File: palette.c

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gnome-xcf-thumbnailer 1.0-1.1
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/* Palette-manipulation functions functions for xcftools
 *
 * Copyright (C) 2006  Henning Makholm
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of version 2 of the GNU General Public License as
 * published by the Free Software Foundation.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, write to the Free Software
 * Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA
 */

#include "palette.h"
#include <assert.h>

#define HASH_SIZE 1711
/* If I remember correctly, this size hash will be able to handle
 * either of
 *  a) the Netscape cube with intensities 0x00, 0x33, 0x66, 0x99, 0xCC, xFF
 *  b) the EGA cube with intensities 0x00, 0x55, 0xAA, 0xFF
 *  c) the "CGA cube" with intensites 0x00, 0x80, 0xFF
 *  d) a full 256-step grayscale
 * without collisions. It will also have a minimal number of collisions
 * (4) on a full 16-to-a-side cube with intensities
 * 0x00, 0x11, 0x22, ..., 0xDD, 0xEE, 0xFF.
 */

unsigned paletteSize ;
rgba palette[MAX_PALETTE] ;
static int masterhash[HASH_SIZE];
static int bucketlinks[MAX_PALETTE];
    
void
init_palette_hash(void)
{
  unsigned i ;
  for( i=0; i<HASH_SIZE; i++ )
    masterhash[i] = -1 ;
  for( i=0; i<MAX_PALETTE; i++ )
    bucketlinks[i] = -1 ;
  paletteSize = 0 ;
}

inline int
lookup_or_intern(rgba color) {
  int *target = &masterhash[color % HASH_SIZE];
  while( *target >= 0 ) {
    if( palette[*target] == color )
      return *target ;
    target = &bucketlinks[*target] ;
  }
#if 0
  fprintf(stderr,"Palette[%u] = %08x (%u --> %d)\n",paletteSize,color,
          color % HASH_SIZE, *target);
#endif
  if( paletteSize >= MAX_PALETTE )
    return -1 ;
  *target = paletteSize ;
  palette[paletteSize] = color ;
  return paletteSize++ ;
}

static inline void
unpalettify_row(rgba *row,unsigned ncols)
{
  index_t *newrow = (index_t*) row ;
  unsigned i ;
  for( i=ncols; i--; ) {
    row[i] = palette[newrow[i]] ;
  }
}

int
palettify_row(rgba *row,unsigned ncols)
{
  index_t *newrow = (index_t*)row ;
  assert(sizeof(index_t) <= sizeof(rgba));
  unsigned i ;
  for( i=0; i<ncols; i++ ) {
    int j = lookup_or_intern(row[i]) ;
    if( j < 0 ) {
      unpalettify_row(row,i);
      return 0 ;
    }
    newrow[i] = j ;
  }
  return 1 ;
}
    
int
palettify_rows (rgba *rows[],unsigned ncols,unsigned nlines)
{
  unsigned i ;
  for( i=0; i<nlines; i++ ) {
    if( !palettify_row(rows[i],ncols) ) {
      while( i-- )
        unpalettify_row(rows[i],ncols);
      return 0 ;
    }
  }
  return 1 ;
}