the color index tables by MAXJSAMPLE in
   * each direction (input index values can be -MAXJSAMPLE .. 2*MAXJSAMPLE).
   * This is not necessary in the other dithering modes.  However, we
   * flag whether it was done in case user changes dithering mode.
   */
  if (cinfo->dither_mode == JDITHER_ORDERED) {
    pad = MAXJSAMPLE*2;
    cquantize->is_padded = TRUE;
  } else {
    pad = 0;
    cquantize->is_padded = FALSE;
  }

  cquantize->colorindex = (*cinfo->mem->alloc_sarray)
    ((j_common_ptr) cinfo, JPOOL_IMAGE,
     (JDIMENSION) (MAXJSAMPLE+1 + pad),
     (JDIMENSION) cinfo->out_color_components);

  /* blksize is number of adjacent repeated entries for a component */
  blksize = cquantize->sv_actual;

  for (i = 0; i < cinfo->out_color_components; i++) {
    /* fill in colorindex entries for i'th color component */
    nci = cquantize->Ncolors[i]; /* # of distinct values for this color */
    blksize = blksize / nci;

    /* adjust colorindex pointers to provide padding at negative indexes. */
    if (pad)
      cquantize->colorindex[i] += MAXJSAMPLE;

    /* in loop, val = index of current output value, */
    /* and k = largest j that maps to current val */
    indexptr = cquantize->colorindex[i];
    val = 0;
    k = largest_input_value(cinfo, i, 0, nci-1);
    for (j = 0; j <= MAXJSAMPLE; j++) {
      while (j > k)             /* advance val if past boundary */
        k = largest_input_value(cinfo, i, ++val, nci-1);
      /* premultiply so that no multiplication needed in main processing */
      indexptr[j] = (JSAMPLE) (val * blksize);
    }
    /* Pad at both ends if necessary */
    if (pad)
      for (j = 1; j <= MAXJSAMPLE; j++) {
        indexptr[-j] = indexptr[0];
        indexptr[MAXJSAMPLE+j] = indexptr[MAXJSAMPLE];
      }
  }
}


/*
 * Create an ordered-dither array for a component having ncolors
 * distinct output values.
 */

LOCAL(ODITHER_MATRIX_PTR)
make_odither_array (j_decompress_ptr cinfo, int ncolors)
{
  ODITHER_MATRIX_PTR odither;
  int j,k;
  INT32 num,den;

  odither = (ODITHER_MATRIX_PTR)
    (*cinfo->mem->alloc_small) ((j_common_ptr) cinfo, JPOOL_IMAGE,
                                SIZEOF(ODITHER_MATRIX));
  /* The inter-value distance for this color is MAXJSAMPLE/(ncolors-1).
   * Hence the dither value for the matrix cell with fill order f
   * (f=0..N-1) should be (N-1-2*f)/(2*N) * MAXJSAMPLE/(ncolors-1).
   * On 16-bit-int machine, be careful to avoid overflow.
   */
  den = 2 * ODITHER_CELLS * ((INT32) (ncolors - 1));
  for (j = 0; j < ODITHER_SIZE; j++) {
    for (k = 0; k < ODITHER_SIZE; k++) {
      num = ((INT32) (ODITHER_CELLS-1 - 2*((int)base_dither_matrix[j][k])))
            * MAXJSAMPLE;
      /* Ensure round towards zero despite C's lack of consistency
       * about rounding negative values in integer division...
       */
      odither[j][k] = (int) (num<0 ? -((-num)/den) : num/den);
    }
  }
  return odither;
}


/*
 * Create the ordered-dither tables.
 * Components having the same number of representative colors may
 * share a dither table.
 */

