jquant1.c

EVM板JPEG实现,Texas Instruments TMS320C54x EVM JPEG

/*
 * jquant1.c
 *
 * Copyright (C) 1991, 1992, 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 1-pass color quantization (color mapping) routines.
 * These routines are invoked via the methods color_quantize
 * and color_quant_init/term.
 */

#include "jinclude.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.
 *
 * This implementation quantizes in the output colorspace.  This has a couple
 * of disadvantages: each pixel must be individually color-converted, and if
 * the color conversion includes gamma correction then quantization is done in
 * a nonlinear space, which is less desirable.  The major advantage is that
 * with the usual output color spaces (RGB, grayscale) an orthogonal grid of
 * representative colors can be used, thus permitting the very simple and fast
 * color lookup scheme used here.  The standard JPEG colorspace (YCbCr) cannot
 * be effectively handled this way, because only about a quarter of an
 * orthogonal grid would fall within the gamut of realizable colors.  Another
 * advantage is that when the user wants quantized grayscale output from a
 * color JPEG file, this quantizer can provide a high-quality result with no
 * special hacking.
 *
 * The gamma-correction problem could be eliminated by adjusting the grid
 * spacing to counteract the gamma correction applied by color_convert.
 * At this writing, gamma correction is not implemented by jdcolor, so
 * nothing is done here.
 *
 * 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.
 */


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

static JSAMPARRAY colormap;	/* The actual color map */
/* colormap[i][j] = value of i'th color component for output pixel value j */

static 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.
 */

static JSAMPARRAY input_buffer;	/* color conversion workspace */
/* Since our input data is presented in the JPEG colorspace, we have to call
 * color_convert to get it into the output colorspace.  input_buffer is a
 * one-row-high workspace for the result of color_convert.
 */


/* Declarations for Floyd-Steinberg dithering.
 *
 * Errors are accumulated into the arrays evenrowerrs[] and oddrowerrs[].
 * These have resolutions of 1/16th of a pixel count.  The error at a given
 * pixel is propagated to its unprocessed 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.
 *
 * In each of the xxxrowerrs[] arrays, indexing is [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.
 * In evenrowerrs[], the entries for a component are stored left-to-right, but
 * in oddrowerrs[] they are stored right-to-left.  This means we always
 * process the current row's error entries in increasing order and the next
 * row's error entries in decreasing order, regardless of whether we are
 * working L-to-R or R-to-L in the pixel data!
 *
 * Note: on a wide image, we might not have enough room in a PC's near data
 * segment to hold the error arrays; so they are allocated with alloc_medium.
 */

#ifdef EIGHT_BIT_SAMPLES
typedef INT16 FSERROR;		/* 16 bits should be enough */
#else
typedef INT32 FSERROR;		/* may need more than 16 bits? */
#endif

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

static FSERRPTR evenrowerrs[MAX_COMPONENTS]; /* errors for even rows */
static FSERRPTR oddrowerrs[MAX_COMPONENTS];  /* errors for odd rows */
static boolean on_odd_row;	/* flag to remember which row we are on */


/*
 * Policy-making subroutines for color_quant_init: 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 (decompress_info_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->color_out_comps; /* number of color components */
  int max_colors = cinfo->desired_number_of_colors;
  int total_colors, iroot, i;
  long temp;
  boolean changed;

  /* 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->emethods, "Cannot quantize to fewer than %d colors",
        (int) temp); */
      send_command(ERR11);
      receive_command();
      exit();
  }

