vglgif.c

在DOS13号中断下开发图形显示程序的例程

/*****************************************************************************
 VGLGIF.C

 vglGif( char* file, char far* buffer, char far* palette,
         int* width, int* height );

 Routine to load a 256 color .GIF file into a memory buffer.  *Only* 256
 color images are supported here!  Sorry, no routines to SAVE .GIFs...
 Memory required is allocated on the fly and no checks are in place.  If you
 don't have enough memory it will likely crash.  It's easy to add the checks
 yourself, just put one after each call to malloc().  If you supply a pointer
 to a palette, it will be filled in.  If you supply a pointer to a width
 and/or height variable, it will be filled in as well.

 Mark Morley
 [e-mail]
*****************************************************************************/

#include <stdio.h>

#define MAX_CODES     4096

static FILE*          fp;
static int            curr_size;
static int            clear;
static int            ending;
static int            newcodes;
static int            top_slot;
static int            slot;
static int            navail_bytes = 0;
static int            nbits_left = 0;
static unsigned char  b1;
static unsigned char  byte_buff[257];
static unsigned char* pbytes;
static unsigned char* stack;
static unsigned char* suffix;
static unsigned int*  prefix;

static unsigned long code_mask[13] =
{
   0L,
   0x0001L, 0x0003L,
   0x0007L, 0x000FL,
   0x001FL, 0x003FL,
   0x007FL, 0x00FFL,
   0x01FFL, 0x03FFL,
   0x07FFL, 0x0FFFL
};

static int pascal
get_next_code()
{
   register int  i;
   static unsigned long ret;

   if( ! nbits_left )
   {
      if( navail_bytes <= 0 )
      {
	 pbytes = byte_buff;
         navail_bytes = getc( fp );
	 if( navail_bytes )
	    for( i = 0; i < navail_bytes; ++i )
               *(byte_buff + i) = getc( fp );
      }
      b1 = *pbytes++;
      nbits_left = 8;
      --navail_bytes;
   }
   ret = b1 >> (8 - nbits_left);
   while( curr_size > nbits_left )
   {
      if( navail_bytes <= 0 )
      {
	 pbytes = byte_buff;
         navail_bytes = getc( fp );
	 if( navail_bytes )
	    for( i = 0; i < navail_bytes; ++i )
               *(byte_buff + i) = getc( fp );
      }
      b1 = *pbytes++;
      ret |= b1 << nbits_left;
      nbits_left += 8;
      --navail_bytes;
   }
   nbits_left -= curr_size;

   return( (int) (ret & *(code_mask + curr_size)) );
}

vglGif( char* file, char far* buffer, char far* pal, int* width, int* height )
{
   unsigned char* sp;
   int            code, fc, oc;
   int            i;
   unsigned char  size;
   int            c;
   unsigned char  buf[1028];
   unsigned char  red;
   unsigned char  grn;
   unsigned char  blu;

   fp = fopen( file, "rb" );
   if( !fp )
      return( 0 );
   fread( buf, 1, 6, fp );
   if( strncmp( buf, "GIF", 3 ) )
   {
      fclose( fp );
      return( 0 );
   }
   fread( buf, 1, 7, fp );
   for( i = 0; i < 768; )
   {
      red = getc( fp );
      grn = getc( fp );
      blu = getc( fp );

      if( pal )
      {
         pal[i++] = red >> 2;
         pal[i++] = grn >> 2;
         pal[i++] = blu >> 2;
      }
      else
         i += 3;
   }
   fread( buf, 1, 5, fp );
   i = getw( fp );
   if( width )
      *width = i;
   i = getw( fp );
   if( height )
      *height = i;
   if( !buffer )
      return( 1 );
   fread( buf, 1, 1, fp );
   size = getc( fp );
   if( size < 2 || 9 < size )
   {
      fclose( fp );
      return( 0 );
   }

   stack = (unsigned char*) malloc( MAX_CODES + 1 );
   suffix = (unsigned char*) malloc( MAX_CODES + 1 );
   prefix = (unsigned int*) malloc( sizeof(int) * (MAX_CODES + 1) );

   curr_size = size + 1;
   top_slot = 1 << curr_size;
   clear = 1 << size;
   ending = clear + 1;
   slot = newcodes = ending + 1;
   navail_bytes = nbits_left = 0;
   oc = fc = 0;
   sp = stack;
   while( (c = get_next_code()) != ending )
   {
      if( c == clear )
      {
	 curr_size = size + 1;
	 slot = newcodes;
	 top_slot = 1 << curr_size;
	 while( (c = get_next_code()) == clear );
	 if( c == ending )
	    break;
	 if( c >= slot )
	    c = 0;
	 oc = fc = c;
         *buffer++ = c;
      }
      else
      {
	 code = c;
	 if( code >= slot )
	 {
	    code = oc;
	    *sp++ = fc;
	 }
	 while( code >= newcodes )
	 {
	    *sp++ = *(suffix + code);
	    code = *(prefix + code);
	 }
	 *sp++ = code;
	 if( slot < top_slot )
	 {
	    *(suffix + slot) = fc = code;
	    *(prefix + slot++) = oc;
	    oc = c;
	 }
	 if( slot >= top_slot && curr_size < 12 )
	 {
	    top_slot <<= 1;
	    ++curr_size;
	 }
	 while( sp > stack )
	 {
	    --sp;
            *buffer++ = *sp;
	 }
      }
   }
   free( stack );
   free( suffix );
   free( prefix );
   fclose( fp );
   return( 1 );
}