LOCAL(void)
create_odither_tables (j_decompress_ptr cinfo)
{
  my_cquantize_ptr cquantize = (my_cquantize_ptr) cinfo->cquantize;
  ODITHER_MATRIX_PTR odither;
  int i, j, nci;

  for (i = 0; i < cinfo->out_color_components; i++) {
    nci = cquantize->Ncolors[i]; /* # of distinct values for this color */
    odither = NULL;             /* search for matching prior component */
    for (j = 0; j < i; j++) {
      if (nci == cquantize->Ncolors[j]) {
        odither = cquantize->odither[j];
        break;
      }
    }
    if (odither == NULL)        /* need a new table? */
      odither = make_odither_array(cinfo, nci);
    cquantize->odither[i] = odither;
  }
}


/*
 * Map some rows of pixels to the output colormapped representation.
 */

METHODDEF(void)
color_quantize (j_decompress_ptr cinfo, JSAMPARRAY input_buf,
                JSAMPARRAY output_buf, int num_rows)
/* General case, no dithering */
{
  my_cquantize_ptr cquantize = (my_cquantize_ptr) cinfo->cquantize;
  JSAMPARRAY colorindex = cquantize->colorindex;
  register int pixcode, ci;
  register JSAMPROW ptrin, ptrout;
  int row;
  JDIMENSION col;
  JDIMENSION width = cinfo->output_width;
  register int nc = cinfo->out_color_components;

  for (row = 0; row < num_rows; row++) {
    ptrin = input_buf[row];
    ptrout = output_buf[row];
    for (col = width; col > 0; col--) {
      pixcode = 0;
      for (ci = 0; ci < nc; ci++) {
        pixcode += GETJSAMPLE(colorindex[ci][GETJSAMPLE(*ptrin++)]);
      }
      *ptrout++ = (JSAMPLE) pixcode;
    }
  }
}


METHODDEF(void)
color_quantize3 (j_decompress_ptr cinfo, JSAMPARRAY input_buf,
                 JSAMPARRAY output_buf, int num_rows)
/* Fast path for out_color_components==3, no dithering */
{
  my_cquantize_ptr cquantize = (my_cquantize_ptr) cinfo->cquantize;
  register int pixcode;
  register JSAMPROW ptrin, ptrout;
  JSAMPROW colorindex0 = cquantize->colorindex[0];
  JSAMPROW colorindex1 = cquantize->colorindex[1];
  JSAMPROW colorindex2 = cquantize->colorindex[2];
  int row;
  JDIMENSION col;
  JDIMENSION width = cinfo->output_width;

  for (row = 0; row < num_rows; row++) {
    ptrin = input_buf[row];
    ptrout = output_buf[row];
    for (col = width; col > 0; col--) {
      pixcode  = GETJSAMPLE(colorindex0[GETJSAMPLE(*ptrin++)]);
      pixcode += GETJSAMPLE(colorindex1[GETJSAMPLE(*ptrin++)]);
      pixcode += GETJSAMPLE(colorindex2[GETJSAMPLE(*ptrin++)]);
      *ptrout++ = (JSAMPLE) pixcode;
    }
  }
}


METHODDEF(void)
quantize_ord_dither (j_decompress_ptr cinfo, JSAMPARRAY input_buf,
                     JSAMPARRAY output_buf, int num_rows)
/* General case, with ordered dithering */
{
  my_cquantize_ptr cquantize = (my_cquantize_ptr) cinfo->cquantize;
  register JSAMPROW input_ptr;
  register JSAMPROW output_ptr;
  JSAMPROW colorindex_ci;
  int * dither;                 /* points to active row of dither matrix */
  int row_index, col_index;     /* current indexes into dither matrix */
  int nc = cinfo->out_color_components;
  int ci;
  int row;
  JDIMENSION col;
  JDIMENSION width = cinfo->output_width;

  for (row = 0; row < num_rows; row++) {
    /* Initialize output values to 0 so can process components separately */
    jzero_far((void FAR *) output_buf[row],
              (size_t) (width * SIZEOF(JSAMPLE)));
    row_index = cquantize->row_index;
    for (ci = 0; ci < nc; ci++) {
      input_ptr = input_buf[row] + ci;
      output_ptr = output_buf[row];
      colorindex_ci = cquantize->colorindex[ci];
      dither = cquantize->odither[ci][row_index];
      col_index = 0;

