gifencod.c

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/*****************************************************************************
 *
 * GIFENCODE.C    - GIF Image compression interface
 * 
 * GIFEncode( FName, GHeight, GWidth, GInterlace, Background,
 *        BitsPerPixel, Red, Green, Blue, GetPixel )
 *
 * Modified for use with RLE interface, Spencer W. Thomas, June, 1990.
 * (replaced fopen with rle_open_f).
 *
 *****************************************************************************/

#include <stdio.h>
#include "rle.h"
#include "rletogif.h"

#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 void
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;
            }
        }
    }
}

/*
 * Write out a word to the GIF file
 */
static void
Putword( w, fp )
int w;
FILE *fp;
{
    fputc( w & 0xff, fp );
    fputc( (w >> 8) & 0xff, fp );
}


/*
 * Return the next pixel from the image
 */
int
GIFNextPixel( getpixel )
ifunptr getpixel;
{
    int r;

    if( CountDown == 0 )
        return EOF;

    CountDown--;

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

    BumpPixel();

    return r;
}

/* public */

void
GIFEncode( FName, GWidth, GHeight, GInterlace, Background,
       BitsPerPixel, Red, Green, Blue, GetPixel )

char *FName;
int GWidth, GHeight;
int GInterlace;
int Background;
int BitsPerPixel;
short int Red[], Green[], Blue[];
ifunptr GetPixel;
{
    FILE *fp;
    int B;
    int RWidth, RHeight;
    int LeftOfs, TopOfs;
    int Resolution;
    int ColorMapSize;
    int InitCodeSize;
    int i;

    /* doesn't support interlace yet */
    if (GInterlace) error("no support for interlace yet");
    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 = rle_open_f( MY_NAME, FName, "w" );

    /*
     * 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]>>8, fp );
            fputc( Green[i]>>8, fp );
            fputc( Blue[i]>>8, 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
     */
    compgif( 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 );

}