gd_topal.c

Linux/Unix下的绘图函数库（Graphic Drawing Library）

/* TODO: oim and nim in the lower level functions;
  correct use of stub (sigh). */

/* 2.0.12: a new adaptation from the same original, this time
	by Barend Gehrels. My attempt to incorporate alpha channel
	into the result worked poorly and degraded the quality of
	palette conversion even when the source contained no
	alpha channel data. This version does not attempt to produce
	an output file with transparency in some of the palette
	indexes, which, in practice, doesn't look so hot anyway. TBB */

/*
 * gd_topal, adapted from jquant2.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 2-pass color quantization (color mapping) routines.
 * These routines provide selection of a custom color map for an image,
 * followed by mapping of the image to that color map, with optional
 * Floyd-Steinberg dithering.
 * It is also possible to use just the second pass to map to an arbitrary
 * externally-given color map.
 *
 * Note: ordered dithering is not supported, since there isn't any fast
 * way to compute intercolor distances; it's unclear that ordered dither's
 * fundamental assumptions even hold with an irregularly spaced color map.
 */

#ifdef ORIGINAL_LIB_JPEG

#define JPEG_INTERNALS

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

#else

/*
 * THOMAS BOUTELL & BAREND GEHRELS, february 2003
 * adapted the code to work within gd rather than within libjpeg.
 * If it is not working, it's not Thomas G. Lane's fault.
 */

/* 
  SETTING THIS ONE CAUSES STRIPED IMAGE
  to be done: solve this
  #define ORIGINAL_LIB_JPEG_REVERSE_ODD_ROWS
 */

#include <string.h>
#include "gd.h"
#include "gdhelpers.h"

/* (Re)define some defines known by libjpeg */
#define QUANT_2PASS_SUPPORTED

#define RGB_RED		0
#define RGB_GREEN	1
#define RGB_BLUE	2

#define JSAMPLE unsigned char
#define MAXJSAMPLE (gdMaxColors-1)
#define BITS_IN_JSAMPLE 8

#define JSAMPROW int*
#define JDIMENSION int

#define METHODDEF(type) static type
#define LOCAL(type)	static type


/* We assume that right shift corresponds to signed division by 2 with
 * rounding towards minus infinity.  This is correct for typical "arithmetic
 * shift" instructions that shift in copies of the sign bit.  But some
 * C compilers implement >> with an unsigned shift.  For these machines you
 * must define RIGHT_SHIFT_IS_UNSIGNED.
 * RIGHT_SHIFT provides a proper signed right shift of an INT32 quantity.
 * It is only applied with constant shift counts.  SHIFT_TEMPS must be
 * included in the variables of any routine using RIGHT_SHIFT.
 */

#ifdef RIGHT_SHIFT_IS_UNSIGNED
#define SHIFT_TEMPS	INT32 shift_temp;
#define RIGHT_SHIFT(x,shft)  \
	((shift_temp = (x)) < 0 ? \
	 (shift_temp >> (shft)) | ((~((INT32) 0)) << (32-(shft))) : \
	 (shift_temp >> (shft)))
#else
#define SHIFT_TEMPS
#define RIGHT_SHIFT(x,shft)	((x) >> (shft))
#endif


#define range_limit(x) { if(x<0) x=0; if (x>255) x=255; }


#ifndef INT16
#define INT16  short
#endif

#ifndef UINT16
#define UINT16 unsigned short
#endif

#ifndef INT32
#define INT32 int
#endif

#ifndef FAR
#define FAR
#endif



#ifndef boolean
#define boolean int
#endif

#ifndef TRUE
#define TRUE 1
#endif

#ifndef FALSE
#define FALSE 0
#endif


#define input_buf (oim->tpixels)
#define output_buf (nim->pixels)