      for (col = width; col > 0; col--) {
        /* Form pixel value + dither, range-limit to 0..MAXJSAMPLE,
         * select output value, accumulate into output code for this pixel.
         * Range-limiting need not be done explicitly, as we have extended
         * the colorindex table to produce the right answers for out-of-range
         * inputs.  The maximum dither is +- MAXJSAMPLE; this sets the
         * required amount of padding.
         */
        *output_ptr += colorindex_ci[GETJSAMPLE(*input_ptr)+dither[col_index]];
        input_ptr += nc;
        output_ptr++;
        col_index = (col_index + 1) & ODITHER_MASK;
      }
    }
    /* Advance row index for next row */
    row_index = (row_index + 1) & ODITHER_MASK;
    cquantize->row_index = row_index;
  }
}


METHODDEF(void)
quantize3_ord_dither (j_decompress_ptr cinfo, JSAMPARRAY input_buf,
                      JSAMPARRAY output_buf, int num_rows)
/* Fast path for out_color_components==3, with ordered dithering */
{
  my_cquantize_ptr cquantize = (my_cquantize_ptr) cinfo->cquantize;
  register int pixcode;
  register JSAMPROW input_ptr;
  register JSAMPROW output_ptr;
  JSAMPROW colorindex0 = cquantize->colorindex[0];
  JSAMPROW colorindex1 = cquantize->colorindex[1];
  JSAMPROW colorindex2 = cquantize->colorindex[2];
  int * dither0;                /* points to active row of dither matrix */
  int * dither1;
  int * dither2;
  int row_index, col_index;     /* current indexes into dither matrix */
  int row;
  JDIMENSION col;
  JDIMENSION width = cinfo->output_width;

  for (row = 0; row < num_rows; row++) {
    row_index = cquantize->row_index;
    input_ptr = input_buf[row];
    output_ptr = output_buf[row];
    dither0 = cquantize->odither[0][row_index];
    dither1 = cquantize->odither[1][row_index];
    dither2 = cquantize->odither[2][row_index];
    col_index = 0;

    for (col = width; col > 0; col--) {
      pixcode  = GETJSAMPLE(colorindex0[GETJSAMPLE(*input_ptr++) +
                                        dither0[col_index]]);
      pixcode += GETJSAMPLE(colorindex1[GETJSAMPLE(*input_ptr++) +
                                        dither1[col_index]]);
      pixcode += GETJSAMPLE(colorindex2[GETJSAMPLE(*input_ptr++) +
                                        dither2[col_index]]);
      *output_ptr++ = (JSAMPLE) pixcode;
      col_index = (col_index + 1) & ODITHER_MASK;
    }
    row_index = (row_index + 1) & ODITHER_MASK;
    cquantize->row_index = row_index;
  }
}


METHODDEF(void)
quantize_fs_dither (j_decompress_ptr cinfo, JSAMPARRAY input_buf,
                    JSAMPARRAY output_buf, int num_rows)
/* General case, with Floyd-Steinberg dithering */
{
  my_cquantize_ptr cquantize = (my_cquantize_ptr) cinfo->cquantize;
  register LOCFSERROR cur;      /* current error or pixel value */
  LOCFSERROR belowerr;          /* error for pixel below cur */
  LOCFSERROR bpreverr;          /* error for below/prev col */
  LOCFSERROR bnexterr;          /* error for below/next col */
  LOCFSERROR delta;
  register FSERRPTR errorptr;   /* => fserrors[] at column before current */
  register JSAMPROW input_ptr;
  register JSAMPROW output_ptr;
  JSAMPROW colorindex_ci;
  JSAMPROW colormap_ci;
  int pixcode;
  int nc = cinfo->out_color_components;
  int dir;                      /* 1 for left-to-right, -1 for right-to-left */
  int dirnc;                    /* dir * nc */
  int ci;
  int row;
  JDIMENSION col;
  JDIMENSION width = cinfo->output_width;
  JSAMPLE *range_limit = cinfo->sample_range_limit;
  SHIFT_TEMPS

