gifencoder.java

图像文件分析

		    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;
		    }
		}
	    }
	}

    static final int EOF = -1;

    // Return the next pixel from the image
    int GIFNextPixel() throws IOException
	{
	byte r;

	if ( CountDown == 0 )
	    return EOF;

	--CountDown;

	r = GetPixel( curx, cury );

	BumpPixel();

	return r & 0xff;
	}

    // Write out a word to the GIF file
    void Putword( int w, OutputStream outs ) throws IOException
	{
	Putbyte( (byte) ( w & 0xff ), outs );
	Putbyte( (byte) ( ( w >> 8 ) & 0xff ), outs );
	}

    // Write out a byte to the GIF file
    void Putbyte( byte b, OutputStream outs ) throws IOException
	{
	outs.write( b );
	}


    // GIFCOMPR.C       - GIF Image compression routines
    //
    // Lempel-Ziv compression based on 'compress'.  GIF modifications by
    // David Rowley ([e-mail])

    // General DEFINEs

    static final int BITS = 12;

    static final int HSIZE = 5003;		// 80% occupancy

    // GIF Image compression - modified 'compress'
    //
    // Based on: compress.c - File compression ala IEEE Computer, June 1984.
    //
    // By Authors:  Spencer W. Thomas      (decvax!harpo!utah-cs!utah-gr!thomas)
    //              Jim McKie              (decvax!mcvax!jim)
    //              Steve Davies           (decvax!vax135!petsd!peora!srd)
    //              Ken Turkowski          (decvax!decwrl!turtlevax!ken)
    //              James A. Woods         (decvax!ihnp4!ames!jaw)
    //              Joe Orost              (decvax!vax135!petsd!joe)

    int n_bits;				// number of bits/code
    int maxbits = BITS;			// user settable max # bits/code
    int maxcode;			// maximum code, given n_bits
    int maxmaxcode = 1 << BITS; // should NEVER generate this code

    final int MAXCODE( int n_bits )
	{
	return ( 1 << n_bits ) - 1;
	}

    int[] htab = new int[HSIZE];
    int[] codetab = new int[HSIZE];

    int hsize = HSIZE;		// for dynamic table sizing

    int free_ent = 0;			// first unused entry

    // block compression parameters -- after all codes are used up,
    // and compression rate changes, start over.
    boolean clear_flg = false;

    // Algorithm:  use open addressing double hashing (no chaining) on the
    // prefix code / next character combination.  We do a variant of Knuth's
    // algorithm D (vol. 3, sec. 6.4) along with G. Knott's relatively-prime
    // secondary probe.  Here, the modular division first probe is gives way
    // to a faster exclusive-or manipulation.  Also do block compression with
    // an adaptive reset, whereby the code table is cleared when the compression
    // ratio decreases, but after the table fills.  The variable-length output
    // codes are re-sized at this point, and a special CLEAR code is generated
    // for the decompressor.  Late addition:  construct the table according to
    // file size for noticeable speed improvement on small files.  Please direct
    // questions about this implementation to ames!jaw.

    int g_init_bits;

    int ClearCode;
    int EOFCode;

    void compress( int init_bits, OutputStream outs ) throws IOException
	{
	int fcode;
	int i /* = 0 */;
	int c;
	int ent;
	int disp;
	int hsize_reg;
	int hshift;

	// Set up the globals:  g_init_bits - initial number of bits
	g_init_bits = init_bits;

	// Set up the necessary values
	clear_flg = false;
	n_bits = g_init_bits;
	maxcode = MAXCODE( n_bits );

	ClearCode = 1 << ( init_bits - 1 );
	EOFCode = ClearCode + 1;
	free_ent = ClearCode + 2;

	char_init();

	ent = GIFNextPixel();

	hshift = 0;
	for ( fcode = hsize; fcode < 65536; fcode *= 2 )
	    ++hshift;
	hshift = 8 - hshift;			// set hash code range bound

	hsize_reg = hsize;
	cl_hash( hsize_reg );	// clear hash table

	output( ClearCode, outs );

	outer_loop:
	while ( (c = GIFNextPixel()) != EOF )
	    {
	    fcode = ( c << maxbits ) + ent;
	    i = ( c << hshift ) ^ ent;		// xor hashing

