jccolor.c

JPEG压缩和解压程序和一些相关的说明文档 内容比较全

/*
 * jccolor.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 input colorspace conversion routines.
 */

#include "commondecls.h"



#define MAXJSAMPLE    255
#define SCALEBITS	16	/* speediest right-shift on some machines */
#define ONE_HALF	((INT32) 1 << (SCALEBITS-1))
#define FIX(x)		((INT32) ((x) * (1L<<SCALEBITS) + 0.5))

/* We allocate one big table and divide it up into three parts, instead of
 * doing eight alloc_small requests.  This lets us use a single table base
 * address, which can be held in a register in the inner loops on many
 * machines (more than can hold all eight addresses, anyway).
 */

#define R_Y_OFF		0			/* offset to R => Y section */
#define G_Y_OFF		(1*(MAXJSAMPLE+1))	/* offset to G => Y section */
#define B_Y_OFF		(2*(MAXJSAMPLE+1))	/* etc. */


/*
 * Initialize for RGB->YCC colorspace conversion.
 */

void
jinit_color_transform (bmp_source_ptr sinfo)
{
  
  INT32 * rgb_gray_tab;
  INT32 i;

  /* Allocate and fill in the conversion tables. */
  sinfo->rgb_gray_tab = rgb_gray_tab = (INT32 *)
    alloc_one_row ( ((size_t) (3*256))* SIZEOF(INT32));

  for (i = 0; i <= MAXJSAMPLE; i++) {
    rgb_gray_tab[i+R_Y_OFF] = FIX(0.29900) * i;
    rgb_gray_tab[i+G_Y_OFF] = FIX(0.58700) * i;
    rgb_gray_tab[i+B_Y_OFF] = FIX(0.11400) * i     + ONE_HALF;
   
  } 
}


void
rgb_gray_convert (bmp_source_ptr sinfo,bmp_destination_ptr dinfo)
{
  
  register int r, g, b;
  register INT32 * ctab = sinfo->rgb_gray_tab;
  register JSAMPROW inptr=sinfo->rgb_buffer;
  register JSAMPROW outptr=dinfo->buffer;
  register JDIMENSION col;
  JDIMENSION num_cols = dinfo->image_width;

  
    for (col = 0; col < num_cols; col++) {
      r = (INT32)(inptr[0]);
      g = (INT32)(inptr[1]);
      b = (INT32)(inptr[2]);
      inptr += 3;
      /* Y */
      outptr[col] = (JSAMPLE)
		((ctab[r+R_Y_OFF] + ctab[g+G_Y_OFF] + ctab[b+B_Y_OFF])
		 >> SCALEBITS);
    }
 }