#endif

#ifdef QUANT_2PASS_SUPPORTED


/*
 * This module implements the well-known Heckbert paradigm for color
 * quantization.  Most of the ideas used here can be traced back to
 * Heckbert's seminal paper
 *   Heckbert, Paul.  "Color Image Quantization for Frame Buffer Display",
 *   Proc. SIGGRAPH '82, Computer Graphics v.16 #3 (July 1982), pp 297-304.
 *
 * In the first pass over the image, we accumulate a histogram showing the
 * usage count of each possible color.  To keep the histogram to a reasonable
 * size, we reduce the precision of the input; typical practice is to retain
 * 5 or 6 bits per color, so that 8 or 4 different input values are counted
 * in the same histogram cell.
 *
 * Next, the color-selection step begins with a box representing the whole
 * color space, and repeatedly splits the "largest" remaining box until we
 * have as many boxes as desired colors.  Then the mean color in each
 * remaining box becomes one of the possible output colors.
 * 
 * The second pass over the image maps each input pixel to the closest output
 * color (optionally after applying a Floyd-Steinberg dithering correction).
 * This mapping is logically trivial, but making it go fast enough requires
 * considerable care.
 *
 * Heckbert-style quantizers vary a good deal in their policies for choosing
 * the "largest" box and deciding where to cut it.  The particular policies
 * used here have proved out well in experimental comparisons, but better ones
 * may yet be found.
 *
 * In earlier versions of the IJG code, this module quantized in YCbCr color
 * space, processing the raw upsampled data without a color conversion step.
 * This allowed the color conversion math to be done only once per colormap
 * entry, not once per pixel.  However, that optimization precluded other
 * useful optimizations (such as merging color conversion with upsampling)
 * and it also interfered with desired capabilities such as quantizing to an
 * externally-supplied colormap.  We have therefore abandoned that approach.
 * The present code works in the post-conversion color space, typically RGB.
 *
 * To improve the visual quality of the results, we actually work in scaled
 * RGB space, giving G distances more weight than R, and R in turn more than
 * B.  To do everything in integer math, we must use integer scale factors.
 * The 2/3/1 scale factors used here correspond loosely to the relative
 * weights of the colors in the NTSC grayscale equation.
 * If you want to use this code to quantize a non-RGB color space, you'll
 * probably need to change these scale factors.
 */

#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 */

/* Relabel R/G/B as components 0/1/2, respecting the RGB ordering defined
 * in jmorecfg.h.  As the code stands, it will do the right thing for R,G,B
 * and B,G,R orders.  If you define some other weird order in jmorecfg.h,
 * you'll get compile errors until you extend this logic.  In that case
 * you'll probably want to tweak the histogram sizes too.
 */

#if RGB_RED == 0
#define C0_SCALE R_SCALE
#endif
#if RGB_BLUE == 0
#define C0_SCALE B_SCALE
#endif
#if RGB_GREEN == 1
#define C1_SCALE G_SCALE
#endif
#if RGB_RED == 2
#define C2_SCALE R_SCALE
#endif
#if RGB_BLUE == 2
#define C2_SCALE B_SCALE
#endif


/*
 * First we have the histogram data structure and routines for creating it.
 *
 * The number of bits of precision can be adjusted by changing these symbols.
 * We recommend keeping 6 bits for G and 5 each for R and B.
 * If you have plenty of memory and cycles, 6 bits all around gives marginally
 * better results; if you are short of memory, 5 bits all around will save
 * some space but degrade the results.
 * To maintain a fully accurate histogram, we'd need to allocate a "long"
 * (preferably unsigned long) for each cell.  In practice this is overkill;
 * we can get by with 16 bits per cell.  Few of the cell counts will overflow,
 * and clamping those that do overflow to the maximum value will give close-
 * enough results.  This reduces the recommended histogram size from 256Kb
 * to 128Kb, which is a useful savings on PC-class machines.
 * (In the second pass the histogram space is re-used for pixel mapping data;
 * in that capacity, each cell must be able to store zero to the number of
 * desired colors.  16 bits/cell is plenty for that too.)
 * Since the JPEG code is intended to run in small memory model on 80x86
 * machines, we can't just allocate the histogram in one chunk.  Instead
 * of a true 3-D array, we use a row of pointers to 2-D arrays.  Each
 * pointer corresponds to a C0 value (typically 2^5 = 32 pointers) and
 * each 2-D array has 2^6*2^5 = 2048 or 2^6*2^6 = 4096 entries.  Note that
 * on 80x86 machines, the pointer row is in near memory but the actual
 * arrays are in far memory (same arrangement as we use for image arrays).
 */

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

/* These will do the right thing for either R,G,B or B,G,R color order,
 * but you may not like the results for other color orders.
 */
#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 */

/* Number of elements along histogram axes. */
#define HIST_C0_ELEMS  (1<<HIST_C0_BITS)
#define HIST_C1_ELEMS  (1<<HIST_C1_BITS)
#define HIST_C2_ELEMS  (1<<HIST_C2_BITS)