  for (row = 0; row < num_rows; row++) {
    /* Initialize output values to 0 so can process components separately */
    jzero_far((void FAR *) output_buf[row],
              (size_t) (width * SIZEOF(JSAMPLE)));
    for (ci = 0; ci < nc; ci++) {
      input_ptr = input_buf[row] + ci;
      output_ptr = output_buf[row];
      if (cquantize->on_odd_row) {
        /* work right to left in this row */
        input_ptr += (width-1) * nc; /* so point to rightmost pixel */
        output_ptr += width-1;
        dir = -1;
        dirnc = -nc;
        errorptr = cquantize->fserrors[ci] + (width+1); /* => entry after last column */
      } else {
        /* work left to right in this row */
        dir = 1;
        dirnc = nc;
        errorptr = cquantize->fserrors[ci]; /* => entry before first column */
      }
      colorindex_ci = cquantize->colorindex[ci];
      colormap_ci = cquantize->sv_colormap[ci];
      /* Preset error values: no error propagated to first pixel from left */
      cur = 0;
      /* and no error propagated to row below yet */
      belowerr = bpreverr = 0;

      for (col = width; col > 0; col--) {
        /* cur 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.
         */
        cur = RIGHT_SHIFT(cur + errorptr[dir] + 8, 4);
        /* Form pixel value + error, and range-limit to 0..MAXJSAMPLE.
         * The maximum error is +- MAXJSAMPLE; this sets the required size
         * of the range_limit array.
         */
        cur += GETJSAMPLE(*input_ptr);
        cur = GETJSAMPLE(range_limit[cur]);
        /* Select output value, accumulate into output code for this pixel */
        pixcode = GETJSAMPLE(colorindex_ci[cur]);
        *output_ptr += (JSAMPLE) pixcode;
        /* Compute actual representation error at this pixel */
        /* Note: we can do this even though we don't have the final */
        /* pixel code, because the colormap is orthogonal. */
        cur -= GETJSAMPLE(colormap_ci[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.
         */
        bnexterr = cur;
        delta = cur * 2;
        cur += delta;           /* form error * 3 */
        errorptr[0] = (FSERROR) (bpreverr + cur);
        cur += delta;           /* form error * 5 */
        bpreverr = belowerr + cur;
        belowerr = bnexterr;
        cur += delta;           /* form error * 7 */
        /* At this point cur 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.
         */
        input_ptr += dirnc;     /* advance input ptr to next column */
        output_ptr += dir;      /* advance output ptr to next column */
        errorptr += dir;        /* advance errorptr to current column */
      }
      /* Post-loop cleanup: we must unload the final error value into the
       * final fserrors[] entry.  Note we need not unload belowerr because
       * it is for the dummy column before or after the actual array.
       */
      errorptr[0] = (FSERROR) bpreverr; /* unload prev err into array */
    }
    cquantize->on_odd_row = (cquantize->on_odd_row ? FALSE : TRUE);
  }
}


/*
 * Allocate workspace for Floyd-Steinberg errors.
 */

LOCAL(void)
alloc_fs_workspace (j_decompress_ptr cinfo)
{
  my_cquantize_ptr cquantize = (my_cquantize_ptr) cinfo->cquantize;
  size_t arraysize;
  int i;

  arraysize = (size_t) ((cinfo->output_width + 2) * SIZEOF(FSERROR));
  for (i = 0; i < cinfo->out_color_components; i++) {
    cquantize->fserrors[i] = (FSERRPTR)
      (*cinfo->mem->alloc_large)((j_common_ptr) cinfo, JPOOL_IMAGE, arraysize);
  }
}


/*
 * Initialize for one-pass color quantization.
 */

METHODDEF(void)
start_pass_1_quant (j_decompress_ptr cinfo, boolean is_pre_scan)
{
  my_cquantize_ptr cquantize = (my_cquantize_ptr) cinfo->cquantize;
  size_t arraysize;
  int i;