  if (cinfo->out_color_space == CS_RGB && nc == 3) {
    /* We provide a special policy for quantizing in RGB space.
     * If 256 colors are requested, we allocate 8 red, 8 green, 4 blue levels;
     * this corresponds to the common 3/3/2-bit scheme.  For other totals,
     * the counts are set so that the number of colors allocated to each
     * component are roughly in the proportion R 3, G 4, B 2.
     * For low color counts, it's easier to hardwire the optimal choices
     * than try to tweak the algorithm to generate them.
     */
    if (max_colors == 256) {
      Ncolors[0] = 8;  Ncolors[1] = 8;  Ncolors[2] = 4;
      return 256;
    }
    if (max_colors < 12) {
      /* Fixed mapping for 8 colors */
      Ncolors[0] = Ncolors[1] = Ncolors[2] = 2;
    } else if (max_colors < 18) {
      /* Fixed mapping for 12 colors */
      Ncolors[0] = 2;  Ncolors[1] = 3;  Ncolors[2] = 2;
    } else if (max_colors < 24) {
      /* Fixed mapping for 18 colors */
      Ncolors[0] = 3;  Ncolors[1] = 3;  Ncolors[2] = 2;
    } else if (max_colors < 27) {
      /* Fixed mapping for 24 colors */
      Ncolors[0] = 3;  Ncolors[1] = 4;  Ncolors[2] = 2;
    } else if (max_colors < 36) {
      /* Fixed mapping for 27 colors */
      Ncolors[0] = 3;  Ncolors[1] = 3;  Ncolors[2] = 3;
    } else {
      /* these weights are readily derived with a little algebra */
      Ncolors[0] = (iroot * 266) >> 8; /* R weight is 1.0400 */
      Ncolors[1] = (iroot * 355) >> 8; /* G weight is 1.3867 */
      Ncolors[2] = (iroot * 177) >> 8; /* B weight is 0.6934 */
    }
    total_colors = Ncolors[0] * Ncolors[1] * Ncolors[2];
    /* The above computation produces "floor" values, so we may be able to
     * increment the count for one or more components without exceeding
     * max_colors.  We try in the order B, G, R.
     */
    do {
      changed = FALSE;
      for (i = 2; i >= 0; i--) {
	/* calculate new total_colors if Ncolors[i] is incremented */
	temp = total_colors / Ncolors[i];
	temp *= Ncolors[i]+1;	/* done in long arith to avoid oflo */
	if (temp <= (long) max_colors) {
	  Ncolors[i]++;		/* OK, apply the increment */
	  total_colors = (int) temp;
	  changed = TRUE;
	}
      }
    } while (changed);		/* loop until no increment is possible */
  } else {
    /* For any colorspace besides RGB, treat all the components equally. */

    /* 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.
     */
    for (i = 0; i < nc; i++) {
      /* calculate new total_colors if Ncolors[i] is incremented */
      temp = total_colors / Ncolors[i];
      temp *= Ncolors[i]+1;	/* done in long arith to avoid oflo */
      if (temp > (long) max_colors)
	break;			/* won't fit, done */
      Ncolors[i]++;		/* OK, apply the increment */
      total_colors = (int) temp;
    }
  }

  return total_colors;
}


LOCAL int
output_value (decompress_info_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 (decompress_info_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));
}


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

METHODDEF void
color_quant_init (decompress_info_ptr cinfo)
{
  int total_colors;		/* Number of distinct output colors */
  int Ncolors[MAX_COMPONENTS];	/* # of values alloced to each component */
  int i,j,k, nci, blksize, blkdist, ptr, val;

  /* Make sure my internal arrays won't overflow */
  if (cinfo->num_components > MAX_COMPONENTS ||
      cinfo->color_out_comps > MAX_COMPONENTS)
  {
/*    ERREXIT1(cinfo->emethods, "Cannot quantize more than %d color components",
        MAX_COMPONENTS); */
      send_command(ERR11);
      receive_command();
      exit();
  }
  /* Make sure colormap indexes can be represented by JSAMPLEs */
  if (cinfo->desired_number_of_colors > (MAXJSAMPLE+1))
  {
/*    ERREXIT1(cinfo->emethods, "Cannot request more than %d quantized colors",
        MAXJSAMPLE+1); */
      send_command(ERR11);
      receive_command();
      exit();
  }
  /* Select number of colors for each component */
  total_colors = select_ncolors(cinfo, Ncolors);

  /* Report selected color counts */
  if (cinfo->color_out_comps == 3)
/*    TRACEMS4(cinfo->emethods, 1, "Quantizing to %d = %d*%d*%d colors",
        total_colors, Ncolors[0], Ncolors[1], Ncolors[2]); */
      send_command(ERR5);
  else
/*    TRACEMS1(cinfo->emethods, 1, "Quantizing to %d colors", total_colors); */