/* These are the amounts to shift an input value to get a histogram index. */
#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)


typedef UINT16 histcell;	/* histogram cell; prefer an unsigned type */

typedef histcell FAR *histptr;	/* for pointers to histogram cells */

typedef histcell hist1d[HIST_C2_ELEMS];	/* typedefs for the array */
typedef hist1d FAR *hist2d;	/* type for the 2nd-level pointers */
typedef hist2d *hist3d;		/* type for top-level pointer */


/* 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 has (#columns + 2) entries; the extra entry at
 * each end saves us from special-casing the first and last pixels.
 * Each entry is three values long, one value for each color component.
 *
 * 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 */

typedef struct
{
#ifdef ORIGINAL_LIB_JPEG
  struct jpeg_color_quantizer pub;	/* public fields */

  /* Space for the eventually created colormap is stashed here */
  JSAMPARRAY sv_colormap;	/* colormap allocated at init time */
  int desired;			/* desired # of colors = size of colormap */
  boolean needs_zeroed;		/* TRUE if next pass must zero histogram */
#endif

  /* Variables for accumulating image statistics */
  hist3d histogram;		/* pointer to the histogram */


  /* Variables for Floyd-Steinberg dithering */
  FSERRPTR fserrors;		/* accumulated errors */

  boolean on_odd_row;		/* flag to remember which row we are on */
  int *error_limiter;		/* table for clamping the applied error */
#ifndef ORIGINAL_LIB_JPEG
  int *error_limiter_storage;	/* gdMalloc'd storage for the above */
#endif
}
my_cquantizer;

typedef my_cquantizer *my_cquantize_ptr;


/*
 * Prescan some rows of pixels.
 * In this module the prescan simply updates the histogram, which has been
 * initialized to zeroes by start_pass.
 * An output_buf parameter is required by the method signature, but no data
 * is actually output (in fact the buffer controller is probably passing a
 * NULL pointer).
 */

METHODDEF (void)
#ifndef ORIGINAL_LIB_JPEG
prescan_quantize (gdImagePtr oim, gdImagePtr nim, my_cquantize_ptr cquantize)
{
#else
prescan_quantize (j_decompress_ptr cinfo, JSAMPARRAY input_buf,
		  JSAMPARRAY output_buf, int num_rows)
{
  my_cquantize_ptr cquantize = (my_cquantize_ptr) cinfo->cquantize;
#endif
  register JSAMPROW ptr;
  register histptr histp;
  register hist3d histogram = cquantize->histogram;
  int row;
  JDIMENSION col;
#ifdef ORIGINAL_LIB_JPEG
  JDIMENSION width = cinfo->output_width;
#else
  int width = oim->sx;
  int num_rows = oim->sy;
#endif

  for (row = 0; row < num_rows; row++)
    {
      ptr = input_buf[row];
      for (col = width; col > 0; col--)
	{
#ifdef ORIGINAL_LIB_JPEG
	  int r = GETJSAMPLE (ptr[0]) >> C0_SHIFT;
	  int g = GETJSAMPLE (ptr[1]) >> C1_SHIFT;
	  int b = GETJSAMPLE (ptr[2]) >> C2_SHIFT;
#else
	  int r = gdTrueColorGetRed (*ptr) >> C0_SHIFT;
	  int g = gdTrueColorGetGreen (*ptr) >> C1_SHIFT;
	  int b = gdTrueColorGetBlue (*ptr) >> C2_SHIFT;
	  /* 2.0.12: Steven Brown: support a single totally transparent
	     color in the original. */
	  if ((oim->transparent >= 0) && (*ptr == oim->transparent))
	    {
	      ptr++;
	      continue;
	    }
#endif
	  /* get pixel value and index into the histogram */
	  histp = &histogram[r][g][b];
	  /* increment, check for overflow and undo increment if so. */
	  if (++(*histp) == 0)
	    (*histp)--;
#ifdef ORIGINAL_LIB_JPEG
	  ptr += 3;
#else
	  ptr++;
#endif
	}
    }
}


/*
 * Next we have the really interesting routines: selection of a colormap
 * given the completed histogram.
 * These routines work with a list of "boxes", each representing a rectangular
 * subset of the input color space (to histogram precision).
 */

typedef struct
{
  /* The bounds of the box (inclusive); expressed as histogram indexes */
  int c0min, c0max;
  int c1min, c1max;
  int c2min, c2max;
  /* The volume (actually 2-norm) of the box */
  INT32 volume;
  /* The number of nonzero histogram cells within this box */