  /* Install my colormap. */
  cinfo->colormap = cquantize->sv_colormap;
  cinfo->actual_number_of_colors = cquantize->sv_actual;

  /* Initialize for desired dithering mode. */
  switch (cinfo->dither_mode) {
  case JDITHER_NONE:
    if (cinfo->out_color_components == 3)
      cquantize->pub.color_quantize = color_quantize3;
    else
      cquantize->pub.color_quantize = color_quantize;
    break;
  case JDITHER_ORDERED:
    if (cinfo->out_color_components == 3)
      cquantize->pub.color_quantize = quantize3_ord_dither;
    else
      cquantize->pub.color_quantize = quantize_ord_dither;
    cquantize->row_index = 0;   /* initialize state for ordered dither */
    /* If user changed to ordered dither from another mode,
     * we must recreate the color index table with padding.
     * This will cost extra space, but probably isn't very likely.
     */
    if (! cquantize->is_padded)
      create_colorindex(cinfo);
    /* Create ordered-dither tables if we didn't already. */
    if (cquantize->odither[0] == NULL)
      create_odither_tables(cinfo);
    break;
  case JDITHER_FS:
    cquantize->pub.color_quantize = quantize_fs_dither;
    cquantize->on_odd_row = FALSE; /* initialize state for F-S dither */
    /* Allocate Floyd-Steinberg workspace if didn't already. */
    if (cquantize->fserrors[0] == NULL)
      alloc_fs_workspace(cinfo);
    /* Initialize the propagated errors to zero. */
    arraysize = (size_t) ((cinfo->output_width + 2) * SIZEOF(FSERROR));
    for (i = 0; i < cinfo->out_color_components; i++)
      jzero_far((void FAR *) cquantize->fserrors[i], arraysize);
    break;
  default:
    ERREXIT(cinfo, JERR_NOT_COMPILED);
    break;
  }
}


/*
 * Finish up at the end of the pass.
 */

METHODDEF(void)
finish_pass_1_quant (j_decompress_ptr cinfo)
{
  /* no work in 1-pass case */
}


/*
 * Switch to a new external colormap between output passes.
 * Shouldn't get to this module!
 */

METHODDEF(void)
new_color_map_1_quant (j_decompress_ptr cinfo)
{
  ERREXIT(cinfo, JERR_MODE_CHANGE);
}


/*
 * Module initialization routine for 1-pass color quantization.
 */

GLOBAL(void)
jinit_1pass_quantizer (j_decompress_ptr cinfo)
{
  my_cquantize_ptr cquantize;

  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_1_quant;
  cquantize->pub.finish_pass = finish_pass_1_quant;
  cquantize->pub.new_color_map = new_color_map_1_quant;
  cquantize->fserrors[0] = NULL; /* Flag FS workspace not allocated */
  cquantize->odither[0] = NULL; /* Also flag odither arrays not allocated */

  /* Make sure my internal arrays won't overflow */
  if (cinfo->out_color_components > MAX_Q_COMPS)
    ERREXIT1(cinfo, JERR_QUANT_COMPONENTS, MAX_Q_COMPS);
  /* Make sure colormap indexes can be represented by JSAMPLEs */
  if (cinfo->desired_number_of_colors > (MAXJSAMPLE+1))
    ERREXIT1(cinfo, JERR_QUANT_MANY_COLORS, MAXJSAMPLE+1);

  /* Create the colormap and color index table. */
  create_colormap(cinfo);
  create_colorindex(cinfo);

  /* Allocate Floyd-Steinberg workspace now if requested.
   * We do this now since it is FAR storage and may affect the memory
   * manager's space calculations.  If the user changes to FS dither
   * mode in a later pass, we will allocate the space then, and will
   * possibly overrun the max_memory_to_use setting.
   */
  if (cinfo->dither_mode == JDITHER_FS)
    alloc_fs_workspace(cinfo);
}

#endif /* QUANT_1PASS_SUPPORTED */