/* * jquant1.c * * Copyright (C) 1991-1996, 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 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 == 8typedef INT16 FSERROR;		/* 16 bits should be enough */typedef int LOCFSERROR;		/* use 'int' for calculation temps */#elsetypedef INT32 FSERROR;		/* may need more than 16 bits */typedef INT32 LOCFSERROR;	/* be sure calculation temps are big enough */#endiftypedef 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;      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],