/*****************************************************************************

 *

 * GIFENCODE.C    - GIF Image compression interface

 *

 * GIFEncode( FName, GHeight, GWidth, GInterlace, Background, 

 *	      BitsPerPixel, Red, Green, Blue, GetPixel )

 *

 *****************************************************************************/

 

#include <stdio.h>



/*

 * Pointer to function returning an int

 */

typedef int (* ifunptr)();



#define TRUE 1

#define FALSE 0



static int Width, Height;

static int curx, cury;

static long CountDown;

static int Pass = 0;

static int Interlace;



/*

 * Bump the 'curx' and 'cury' to point to the next pixel

 */

static

BumpPixel()

{

	/*

	 * Bump the current X position

	 */

	curx++;



	/*

	 * If we are at the end of a scan line, set curx back to the beginning

	 * If we are interlaced, bump the cury to the appropriate spot,

	 * otherwise, just increment it.

	 */

	if( curx == Width ) {

		curx = 0;



	        if( !Interlace ) 

			cury++;

		else {

		     switch( Pass ) {

	     

	               case 0:

        	          cury += 8;

                	  if( cury >= Height ) {

		  		Pass++;

				cury = 4;

		  	  }

                          break;

		  

	               case 1:

        	          cury += 8;

                	  if( cury >= Height ) {

		  		Pass++;

				cury = 2;

		  	  }

			  break;

		  

	               case 2:

	                  cury += 4;

	                  if( cury >= Height ) {

	                     Pass++;

	                     cury = 1;

	                  }

	                  break;

			  

	               case 3:

	                  cury += 2;

	                  break;

			}

		}

	}

}



/*

 * Return the next pixel from the image

 */

GIFNextPixel( getpixel )

ifunptr getpixel;

{

	int r;



	if( CountDown == 0 )

		return EOF;



	CountDown--;



	r = ( * getpixel )( curx, cury );



	BumpPixel();



	return r;

}



/* public */



GIFEncode( FName, GWidth, GHeight, GInterlace, Background, 

	   BitsPerPixel, Red, Green, Blue, GetPixel )

	 

char *FName;

int GWidth, GHeight;

int GInterlace;

int Background;

int BitsPerPixel;

int Red[], Green[], Blue[];

ifunptr GetPixel;



{

	FILE *fp;

	int B;

	int RWidth, RHeight;

	int LeftOfs, TopOfs;

	int Resolution;

	int ColorMapSize;

	int InitCodeSize;

	int i;



	Interlace = GInterlace;

	

	ColorMapSize = 1 << BitsPerPixel;

	

	RWidth = Width = GWidth;

	RHeight = Height = GHeight;

	LeftOfs = TopOfs = 0;

	

	Resolution = BitsPerPixel;



	/*

	 * Calculate number of bits we are expecting

	 */

	CountDown = (long)Width * (long)Height;



	/*

	 * Indicate which pass we are on (if interlace)

	 */

	Pass = 0;



	/*

	 * The initial code size

	 */

	if( BitsPerPixel <= 1 )

		InitCodeSize = 2;

	else

		InitCodeSize = BitsPerPixel;



	/*

	 * Set up the current x and y position

	 */

	curx = cury = 0;



	/*

	 * Open the GIF file for binary write

	 */

	fp = fopen( FName, "wb" );



	if( fp == (FILE *)0 ) {

		printf( "error: could not open output file\n" );

		exit(1);

	}



	/*

	 * Write the Magic header

	 */

	fwrite( "GIF87a", 1, 6, fp );



	/*

	 * Write out the screen width and height

	 */

	Putword( RWidth, fp );

	Putword( RHeight, fp );



	/*

	 * Indicate that there is a global colour map

	 */

	B = 0x80;	/* Yes, there is a color map */



	/*

	 * OR in the resolution

	 */

	B |= (Resolution - 1) << 5;



	/*

	 * OR in the Bits per Pixel

	 */

	B |= (BitsPerPixel - 1);



	/*

	 * Write it out

	 */

	fputc( B, fp );



	/*

	 * Write out the Background colour

	 */

	fputc( Background, fp );



	/*

	 * Byte of 0's (future expansion)

	 */

	fputc( 0, fp );



	/*

	 * Write out the Global Colour Map

	 */

     	for( i=0; i<ColorMapSize; i++ ) {

		fputc( Red[i], fp );

		fputc( Green[i], fp );

		fputc( Blue[i], fp );

	}



	/*

	 * Write an Image separator

	 */

	fputc( ',', fp );



	/*

	 * Write the Image header

	 */



	Putword( LeftOfs, fp );

	Putword( TopOfs, fp );

	Putword( Width, fp );

	Putword( Height, fp );



	/*

	 * Write out whether or not the image is interlaced

	 */

	if( Interlace )

		fputc( 0x40, fp );

	else

		fputc( 0x00, fp );



	/*

	 * Write out the initial code size

	 */

	fputc( InitCodeSize, fp );



	/*

	 * Go and actually compress the data

	 */

	compress( InitCodeSize+1, fp, GetPixel );



	/*

	 * Write out a Zero-length packet (to end the series)

	 */

	fputc( 0, fp );



	/*

	 * Write the GIF file terminator

	 */

	fputc( ';', fp );



	/*

	 * And close the file

	 */

	fclose( fp );

	

}



/*

 * Write out a word to the GIF file

 */

static

Putword( w, fp )

int w;

FILE *fp;

{

	fputc( w & 0xff, fp );

	fputc( (w / 256) & 0xff, fp );

}