	    if ( htab[i] == fcode )
		{
		ent = codetab[i];
		continue;
		}
	    else if ( htab[i] >= 0 )	// non-empty slot
		{
		disp = hsize_reg - i;	// secondary hash (after G. Knott)
		if ( i == 0 )
		    disp = 1;
		do
		    {
		    if ( (i -= disp) < 0 )
			i += hsize_reg;

		    if ( htab[i] == fcode )
			{
			ent = codetab[i];
			continue outer_loop;
			}
		    }
		while ( htab[i] >= 0 );
		}
	    output( ent, outs );
	    ent = c;
	    if ( free_ent < maxmaxcode )
		{
		codetab[i] = free_ent++;	// code -> hashtable
		htab[i] = fcode;
		}
	    else
		cl_block( outs );
	    }
	// Put out the final code.
	output( ent, outs );
	output( EOFCode, outs );
	}

    // output
    //
    // Output the given code.
    // Inputs:
    //      code:   A n_bits-bit integer.  If == -1, then EOF.  This assumes
    //              that n_bits =< wordsize - 1.
    // Outputs:
    //      Outputs code to the file.
    // Assumptions:
    //      Chars are 8 bits long.
    // Algorithm:
    //      Maintain a BITS character long buffer (so that 8 codes will
    // fit in it exactly).  Use the VAX insv instruction to insert each
    // code in turn.  When the buffer fills up empty it and start over.

    int cur_accum = 0;
    int cur_bits = 0;

    int masks[] = { 0x0000, 0x0001, 0x0003, 0x0007, 0x000F,
		    0x001F, 0x003F, 0x007F, 0x00FF,
		    0x01FF, 0x03FF, 0x07FF, 0x0FFF,
		    0x1FFF, 0x3FFF, 0x7FFF, 0xFFFF };

    void output( int code, OutputStream outs ) throws IOException
	{
	cur_accum &= masks[cur_bits];

	if ( cur_bits > 0 )
	    cur_accum |= ( code << cur_bits );
	else
	    cur_accum = code;

	cur_bits += n_bits;

	while ( cur_bits >= 8 )
	    {
	    char_out( (byte) ( cur_accum & 0xff ), outs );
	    cur_accum >>= 8;
	    cur_bits -= 8;
	    }

	// If the next entry is going to be too big for the code size,
	// then increase it, if possible.
       if ( free_ent > maxcode || clear_flg )
	    {
	    if ( clear_flg )
		{
		maxcode = MAXCODE(n_bits = g_init_bits);
		clear_flg = false;
		}
	    else
		{
		++n_bits;
		if ( n_bits == maxbits )
		    maxcode = maxmaxcode;
		else
		    maxcode = MAXCODE(n_bits);
		}
	    }

	if ( code == EOFCode )
	    {
	    // At EOF, write the rest of the buffer.
	    while ( cur_bits > 0 )
		{
		char_out( (byte) ( cur_accum & 0xff ), outs );
		cur_accum >>= 8;
		cur_bits -= 8;
		}

	    flush_char( outs );
	    }
	}

    // Clear out the hash table

    // table clear for block compress
    void cl_block( OutputStream outs ) throws IOException
	{
	cl_hash( hsize );
	free_ent = ClearCode + 2;
	clear_flg = true;

	output( ClearCode, outs );
	}

    // reset code table
    void cl_hash( int hsize )
	{
	for ( int i = 0; i < hsize; ++i )
	    htab[i] = -1;
	}

    // GIF Specific routines

    // Number of characters so far in this 'packet'
    int a_count;

    // Set up the 'byte output' routine
    void char_init()
	{
	a_count = 0;
	}

    // Define the storage for the packet accumulator
    byte[] accum = new byte[256];

    // Add a character to the end of the current packet, and if it is 254
    // characters, flush the packet to disk.
    void char_out( byte c, OutputStream outs ) throws IOException
	{
	accum[a_count++] = c;
	if ( a_count >= 254 )
	    flush_char( outs );
	}

    // Flush the packet to disk, and reset the accumulator
    void flush_char( OutputStream outs ) throws IOException
	{
	if ( a_count > 0 )
	    {
	    outs.write( a_count );
	    outs.write( accum, 0, a_count );
	    a_count = 0;
	    }
	}

    }

class GifEncoderHashitem
    {

    public int rgb;
    public int count;
    public int index;
    public boolean isTransparent;

    public GifEncoderHashitem( int rgb, int count, int index, boolean isTransparent )
	{
	this.rgb = rgb;
	this.count = count;
	this.index = index;
	this.isTransparent = isTransparent;
	}

    }