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src/Source/LibJPEG/jquant1.c  view on Meta::CPAN

 * 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;
  static const int RGB_order[3] = { RGB_GREEN, RGB_RED, RGB_BLUE };

  /* 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;